Climate ChangeV. R A M A N AT H A NUC San Diego, Scripps Institution of Oceanography

C H A P T E R C O N T E N T S

Learning Objectives

Overview

Entering the Age of Humans

he Atmospheric an et an Its arming E ect

hy an How Is C imate Changing

Impacts of Climate Change

Summary hat Have e Learne So ar

Supplementary Readings

Sources for the Figures

Sources for the Text

Learning Objectives

. Explain the basic concepts of climate change science.You will learn how some atmospheric gases emitted by human ac-tivities spread around the planet like a blanket and how that blanket traps infrared radiation (heat radiation) and warms the planet. Such gases are popularly called greenhouse gases. A basic knowledge of climate science will help motivate you to solve the problem of in-creased greenhouse gases. You will be able to explain the underlying scienti c principles of climate change to others, including skeptics.

. Discuss the anthropogenic drivers of climate change.ext you will learn how our various activities driving, ying, cook-

ing, heating and cooling homes, and producing food—contribute to climate change. Because these drivers are related to human activities, we call them anthropogenic drivers of climate change (anthropogenic is the scienti c term for human-caused ).

. Explain how and why the climate is changing.By this point, you will know how the greenhouse gases emitted by human activities are expected to change the climate, based on phys-ical principles. The third learning objective is to understand how the climate is changing now and how our observations of the weather are matching predictions from climate models.

. Describe the likely climate changes and their projected impacts.The fourth objective is to use the knowledge you have gained thus far to describe the potential impacts of the warming on aspects of climate that a ect us all, including heat extremes, droughts, oods, sea level rise, and melting sea ice and glaciers. It is having huge impacts on almost everything we know. Climate change is causing new weather extremes such as oods, heat waves, and droughts, with negative e ects on public health. It is better termed as climate disruption. We will track down the various impacts we are already experiencing today and project the impacts that are likely to occur in the future if we continue on our current path of unsustainable greenhouse gas emissions.

Overview

The climate system is dynamic and has undergone major changes in the history of the planet. Over the last million years, the Earth s climate has cycled between cool glacial periods and warm interglacial periods. These cycles have occurred about every , years over at least the past , years. Beginning , years ago, the Earth transitioned to the current interglacial warm epoch called the Holocene. Before the nineteenth century, climate change on the planet was mainly a naturally occurring phenomenon caused by changes in the Earth s orbit around the sun, changes in the amount of solar radiation reaching the Earth, volcanic activity, and natural patterns of heat exchange between the land, the ocean, and the atmosphere.

Since the dawn of the industrial era, a new global causal factor has been added to this list: humans. We emit carbon dioxide and other pollutant gases when we burn fossil fuels and do many other things, such as refrigerate our food and fertilize our crops. These pollutants have drastically altered the heat-trapping properties of the atmosphere. In the case of carbon dioxide, the changes are irreversible on time scales of thousands of years or more. These pollutant gases now cover the Earth like a blanket, trapping infrared heat and warming the planet. The climate has already warmed by °C since the preindustrial era and in another years (from ) will reach levels not seen in the past

, years. Climate scientists have concluded that if the current rate of emissions continues, the planet will warm to levels not observed in the last million years or more. ot only is the amount of the warm-ing unprecedented, but the rate of change is also orders of magnitude larger than that of past natural variations.

How do we know this is true Climate change science is intensely data driven and has undergone the traditional scrutiny and rigor of scienti c methods. It has taken thousands of peer-reviewed studies over more than years and large uantities of data collected from ships, surface stations, aircra , and satellites to arrive at the conclusions described in this chapter. The ndings reported here are based on analyses of literally trillions of bytes of data by thousands of scientists from around the world as reported in thousands of peer-reviewed publications. These studies have been reviewed by science academies in the United States

and around the world since the s, culminating in the formation of the Intergovernmental Panel on Climate Change (IPCC) by the United

ations in . Hundreds to thousands of scientists contribute to the IPCCs periodic reports that assess, evaluate, and update the science and the data. References to some of these reports are provided at the end of this chapter.

The most important messages in this chapter are that ( ) anthro-pogenic emissions are causing climate change at a magnitude and rate that are unprecedented over at least the past million years, and ( ) this human-caused climate change is likely to have severe impacts on both the natural environment and human society. Finally, it is particularly im-portant to recognize that we still have time to act to re ce h man ca se climate change and moderate or avoid the most serious impacts—if we start acting now ( ). The purpose of this book is to empower you and give you the tools you will need to act as climate warriors innovating and implementing climate solutions.

This chapter will also help you address uestions that people you know, including climate change skeptics, might ask you. For example, how do you know the climate is changing Even if it is changing, how do you know the change is caused by human activities And whom do you believe

Your answer to the last uestion should be simple: we do not have to believe anyone. We have the data. Thousands of scientists have an-alyzed and interpreted observed data from peer-reviewed studies, so these are facts, not beliefs. The real issue we want to address is the following: if we continue along the current path of unsustainable pollu-tion, what does the future hold for us How will the planet look a few decades from now And what s in store by the end of the century The projections made by various scienti c institutions are summarized in this chapter.

Hopefully these scienti cally projected scenarios will give you the reasons, as well as the motivation, to solve the problem in a timely man-ner. As you already know, this book is about solving the climate change problem. A hopeful message comes out of the ndings summarized in this chapter: there is still time to solve the problem of climate change and stabilize the climate below dangerous levels of warming.

The Intergovernmental Panel on Climate Change

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Increase in atmospheric concentration of carbon dioxide (CO ), methane (CH ), and nitrous oxide (N O) with time. Reproduced from IPCC.

80010001200140016001800

CH4

(ppb

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270280290300310320330

280

300

320

340

360

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CO2

(ppm

)

1850 1900 1950 2000Year

Globally averaged greenhouse gas concentrations

Before diving into the problem of anthropogenic climate change, it s helpful to understand what the planet s climate was like before modern humans (Homo sapiens) became a major force.

The history of Homo sapiens extends as far back as , years ago. However, it was only in the last , years, with the advent of the Agricultural Revolution, that Homo sapiens began modifying the land surface for growing food. The Agricultural Revolution roughly coincides with a warm interglacial that began about , years ago and con-tinues to the present. Scientists call this period of warm and relatively stable climate the Holocene. Although there were signi cant regional climate uctuations, the stable climate of the Holocene enabled the Agricultural Revolution. During this period humans transitioned from hunting and gathering to agriculture. Cities, writing, and major human civilizations all developed during this time. The last years of the Holocene ushered in the Industrial Revolution, and with that, climate pollution began to increase dramatically.

Entering the Age of Humans

. . Homo sapiens history. Image from V. Ramanathan.

The “Great Acceleration”The Industrial Revolution, which started in Britain and evolved during AD

, was powered by fossil fuels. ow nearly all nations in the world consume them. Beginning around , a er World War II, the production and consumption of goods increased dramatically, leading to accelerated burning of fossil fuels. This acceleration is illustrated in every measure we can think of: population, gross domestic product (GDP), water use, fertilizer consumption, the number of motor vehicles in circulation, and many more. Because of this, scientists call the period from to the present the Great Acceleration.

Not only did the Great Acceleration increase production, consump-tion, and population, it also le a huge imprint on the ecosystem. For example, the rate of change in carbon dioxide concentration, which was already steadily increasing, accelerated in . By the s, this increase in carbon dioxide had already led to a signi cant rise in global temperatures, which in turn brought huge ecological impacts. Depletion of the ozone layer again became a major factor in the s. A er

, the biodiversity of species started decreasing and the extinction of species accelerated.

. . ects of the reat cce eration on six di erent measures. s human popu ation and ross domestic product ( ) ha e risen sharp y,

so ha e water use, carbon dioxide emissions, deforestation, and species extinctions. dapted from te en et a . .

Figure . . shows a few examples of the rapid increase in human activity and its ecological impacts during the Great Acceleration. Notice that nearly every measure—population, total real GDP, water use, bio-sphere degradation (species extinctions), and the loss of tropical rain forests—takes o dramatically beginning around .

No part of the planet remains una ected by the Great Acceleration. The planet we see today is vastly di erent from the planet our ancestors inherited a century ago.

The AnthropoceneWe are still in the Holocene climatologically, but the impacts of the Industrial Revolution and Great Acceleration, combined with a massive population increase, have caused many scientists and other thinkers to argue that we have entered a new period called the Anthropocene, or age of humans. Humans have emerged as a major force modifying the environment. The activities of all humans combined are comparable to geological forces, such as earth uakes and volcanoes, that modify the Earth s surface and atmosphere on large scales and in visible ways.

Scientists have not yet agreed on the time period that marks the be-ginning of the Anthropocene. Many propose the period started some-time during the Industrial Revolution. Others argue that it began with the advent of the Agricultural Revolution about , years ago. Still others suggest that its start date should be set to , the date of the rst atomic explosions. Irrespective of how scientists settle the starting

date of the Anthropocene, it s the Great Acceleration that began in the s that marks the beginning of Homo sapiens truly massive global

imprint on the planet. The climate has already warmed by C since . If we continue with business-as-usual consumption of fossil fuels,

the warming between and could exceed C.

hy worry about a warming of °CWe know that the planet has been both signi cantly warmer and signi -cantly cooler in the past. Why bother about a human-induced warming of C How do we judge whether this amount of warming is large or small One way is by looking at the glacial to interglacial cycles the planet has undergone many times over the last . million years. The

di erence in global average temperature between a glacial and an in-terglacial period is only about C to C, but this di erence results in a dramatically di erent climate. At the coldest point of the last glacial period (the Ice Age, about , years ago), an ice sheet covered most of what is now Canada and the northern United States. This ice sheet was about kilometer thick over the current location of New York City. In contrast, during the previous warm interglacial period about , years ago, called the Eemian interglacial, there was substantially less ice in Greenland and Antarctica than at present, and global sea levels were at least meters higher. The temperature at that time was only about

C warmer than the preindustrial average of the early s. It s clear that a C increase in the global temperature is, in fact, a big deal.

It s also important to understand that the planet is already in its warm state (interglacial period), having warmed by about C from the glacial temperatures of about , years ago. Warming it by another

C would push the planet, along with all of its ecosystems and glaciers, beyond any temperature experienced in the last million years. In short, more than just the C warming, the fact that this warming is happening on top of the current warm interglacial period is the bigger concern.

Finally, past changes in global temperature by as much as C oc-curred over periods of thousands of years or more. Compare that to the projected Anthropocene warming, which will happen within a single century. This rate of warming is at least times faster than naturally occurring changes—and far too rapid for social systems, natural species, and ecosystems to adapt.

The Atmospheric Blanket an Its arming E ect

The Earth s atmosphere is an extremely thin shell compared with the size of our planet. The primary gases in the atmosphere by volume are nitrogen ( . ), oxygen ( . ), and argon ( . ). These gures don t include water vapor, which varies signi cantly with location and altitude but averages about . of the atmosphere globally. Other naturally oc-curring gases include carbon dioxide (designated by chemists as CO ), ozone, and methane, which all occur in trace amounts. Although CO , methane, and ozone occur naturally, human activities are increasing their concentrations.

This blanket of atmosphere sustains life in many fundamental ways. First, it is vital to the cycle of plant and animal life. Plants grow by taking carbon dioxide from the atmosphere. In the process of photosynthesis, they use energy from the sun to synthesize carbon dioxide with water and release oxygen to the atmosphere. Plants incorporate the carbon into sugars that store energy and into structural materials such as cel-lulose, while they release the oxygen back into the atmosphere. When animals, including human beings, consume plant material (or other an-imals), they digest it: that is, they take in oxygen, which reacts with the food to release its stored energy. In the process, animals and humans convert some of the carbon back into carbon dioxide and re-exhale it into the atmosphere.

Water vapor is a crucial part of the atmospheric composition. Water vapor is produced primarily from evaporation from the oceans, surface soils, and subsurface a uifers and then spreads around the planet. It is the water vapor in the atmosphere that forms clouds and rain, thus creating rivers, lakes, glaciers, and ski slopes.

Most important for our purposes, the atmosphere plays a crucial role in determining the temperature of our planet, as we will see in our discussion of the greenhouse e ect. Water vapor, carbon dioxide, and

other greenhouse gases warm our planet. Without these greenhouse gases, our planet would be about as cold as Mars—far too cold to sup-port li uid water and life. At the other extreme, without clouds, ice, and snow to re ect sunlight, the planet would be so hot that it would be unlivable.

Thus, the composition of the atmosphere keeps the Earth s tem-perature at just the right level for water to be present in the planet in all three phases: gaseous water vapor, li uid water, and solid ice and snow crystals. The presence of all three forms of the water molecule is essential for the survival of Homo sapiens and most other species on Earth. The atmosphere thus protects life.

The natural greenhouse e ectOur planet s fundamental energy source is incoming radiation from the sun, which we will refer to as incoming solar energy. Not all of this solar energy is absorbed by the planet. About of it is re ected back into space by the atmosphere, the land surface, and the sea surface. The percentage of solar radiation re ected back into space is called the

. . tmosphere a thin, fra i e she . he arious ways the atmosphere sustains ife on the p anet. Image by V. Ramanathan. Icons designed by reepi from aticon.com.

albedo. The primary climate variables responsible for the Earth s albedo are clouds, snow cover, ice sheets, sea ice, glaciers, and oxygen and nitrogen in the atmosphere. In general, whiter substances (clouds, ice, and snow) re ect more solar radiation. The scattering of sunlight by oxygen and nitrogen gives the sky its blue color.

A er of the incoming solar radiation is re ected back to space, the Earth absorbs the remaining , which heats the land, ocean sur-face, and atmosphere. In response, the surface and the atmosphere radiate (i.e., give o ) this heat by emitting infrared radiation. This infrared radiation is commonly referred to as heat energy because the infrared radiation emitted by any substance depends on its tempera-ture. The higher an object s temperature, the more heat energy it emits.

However, not all of the emitted heat energy can escape to space. The greenhouse gases in the intervening atmosphere absorb (trap) some of this heat energy. As a result, the heat energy leaving the planet is re-duced by the intervening atmosphere. It is this trapping of heat energy that otherwise would have escaped to space through the atmosphere that is referred to as the greenhouse e ect.

. . he reenhouse e ect. Adapted from NASA.

Now, let s see how this trapping e ect warms the surface. The greenhouse gases in the atmosphere trap heat energy and reradiate some of it back to the surface. The surface absorbs this reradiated heat energy, causing it to warm some more. The Earth will continue to warm until it reaches a temperature at which the net incoming solar energy e uals the heat energy emitted to space by the warmer surface and the atmosphere.

Thus, the surface temperature of a planet is primarily determined by two factors: the net amount of incoming solar energy it receives, and the heat-trapping properties of any greenhouse gases in its atmosphere. Increasing the concentration of greenhouse gases shi s the balance between incoming solar energy and outgoing heat energy, re uiring the planet to become warmer and emit more heat energy to restore e uilibrium.

“Blanket” is a better metaphorThe trapping of heat by the Earth s atmosphere is typically referred to as the greenhouse e ect. This metaphor compares the heat-trapping gases in the at-mosphere to the glass panes of a greenhouse, which allow solar radiation to enter but slow down outgo-ing infrared heat radiation. Although this name has become standard, it s not the best metaphor for un-derstanding the e ects of climate pollutants. In fact, the main reason it s warmer inside a real greenhouse is not because it traps radiated heat energy, but be-cause its walls and roof keep warm air from escaping and colder outside air from entering.

A more scienti cally accurate metaphor for the warming e ect of the atmosphere is the blanket e ect. On a cold night, a blanket (the atmosphere) warms us by trapping some of the heat energy ra-diated by the body (the planet s surface) and thus prevents some of it from escaping to the rest of the room (space). However, following well-established tradition, we will retain the terms greenhouse e ect and greenhouse gases throughout this book.

. . an et metaphor. Photograph by

atthe enry on nsp ash.

hat are the natural greenhouse gasesSo, what are the gases responsible for this natural heat-trapping e ect Most of the Earth s atmosphere is made up of gases, primarily nitrogen and oxygen, that do not trap heat energy and do not contribute to the greenhouse e ect. The term greenhouse gases refers to the small fraction of gases that do have the ability to trap infrared heat energy.

The dominant greenhouse gas in the Earth s atmosphere is water vapor. Next is carbon dioxide. Other naturally occurring greenhouse gases in the atmosphere include methane, ozone, and nitrous oxide. Concentrations of these gases are extremely small when compared with oxygen and nitrogen, but they play a crucial role in regulating climate and climate change. They have a much larger role in determining the Earth s climate than their tiny concentrations would suggest.

As we noted earlier, water exists in the atmosphere not only in the form of gaseous water vapor, but also in the form of clouds (li uid water droplets and ice crystals). Clouds also provide a large greenhouse e ect, almost comparable to that of CO . However, clouds also re ect solar energy. The re ective e ect of clouds is about twice as large as their greenhouse e ect. Thus clouds, in spite of trapping signi cant amounts of heat, have a large net cooling e ect on the planet.

Experimental validation of the atmospheric greenhouse e ectHow do we know the greenhouse e ect is real One way is to look at the energy absorbed and emitted by the Earth. Satellites routinely measure the incoming solar energy and the outgoing heat energy from the planet. Independently, the heat energy emitted by the surface has been estimated using observed surface temperatures on land and sea.

A note about units: scientists measure energy in units called joules. To describe incoming and outgoing energy for the Earth, scientists use watts. A watt is a unit describing the rate at which energy is emitted or absorbed watt is e ual to a rate of joule per second. To give a familiar example, a -watt lightbulb, when lit, emits joules of heat and light energy per second. Scientists measure the rate of incoming solar energy and emitted heat energy for a planet in terms of the energy

rate per unit of its surface area. This is expressed as watts per square meter of the planet s surface, denoted in short form as W/m (where the slash means per ).

Globally, measurements show that the Earth s surface emits heat energy at W m . However, satellite measurements during the s showed that the heat energy escaping to space through the atmosphere was only W m . Thus, the atmosphere traps about one-third of the surface-emitted heat energy. Clouds decrease the energy that escapes by an additional W m ; thus, the net heat energy escaping to space (with clouds) is W m .

We can use another, more whole-system approach to validate the greenhouse e ect: comparing planet Earth with its neighbors, enus and Mars. On one hand, the average surface temperature of Earth is C. The enusian surface, on the other hand, is searing hot at C—well above the melting point of lead. Why is this the case The rst obvious suggestion would be that enus is hot because it is so close to the sun. Indeed, enus is close to the sun, and its incoming solar energy is W m , compared with W m for Earth.

But there is a second factor to consider: enus is completely cloud covered and as a result re ects as much as of its incoming solar energy (that is, the albedo of enus is ). Taking this into account, we nd that enus actually absorbs solar energy of W m , slightly less

. . Comparati e statistics of the incomin so ar ener y and surface temperatures of Venus, Earth, and Mars. Adapted from NASA.

than the amount of solar energy that Earth absorbs ( W m ). On that basis, we would expect enus to be cooler than the Earth.

The only remaining explanation for enus s searing hot surface tem-perature is the greenhouse e ect of the CO in its atmosphere. It turns out that the concentration of CO on enus is about , times more than that on Earth, creating a superstrong CO greenhouse e ect, which maintains enus s hot temperature.

Mars, on the other hand, is much farther from the sun and receives less than half the solar energy that Earth receives. Mars is nearly cloud-free (except for some dust clouds), and its albedo is only . The net e ect is that the solar energy that Mars absorbs ( W m ) is only half of that absorbed by Earth. This is the primary reason for the frigid average temperature on Mars ( C). Mars s atmosphere is mostly CO , and the amount of CO on Mars is actually about times larger than that on Earth, but the stronger greenhouse e ect is not enough to compensate for the lower incoming solar energy.

Earth in the Goldilocks zoneThe above exercise illustrates an important message about the optimal climate on Earth. The surface temperature is determined by a delicate balance between the amount of incoming solar energy, the re ected solar energy, and the greenhouse gases in the atmosphere.

As we saw earlier, water vapor has the strongest warming e ect of the naturally occurring greenhouse gases. At the same time, clouds made up of condensed water vapor have a net cooling e ect. If water vapor plays such a signi cant role in our climate, why do discussions of climate change mostly focus on emissions of carbon dioxide Where does the water vapor greenhouse e ect t in this picture

While carbon dioxide is emitted by geological processes (and more recently, human activities), the concentration of water vapor is primarily governed by surface and atmospheric temperatures. The warmer the at-mosphere, the higher the concentration of water vapor, assuming there is an abundant source (such as oceans or water cooked out from minerals deep in the Earth s interior).

Because the concentration of water vapor depends on temperature,

climate scientists refer to it as a climate feed-back that ampli es warming, rather than a direct cause of warming. If there were no carbon dioxide in the Earth s atmosphere, tem-peratures would fall to the point that most of the water vapor would condense or crystallize out of the Earth s atmosphere as well. With-out carbon dioxide, there would be very little water vapor greenhouse e ect and the Earth would be much cooler, if not frozen.

Thus, the Earth seems to have just the right amount of incoming solar radiation, clouds, and CO to maintain an e uitable climate. Among the three planets, Earth is the only one whose temperature is not too hot, not too cold, but just right for Goldilocks s porridge—and for life.

CO increased by human activitiesWhile the natural greenhouse e ect is vital for maintaining life on Earth, humans have added an enormous amount of carbon dioxide to the thin shell of the atmosphere since the dawn of the Industrial Revolution. As of , we have dumped , , , , ( . trillion) tons of carbon dioxide into the atmosphere over the past years. About of that carbon dioxide still remains in the air today. That leaves a blanket of human-generated carbon dioxide in our thin atmospheric shell whose sheer weight is astounding— billion tons. That s e uivalent to the weight of about billion cars circling the planet all the time.

How do we know the weight of the human-made CO From direct measurements initiated by Charles David Keeling of the Scripps Institu-tion of Oceanography (UC San Diego). This wiggly curve (Figure . . ) is called the Keeling Curve and shows the concentration of carbon dioxide in the atmosphere. When Keeling rst started making measurements in

, the atmospheric carbon dioxide concentration was parts per million (abbreviated as ppm). That is, out of every million mole-cules in the atmosphere, were carbon dioxide molecules in .

. . o di oc s princip e. Reproduced from NOAA.

Passing a major thresholdIn the year , we passed a major threshold—one that we should not be passing. Based on measurements of ancient air bubbles trapped in ice and other data, scientists estimate that the concentration of CO be-fore was ppm. That concentration has since increased steadily, shooting past ppm by , ppm by , and ppm by

. Carbon dioxide concentration was about ppm in , mean-ing that humans have now increased the overall concentration of carbon dioxide by nearly since the preindustrial era. Crossing the threshold of parts per million signi es that the planet could be transitioning into an era of major climate changes.

The increase is seen everywhere on the planet: the ocean surface, mountaintops, and deserts. Whether the data are collected in Hawaii, the Arctic, or the Antarctic, the ndings are the same. Basically, the additional CO has covered the planet like a blanket.

. . he ee in Cur e shows the increase in CO from to . Reproduced from the Scripps CO Program from the Scripps Institution of

Oceanography.

How did that happen Air travels fast. Pollution from North America travels to Europe in days; pollution from Asia travels to North America in a week; and pollution from South America travels to the Antarctic in a few weeks. Air takes a few years to travel from the Arctic to the Antarctic. Travel times for pollution are much shorter than the lifetime of the CO molecule in the air, which is to , years. That s why the CO increase is found everywhere on the planet. Carbon dioxide is what scientists refer to as a well-mixed gas—one that remains in the atmosphere much longer than the time it takes to spread around the world.

What is the take-home messageThe atmosphere connects every part of the world with every other part in a matter of days or weeks. Therefore, we can only solve the climate change problem through global cooperation.

Greenhouse gases as pollutantsWhy do we call carbon dioxide and other anthropogenic greenhouse gases pollutants Pollute means contaminate something with a harm-ful or toxic substance. Carbon dioxide is a natural component of the atmosphere and a vital part of the respiration cycle that sustains life, so how can it be a pollutant Although carbon dioxide and most other greenhouse gases exist naturally in the atmosphere, human emissions are increasing their concentrations, causing warming that will most de -nitely have harmful impacts, as we will see later in this chapter. The harmful impacts of these emissions make it appropriate to refer to greenhouse gases as pollutants.

What are the sources for the observed increase in COMany human activities that address our basic needs, development, and well-being are sources of greenhouse gases. Most of the energy used by society since the Industrial Revolution has come from fossil fuels: coal, oil, and natural gas. Burning fossil fuels emits the largest amount of CO by far, contributing an estimated billion tons in .

Major anthropogenic sources of carbon dioxide include the following:

Using fossil fuels to produce electricity: In , of electricity worldwide was gen-erated by burning fossil fuels, including from coal and

from natural gas. Coal emits roughly twice as much CO per unit of electricity gen-erated as natural gas, so burn-ing coal to generate electricity is particularly concerning.

Transportation: There are about billion motor vehicles in use around the world, the vast majority of which use oil-based fuels. Aviation and commercial shipping are also major emitters of carbon dioxide.

Residential and commercial buildings and activities: In addition to indirect emissions from electricity use, buildings can be a direct source of CO emissions, primarily through heating. In developed countries, natural gas is fre uently used for space heating, water heating, and cooking. The least a uent billion, with limited access to fossil fuels, fre uently burn wood or animal dung for heating and cooking, which also release CO .

Industrial processes: A range of industrial processes, in particular cement and steel production, emit signi cant amounts of CO . Ce-ment production alone is estimated to have been responsible for billion tons of CO emissions in .

Land use: Changes in land use, in particular burning forests to clear land for farming, grazing, or housing, also emit signi cant amounts of carbon dioxide. Over the decade , CO emissions from land use averaged about billion tons per year.

. . ources of reenhouse as emissions. Reproduced from N P RI Arenda .

Waste andwastewater

Source: IPCC, 2007.

Residential and commercial buildings

Energy supply25.9 %

Transport13.1 %

Industry19.4 %

Agriculture13.5 %

Forestry17.4 %

7.9 %

2.8%

Global greenhouse gas emissions by sector (2004)

How much additional heat energy is trapped by the -billion-ton CO blanket

As of , the heat-trapping e ect of human-emitted carbon dioxide was about terawatts ( terawatt e uals , billion watts—that s a followed by zeros). This represents about times our total global rate of energy consumption To understand the enormity of terawatts, let s look at another statistic. The heat energy trapped by our human-made blanket is e uivalent to burning trillion -watt light-bulbs every second, every day, every month, every year. We are trap-ping an enormous amount of heat in the land, oceans, and atmosphere. Based on fundamental physics, the temperature of the planet and the atmosphere will be forced to increase until the extra terawatts are radiated away into space. If we continue to increase the concentration of CO in the atmosphere, even more heat will be trapped, forcing the planet to warm even further. This, in a nutshell, is the cause of global warming.

Is CO the only important anthropogenic greenhouse gasUntil , we thought that CO was the only source of anthropo-genic warming. Then the greenhouse e ect of chloro uorocarbons (CFCs)—a group of arti cially produced molecules used as refrigerants, solvents, and propellants—was discovered in . Soon a er, a host of other anthropogenic gases (more than ) were added to the list of climate-warming gases. The most important of these, in terms of their warming impact, are methane, ozone, nitrous oxide, and an-other group of refrigerants known as hydro uorocarbons (HFCs). The sources of these pollutants are the following:

Methane is the main natural gas that we use for power generation, heating, and cooking. Natural gas leaks (called fugitive emissions ) at production and processing facilities and through distribution pipes are a signi cant source of methane emissions. Another major source is methane produced by bacteria in the guts of cattle, sheep, and goats. Wet rice agriculture (rice paddy elds), wood burning, land lls, and sewage water treatment plants are among the other signi cant sources.

CFCs (chloro uorocarbons) and HFCs (hydro uorocarbons) are ar-ti cially produced for refrigeration and air conditioning. CFCs have been phased out by international treaty since the late s, but work to phase out HFCs is just beginning.

Ozone is not directly emitted by human activities, but fossil-fuel power plants and automobile engines emit gases known as ozone precursors (methane, nitrogen oxides, and volatile organic com-pounds) that react with sunlight to produce ozone in the lower atmosphere.

Nitrous oxide is released by bacteria in the soil. Nitrogen-based fer-tilizers used in agriculture increase the activity of soil bacteria and their nitrous oxide emissions.

The IPCC estimates that as of (the IPCC data is available for only up to ), CO has trapped . W m of heat, which is terawatts when integrated over the surface area of the whole planet. All of the anthropogenic non-CO gases have added another . W m , bringing the total heat trapped by all anthropogenic greenhouse gases to W m (about , terawatts).

Is the climate responding to this added heatUndoubtedly, the climate is responding to this added heat, according to data that scientists have collected at the surface, in the atmosphere using aircra and balloons, and from space using satellites. The entire atmosphere, most of the land surface, and the oceans to depths of as much as a kilometer have warmed to unprecedented levels compared with the temperatures of the last , years. In the following section, we will review the vast amount of past climate data that provide uanti-tative answers about the magnitude of the climate s response.

Why and How Is Climate Changing?

You have so far learned how certain pollutant gases behave like a blan-ket, trapping heat and causing global warming. In this section, we will document the evidence that these gases are changing our climate and do a deep dive into why and how the climate is changing.

Distinguishing between weather and climateWeather is what is happening at any given time or on a short time scale of a few weeks or less. For example, there may be rain today, sunshine tomorrow, and a storm a few days later. These kinds of day-to-day short-term changes are what we call weather. Climate describes conditions over a longer term. For example, in many regions winter is colder and drier than summer. Summer might bring monsoons to some regions. These are descriptions of climate, which is essentially a longer-term average of weather. The greenhouse e ect causes warming and other changes to climate on time scales of seasons or longer. A warmer climate in turn leads to other changes, such as extreme weather events (heat waves, droughts, extreme storm events). It s in this context that we talk about climate change.

Distinguishing between global warming and climate changeUntil about two decades ago, scientists used to refer to the increase in temperature due to increases in CO as global warming. However, this term does not describe all of the impacts that go along with warm-ing, such as extreme weather and rising sea levels. Moreover, in the

s and early s, global warming became a politicized phrase and issue, particularly in the United States. Scientists have responded by avoiding the phrase global warming and replacing it with the phrase climate change. Both terms are used in this text because global warming and climate change are distinct processes. By de nition, global warming

refers speci cally to the warming e ect of anthropogenic gases on the planet. This global warming in turn leads to broader climate change, which includes changes in winds, storms, rainfall, and humidity.

Why is the planet warmingAs brie y described in the earlier sections, the Earth has been warming since the s. The warming has not been constant or steady, how-ever. As we will see, the evidence indicates that most of this warming is caused by human activities that release pollutant greenhouse gases into the atmosphere, thickening the natural greenhouse blanket. The gases began increasing in the s, but the Great Acceleration in consump-tion that began in the s (Section . and Figure . . ) contributed

. . Human acti ities that ead to c imate po ution. op e image by A an iniry from Pe e s. op right image reproduced from Pi abay. ottom e image by V. Ramanathan. ottom right image reproduced from Pi abay.

to a steeper increase in the concentration of many gases during the last half of the twentieth century.

The four images in Figure . . reveal the interconnectedness of the climate change problem. The woman cooking with rewood (that was my grandmother s kitchen in south India) could lose her source of food because of changes in climate, such as droughts, caused by CO emitted for the most part in developed countries. Likewise, the smoke coming from that woman s kitchen in south India—as well as from cars in the US and power plants in China—could melt glaciers thousands of kilometers away. It is imperative to keep in mind that pointing ngers at each other will not solve the climate change problem. We are all in this together and together we must solve this problem.

We have already identi ed carbon dioxide as a major anthropogenic greenhouse gas. Carbon dioxide is a signi cant concern in part because of its long lifetime in the atmosphere. Roughly half the emitted CO will be taken out of the atmosphere in less than a decade by the land biosphere (trees, plants, and soil) and by the ocean, but the remaining half will stay in the air for at least years, and about of the CO will stay in the atmosphere for years or more. You, your children, your grandchildren, and future generations yet to be born will still be inhaling the carbon dioxide emitted by your car today.

The impact of aerosolsOne important point to note from Figure . . is that the visible smoke and smog shown in the images is made up in part of tiny particles called aerosols; carbon dioxide and other greenhouse gases cannot be seen by the human eye. Most of these aerosols re ect sunlight and have a cooling e ect, but black carbon aerosols (a major component of soot) absorb solar radiation entering the atmosphere and have a warming e ect. This trapping of incoming solar radiation should not be confused with the trapping of outgoing infrared radiation emitted by the surface. O en black carbon is referred to as a greenhouse gas. This is wrong on two counts: black carbon is not a gas, and black car-bon warms the climate by absorbing solar energy rather than infrared energy from the planet. Black carbon is mainly produced by incomplete combustion. Major anthropogenic sources include internal combustion

engines in vehicles (particularly diesel-powered vehicles) and the burn-ing of solid coal, rewood, crop residues, and animal dung (for heating and cooking).

Fertilizing agricultural elds and burning fossil fuels and biomass fuels (for example, wood) also contribute to other types of aerosol par-ticles, such as sulfates, nitrates, and organics. Unlike black carbon, these other aerosol particles primarily re ect sunlight and have a cooling ef-fect. Although some of this cooling is o set by black carbon s warming, the net e ect of all aerosols combined is one of cooling. This cooling has been estimated to o set about a third of the warming caused by an-thropogenic greenhouse gases, but the net impact of human emissions still warms the planet.

Super pollutantsAs of , non-CO pollutants (non-CO greenhouse gases and black carbon) contribute about of the total anthropogenic warming ef-fect. These non-CO greenhouse gases and black carbon particles are also called super pollutants. This is because, per molecule, their warm-ing e ects are much larger than that of CO . For example, methane is

times more potent than CO at warming the planet; nitrous oxide is times more potent; HFCs and CFCs are a few thousand to ,

times more potent; and black carbon is , times more potent (also Box . . ). These non-CO pollutants have powerful warming e ects, but methane, ozone, HFCs, and black carbon are also called short-lived climate pollutants (SLCPs) because their lifetimes in the air range from less than a week (black carbon), to a month (ozone), to a decade or two (methane and HFCs), compared with the century to millennial time scales of CO . These relatively short atmospheric lifetimes will be an important factor when we begin to look at climate solutions.

Warming trendsSigns of warming can be seen on the land and sea surface as well as in the atmosphere and the deeper oceans. The globally averaged surface temperature shown in Figure . . reveals a persistent warming that began in and continues until the present ( ), with some ups and downs. Most of the C warming experienced since the beginning

Bo Global Warming Potential of Greenhouse Gases

a h greenho se gas has a i erent apa it to trap heat in the atmosphere ne a e an meas re this is thro gh global warming potential (GWP) hi h ompares the heat-trapping e e t o a gas to the e e t o an e al mass o ar on io i e

i erent gases sta in the atmosphere or i erent time perio s s ientists all the time a parti lar gas remains its lifetime in e the arming e e t o a gas epen s in part on ho long it sta s in the atmosphere glo al arming potential m st e e ne or a spe i time perio s all ears or ears

The ta le elo lists the - ear glo al arming potential ( P ) or three o the most important greenho se gases or e ample the - ear P o methane is given as ( ith a range o to ) This means that i e ere to emit e al masses o methane an ar on io i e into the atmosphere at the same time the methane o l trap times as

m h heat energ as the ar on io i e over a perio o ears

Greenhouse gas Chemical FormulaLifetime in the

Atmosphere (years)GWP

( years)

Methane CH

itro s i e O

H C- a CH C

H C- a is a ommonl se re rigerant an is given as an e ample o a h ro oro ar on (H C) There are o ens o i erent H Cs in se ith P val es ranging rom a e h n re to several tho san

e an se glo al arming potentials to e ne e ivalent emissions in terms o CO ientists all this the CO equivalent t pi all ritten as CO e or CO e or e ample

sin e methane has a P o the release o ton o methane o l have a arming e e t ompara le to tons o CO This might e es ri e as the a ition o tons o CO e hen loo ing at greenho se gas emission n m ers it s important to note hether the re

e presse in tons CO or tons CO eo ma have noti e that ar on io i e is not in l e in the ta le its P is

e nition lso as e ill see ar on io i e is remove rom the atmosphere a variet o i erent pro esses so it s not possi le to e ne a single li etime or CO

lso noti e that t o important greenho se gases ater vapor an o one are not in l e in the ta le That s e a se their li etimes in the atmosphere are e tremel short onl a e a s or ee s so it s not meaning l to e ne a - ear glo al arming potential or them

Climate Change The Physical Science Basis. Contri tion o or ing ro p I to the i h ssessment eport o the Intergovernmental Panel on Climate Change to er T et al (e s ) i etimes a iative ien ies an Metri al es Ta le Cam ri ge niversit Press e or P

The missions Gap eport nite ations nvironment Programme airo i en a

of the twentieth century happened a er the Great Acceleration began in the s.

A similar pattern is observed in the ocean to a depth of at least meters. The warming can be seen over the whole globe with very few exceptions. Most every region has experienced the warming, but it is not uniform. For example, the land surfaces have warmed more than the sea surface. This is expected since the land surface has less thermal inertia than the sea and hence warms more rapidly than the ocean. The Northern Hemisphere has warmed more than the Southern Hemi-sphere, again largely because of the ocean s in uence: the spatial extent of the ocean is not as great in the Northern Hemisphere. The northern polar regions have warmed twice as much as the global average: more than C compared with the global average of C.

. . Chan es in oba a era ed and and sea surface temperatures since , re ati e to a era e. Adapted from NASA/GISS.

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1910

1940

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pera

ture

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nge (

C)

Year

Global Land-Ocean Temperature

Annual Mean

Smoothed Curve

If we keep adding climate pollutants at the present rate, global tem-peratures will continue to increase to more than C by , and to a catastrophic C to C by end of this century.

How do we know the warming is due to human activitiesA large amount of evidence and many lines of evidence-based reasoning have led scientists to conclude une uivocally that the warming is caused by the increase in the thickness of the greenhouse blanket of CO , meth-ane, CFCs, ozone, and nitrous oxide. There are two primary grounds for this conclusion:

. Natural changes are much too small to produce the observed warming. There are three main ways that natural changes can contribute to climate change. First, changes in processes within the sun can cause variations in incoming solar energy. However, incoming solar energy has been regularly monitored by satellites since the late s, and the observed variations in incoming solar energy are about a factor of lower than the W m increase caused by anthropogenic thickening of the greenhouse gas blanket. Even more signi cantly, changes in solar output over the last couple of decades have been in the opposite direction. That is, the sun s energy output has decreased slightly, which would tend to cause cooling, not warming. A second natural factor that can a ect climate is variation in the Earth s orbit around the sun. These orbital changes play a signi cant role in climate changes on time scales of , years or more (for example, the cycles between glacial and interglacial periods), but they have negligible e ects on time scales of a century or so. They are simply too slow to be responsible for the warming observed over the past few decades. The third natural factor that can cause climate changes is volcanic eruptions. olcanoes put out sulfur gases that get converted into re ective aerosol particles in the atmosphere. By re ecting solar energy back into space, these particles cool the climate. olcano-induced cooling is real but lasts for less than years. The change in re ected solar radiation due to volcanoes and the resulting temperature changes have been measured

from both surface instruments and satellites. For example, the eruption of Mount Pinatubo in the Philippines in produced a measurable drop in global temperatures for at least years. As the sulfate particles are gradually removed from the atmosphere, temperatures tend to return to previous levels. Although volcanoes do emit carbon dioxide, these emissions are less than of human-generated CO . Scientists have concluded that apart from temporary cooling, volcanoes have had very little e ect on the rapid warming trend observed since the s.

. Models can simulate the observed warming only if they include human activities. The most sophisticated climate models to date account for both natural variations and the human-caused increase in greenhouse gases. Model runs that include only natural variations show year-to-year uctuations in temperatures, but they completely fail to reproduce the current warming trend. Only when models include the anthropogenic thickening of the greenhouse blanket do they reproduce the observed warming of the planet. We can see this in Figure . . . The black lines represent observations, the blue regions represent the range of predicted temperatures from models that include only natural

. . Obser ed temperatures compared with those from mode s usin on y natura factors and with those from mode s usin both natura and anthropo enic factors. Reproduced from IPCC.

factors, and the pink regions indicate the range of projections from models that include both natural and anthropogenic factors. The observed warming is far outside the range of projections that include only natural factors, but it is well within the range of projections that include anthropogenic factors as well. This leads climate scientists to conclude that anthropogenic changes are the dominant factor in recent warming.

Why trust the modelsThis leads us to a uestion: Why should we trust the models A er all, they are just computer calculations. How do we know they accurately re ect the real world

Scientists trust the models in attributing the observed warming to human activities because, in general, the model projections are con-sistent with the observed changes. Models have successfully predicted many changes that were later observed, a few of which are listed below:

In , models were used to predict that CO -induced warming would be detected by the year . Indeed, in the compre-hensive report written by over , scientists for the IPCC was the rst to formally conclude that there was a discernible warming in

the observed records.

Models predicted that warming induced by greenhouse gases would penetrate to the deeper oceans. Scientists have deployed thousands of underwater probes in every major ocean basin, and their mea-surements show that warming temperatures have penetrated to at least meters below the surface.

Models predicted that the greenhouse-gas-induced warming would extend to the entire lower atmosphere (from the surface up to above kilometers). This has been con rmed by balloon and sat-ellite data.

The predictions suggested that a warmer atmosphere would be-come more humid and that the increase in water vapor would in turn amplify the warming because water vapor is a powerful greenhouse gas. Humidity data collected by weather balloons and microwave instruments

on satellites con rmed that water vapor has increased with the increase in temperature since the s.

In the late s, a Russian meteorologist predicted that as the planet warmed, sea ice and snow would retreat, making the surface less re ective and exposing the darker ocean below to solar energy. This reduced re ectivity would increase the solar energy absorbed by the Arctic Ocean, amplifying the warming. Indeed, satellite data have shown that the Arctic sea ice has retreated signi cantly since the late s, followed by an increase in solar energy absorption by the Arctic Ocean and ampli ed warming. The Arctic region has warmed by almost . C, compared with the global average warming of C.

But models are tested not just by their ability to successfully forecast changes in climate that are later observed. A typical test for modern cli-mate models is their ability to reproduce past climate observations, such as the temperature record for the twentieth century. This process is called hindcasting. The ability of models to pass such tests increases sci-entists con dence that they include the factors necessary to determine the causes of observed climate change, as well as to project changes likely to occur in the future.

Based on the results from models and other observations and analy-ses, the most recent report of the US Global Change Research Program, composed of federal departments and agencies, concluded in that it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid- th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.

Projecting future warming: climate feedbacksAs we have seen, we have a good scienti c understanding of tempera-ture increases over the past century. Warming is driven primarily by increases in concentrations of greenhouse gases. This warming has been partially o set by the net cooling e ect of aerosols.

Past and future warming is governed by climate feedbacks, which happen when the climate system responds to temperature increases in ways that can either amplify or moderate warming. Three of the most

Bo Climate Change: Is the Science Settled?

Me ia overage o limate iss es sometimes gives the impression that there is signi ant s ienti e ate a o t limate hange In realit the s ienti omm nit largel agrees a o t limate hange oth the a t that it is o rring an why it is o rring This n erstan ing o the me hani s o limate hange is ase on n amental ph si s an ell-es-ta lishe s ienti prin iples e a ress some o the most ommon

estions a o t the s ienti onsens s on limate hange here

What fraction of the warming is due to human activities

M est estimate or more Ho i I arrive at s h a n m er The s ien e tells s that the variations in nat ral limate or ing (that is solar an vol ani a tivities) are too small to a o nt or the o serve arming tren s an at times ontrar to them rther oth pe agogi al an om-ple limate mo els are a le to sim late the o serve arming magnit e ( C to C) onl i the in l e the o serve il p o greenho se gases sin e ee o or etails o these al lations

So is the science settled

The ans er epen s on hat aspe t o limate hange s ien e o as a o t ome o the most important estions have een ans ere ith a high egree o on en e as s mmari e in Ta le

What aspect of the science is not settled

Pre i tions o future arming are less ertain In the rst pla e e o not no ho m h limate poll tion h mans ill emit over the oming e-a es ven or a parti lar emissions s enario ho ever limate mo els

give a i e range o estimates ome o the ma or reasons or this range in l e var ing assessments o a tors s h as aerosols lo ee a s an other ee a s e to the response o soils an plants to arming temperat res

ith s ient arming there is also the possi ilit o a r pt an irreversi le hanges i glo al temperat res ross tipping points that an p sh the limate into ne states amples o tipping points in l e sig-ni ant methane releases rom melting perma rost or large-s ale hanges in o ean ir lation n ort natel the temperat re threshol s or these tipping points are not ell n erstoo

These ee a s an nami pro esses mean that e m st present an on l sion regar ing the arth s t re arming as a pro a le range rather than a single val e

important feedbacks we need to consider in relation to climate change in the twentieth and twenty- rst centuries are the following:

. Water vapor feedback: We have already discussed this feedback earlier in the chapter. When the temperature of the atmosphere increases, it holds more water vapor. Since water vapor is a greenhouse gas, this feedback acts to amplify warming, resulting in temperature changes that are roughly twice as large as would be expected from the increase in greenhouse gases alone.

. Ice-albedo feedback: As described in the previous section, increasing temperatures reduce snow and sea ice cover, which decreases albedo and ampli es warming. This feedback has its strongest e ect in the Arctic, which is why this region has warmed substantially more than the global average.

. Cloud feedbacks: Clouds can a ect temperatures in two di erent ways. Clouds re ect sunlight, which tends to cool the Earth. However, the li uid water or ice crystals in clouds also trap infrared radiation, causing warming. It turns out that low, thick clouds have a net cooling e ect, while high cirrus clouds have a net warming e ect. Thus, the overall feedback from clouds depends on whether a warmer world would have more low,

. . ummary of scienti c consensus

Question Reply

Is the atmosphere getting more poll te Yes

re the greenho se gases CO methane an others in reasing

Yes

re the in reases e to h man a tivities Yes

Is the limate arming Yes

Is the arming in part e to h man a tivities

Yes

hat ra tion o the arming is e to h man a tivities

hat h man a tivities are responsi le or the arming

Increase in CO other greenho se gases an lac car on particles e to h man activities

Can e ma e precise pre ictions o t re temperat res

o e can onl provi e pro a ilistic val es

Bo Can Climate Science Account for the Observed Warming Trends?

Certainl let me al o thro gh a little it o math e ve alrea men-tione that scientists meas re incoming solar ra iation an o tgoing heat in nits o atts per s are meter ( m ) The ph sics ehin the heat-trap-

ping e ect o greenho se gases is ell n erstoo so e can calc late the im alance the create in o tgoing vers s incoming ra iation cientists call this im alance greenhouse gas forcing or the amo nt o greenho se gases in the atmosphere in e n that the orcing is a o t m

Ho m ch arming o l that m orcing e e pecte to ca se To calc late this e ivi e the greenho se gas orcing a n m er calle the climate feedback parameter O r est estimate o this n m er is (m C) rea as atts per s are meter per egree Celsi s This means that a orcing o m o l e e pecte to raise the glo al temperat re C Th s e are a le to erive the theoretical arming that e sho l have seen rom greenho se gases alone ivi ing m

(m C) res lting in an e pecte arming o C Ho ever e have onl o serve C here is this i erence coming rom

irst not all o the arming appears at the arth s s r ace appro i-matel C is store the oceans lso greenho se gas orcing is not the hole stor o t C o the e pecte arming is reverse aero-sol cooling an C is reverse changes in s r ace al e o mainl e to clearing o orests or agric lt re an gra ing hen e s tract o t

arming that as store the oceans or reverse ( ) e arrive at C e pecte arming or the s r ace (Ta le ) This is a

goo match or the C arming that has een o servehat a o t nat ral actors s ch as changes in the energ ra iate

the s n volcanic er ptions or nat ral varia ilit e to heat e changes et een the oceans atmosphere an lan These actors have een e -

amine care ll an the concl sion is that the co l ca se the glo al temperat re to var p or o n as m ch as C In short nat ral actors alone are ar too small to acco nt or the o serve C arming

e can onl acco nt or the o serve arming incl ing the e ects o anthropogenic greenho se gas emissions

M hre et al nthropogenic an nat ral ra iative orcing In Climate Change The Physical Science Basis Contri tion o or ing ro p I to the i h

ssessment eport o the Intergovernmental Panel on Climate Change toc er T et al (e s ) Cam ri ge niversit Press e Yor Y https ipcc ch site assetsploa s Chapter I p

thick clouds or more high cirrus clouds. Including cloud e ects in computer models is di cult because of their relatively small size and complex formation processes. The current scienti c consensus is that, overall, cloud feedbacks are likely to have a small amplifying e ect on warming. However, cloud feedbacks continue to be one of the largest sources of uncertainty in computer projections of future temperatures.

. . heoretica warmin brea down

Factor Warming

reenho se gas orcing ( ) m

Climate ee ac parameter m C

Theoretical arming e sho l have seen ith st greenho se gas orcing ( greenho se gas orcing ivi e the climate ee ac

parameter )

C

O serve arming C

Ocean heat storage This is the heat energ store in the ocean an it ill e release as s r ace arming in a e eca es

C

Mas ing aerosol cooling C

r ace al e o changes C

Impacts of Climate Change

Climate change a ects all aspects of life on the planet, including ecosys-tems, social systems, economics, public health, urban systems, and rural systems. The observed warming of C is already having an impact on these systems. With unchecked emissions, warming could reach unman-ageable levels this century. It may be better to call it climate disruption rather than climate change.

As shown in Figure . . , the Earth s climate has varied signi cantly over the last , years. Global records for this period are not avail-able, but proxy records such as tree rings and pollen suggest that the northern half of the Northern Hemisphere experienced signi cant warm-ing ( . C) during the Medieval Warm Period from AD to . While Europe enjoyed the warmth and ikings traveled westward to found settlements in Greenland, other regions, including the American Southwest, su ered from megadroughts and heat waves. The unlucky regions included North America, Central and South America, and north-ern China. The legendary city and massive temple complex of Angkor Wat in Cambodia were abandoned largely because of decades-long megadroughts interrupted by occasional episodes of intense rainfall and ooding. The Medieval Warm Period was followed by the Little Ice Age

from about the mid- s to the mid- s, which saw widespread cooling over the North Atlantic and Europe, with global temperatures on the order of . C cooler than in the mid-twentieth century. The Thames River in London froze over multiple times during this period.

These climate events serve to illustrate the strong vulnerability of civilizations to climate change. However, the large climate changes experienced during the past , years cannot be assumed to be a reliable guide for expected climate changes in the coming decades, in part because the Medieval Warm Period was neither global nor wide-spread, even over the Northern Hemisphere. We will begin with the

documented impacts of twentieth-century warming on a global scale. As we will see, temperature changes during the twentieth and twenty- rst centuries have been larger than those of either the Medieval Warm Period or the Little Ice Age, with signi cant climate impacts. A er that, we will look at the projected impacts of continued warming during the twenty- rst century.

Current impacts: twentieth and early twenty- rst centuriesAbout two-thirds of the C warming recorded since the beginning of the twentieth century has occurred in the past four decades, starting

. . Obser ed temperature chan es durin the ast , years compared with the predicted chan es from to . Reproduced from

N P/GRI Arenda .

Variations in the Earth’s surface temperature: year 1000 to 2100Deviation in oCelsius (in relation to 1990 value)

Source: UNEP&GRID/Arendal, Vital Climate Graphics update, 2005.

6.0

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Bars show the range in year 2100produced byseveral models

Year

A1BA2B1B2

Observations, Norhern Hemisphere, proxy data

Scenarios

Globalinstrumentalobservations

Projections

around . As of this writing ( ), to has been the hot-test -year period on record.

As previously discussed, a warmer atmosphere holds more water vapor, so as the planet warms, it becomes more humid. Warmer tem-peratures also increase the overall cycle of water evaporation and pre-cipitation, making drier regions even drier and wetter regions wetter. Dry areas worldwide have increased from about of the Earth s land surface during the mid-twentieth century to about by the rst de-cade of the twenty- rst century.

The last two decades have also witnessed record increases in ex-treme weather events. The incidence of very strong hurricanes (category and ) has increased at the rate of about per degree of global

averaged warming. The number of disastrous oods has increased from less than per year during the mid-twentieth century to more than per year during the rst decade of the twenty- rst century.

How do we know the increase in extreme weather is due to anthro-pogenic global warming The science of attributing individual extreme events to climate change has improved signi cantly. Multiple factors are involved in any extreme event, so it s not possible to say that a speci c weather event was caused by global warming, but we can determine how much more likely that kind of event is, given the in-creased temperatures. For example, the record Russian heat wave of

, which claimed , lives, as well as many of the major storms and droughts witnessed in , have all been statistically attributed to global warming with about certainty. That is, there is a four in ve ( ) chance that the Russian heat wave would not have occurred

in the absence of human-induced climate change. It s estimated that widespread warming and rising humidity increased the probability of extreme weather, particularly heat waves, by a factor of or more from to . An analysis of reports on extreme weather events from to mid- suggests that about two-thirds of these extreme weather events were made more likely, or more severe, by anthropogenic climate change.

The impacts of global warming can also be seen in its e ects on ice and sea levels around the planet. Since , the summer extent of Arctic sea ice has decreased by as much as to . Glaciers

Bo Observed Impacts of Global Warming (Late Twentieth and Early Twenty-First Centuries)

Impacts on ecosystems

• s e sa earlier in this chapter trees an other plants a sor an store car on io i e rom the atmosphere Prior to the t ent - rst cent r tropical orests

acte as a net a sor er (sin ) o car on io i e or e ample a o ng gro ing tree o l a sor car on in car on io i e hile a ing tree o l release that car on ac to the air Ho ever ring the rst eca e o the t ent - rst cent r tropical orests ecame a net so rce o CO eca se o egra ation rom ro ght an arming

• Corals get most o their energ rom single-cell photos nthetic organisms that live in their tiss es Ho ever i ater temperat res are too arm the corals e pel these photos nthesi ing organisms an are le as hite s eletons This is calle coral leaching I arm con itions persist or ee s or months the coral ma ie Coral leaching e to arming is happening in most coral ree s the most severe glo al leaching event in recor e histor occ rre rom to

ring this perio it is estimate as m ch as hal o the coral in stralias reat arrier ee as ille

Impacts on human societies and human health

• arming an ro ghts have increase ater eman over o cropping area a o t to per eca e since contri ting to signi cant re ctions in heat iel an increase in plant iseases

• verse health impacts o climate change s ch as heat stress have een oc -mente e tensivel The ancet Commissions hich consists o international e -perts in p lic health air poll tion an climate change concl e in that the e ects o climate change are eing elt to a an t re pro ections rep-resent an naccepta l high an potentiall catastrophic ris to h man health

• Threats to health oth ph sical an mental also arise rom ecreases in oo sec rit an ater availa ilit These threats incl e increases in ater orne iseases s ch as chil hoo gastrointestinal iseases ca se oo s e to orl i e increases in temperat re an h mi it insect- orne iseases s ch

as malaria eng e ever me isease an chi ng n a are migrating o tsi e the tropics an to higher altit es

• The n m er o people isplace eca se o eather e tremes has increase to million people

are melting worldwide. Major ice sheets, particularly in Greenland and West Antarctica, are losing mass at a signi cant rate. Sea levels are rising at a rate of about millimeters per year because of the melting of glaciers and ice sheets and the expansion of seawater as the ocean warms. The ocean is also becoming more acidic because of absorption of CO , which produces carbonic acid.

The changes described above have had signi cant impacts on natu-ral ecosystems as well as human society and human health. A few of the observed impacts are detailed in Box . . .

Climate change to climate disruptionFor the rst time, the statistical barrier against identi cation of climate change as causal factor for extreme weather events was overcome. The scienti cally cautious American Meteorological Association (AMS) issued the remarkable statement:

For years scientists have known humans are changing the risk of some extremes. But nding multiple extreme events that weren t even possible without human in uence makes clear that we re experiencing new weather, because we ve made a new climate.

The United Nations O ce for Disaster Risk Reduction estimates that from to , weather-related disasters have claimed , lives; furthermore . billion people have been injured, made homeless, or re uired emergency assistance. In addition, the UN agency estimates the number of disasters during the latter half of the -year period was double that of the rst -year period. Climate change is thus bringing new weather extremes and fatal catastrophes—meaning that climate change is better termed climate disruption. Unchecked climate change is likely to become unmanageable. That could happen in a matter of few to several decades as discussed next.

The next three decades: impacts of °C warmingAs of , we have already emitted trillion tons of carbon dioxide. As discussed earlier in this chapter, nearly half of that amount is still in the atmosphere— billion tons of CO , trapping terawatts of heat energy. Since , we have added another billion tons, bringing the

total to . trillion tons as of . Even if we were to stop emissions immediately, the Earth would warm by another . C by to com-pensate for the heat energy trapped by the already emitted CO , along with non-CO pollutants. Emissions to date have already committed us to this . C rise in temperature, which would bring the total warming since to . C. For comparison, even the C of warming experi-enced during the Eemian interglacial , years ago was su cient to increase sea level by to meters.

At current emission growth levels, under a business as usual sce-nario, we will add another trillion tons of carbon dioxide to the atmo-sphere within the next years, by about . This additional carbon dioxide is likely (with a probability of at least ) to mean that total warming will exceed C before . At that point, the decadal rate of climate change will be three times faster than the pace experienced until now. Most climate scientists and ecologists concur that . C to C represents the warming threshold for dangerous climate impacts.

The impacts of C warming would be uite severe. Rising tempera-tures will result in an increase in the fre uency and duration of severe heat waves. It s estimated that with C warming, well over billion people—about of the human population by —would experi-ence summer mean temperatures hotter than the current record hottest summers in one out of every two years. Moreover, about . billion people would be exposed to lethal heat for more than days a year. Increasing temperatures will also lead to more droughts and wild res, as well as increases in severe storms and ooding.

One impact with truly global conse uences is sea level rise. Even in the unlikely event that warming is stabilized at . C, sea level rise will continue for centuries because of ongoing melting of the Green-land and West Antarctic ice sheets. Studies of data for the past million years suggest that a C warming (e uivalent to the Eemian warming) is su cient to lead to an eventual sea level rise of to meters over several centuries, and a C warming could lead to a rise of to meters. Since more than of the population will be living in coastal cities by the end of this century, sea level rise of such magnitudes has enormous negative implications for displacement and mass migration

of people, disruption of social systems, and exacerbation or creation of international con icts.

Box . . provides some additional examples of the impacts of C warming.

The late twenty- rst century: warming of °C or moreBy the end of this century, a business-as-usual path with unchecked emis-sions will lead to warming that could exceed C. As we discussed in Section . , projections by models give a range of possible future tem-peratures because of di ering model treatments of climate feedbacks that can either amplify the warming or moderate it. These feedbacks, as discussed earlier, include increasing water vapor in the atmosphere and the melting of Arctic sea ice, replacing the re ective ice surface with open ocean waters that absorb additional solar radiation and amplify

Bo Impacts of C Warming

• Highl pop late regions s ch as the eastern an estern nite tates Mi le ast o th sia an China co l e perience heat aves orse than the most severe ssian heat ave o hen temperat res reache C ( )

• o t million a itional people ill e e pose to eng e chi ng n a an man other vir ses eca se o the e pan e range o isease-carr ing mos itoes

• Mo erate to severe i esprea ro ghts an res ill occ r orl i e oth r ral an r an pop lations ill e a ecte

air poll tion loss o propert an lan egra ation that re ces oo pro ction an contri tes to volatile oo prices

• loo s an storms ill ecome more re ent an or intense cientists are still e ating hether the re enc o h rricanes ill increase t the storms that o occ r are e pecte to e

stronger meaning an increase in the strongest (categor an ) h rricanes

• The climate co l reach a tipping point hen orest recover time increases to more than months an the intervals et een ro ghts ecrease to less than months I that happens orests

ma not recover rom ro ghts an res

Bo angerous to E istential isk: Categories of Projected Warming

The green c rve on the le la ele ol tions represents a scenario in hich emissions are c rtaile or phase o t completel emplo ing the ten sol tions escri e in Chapter The other three c rves in re an ro n represent scenarios ith nchec e emissions

or the re an ro n c rves aseline meaning no sig-ni cant mitigation e orts CI car on intensit re erring to the amo nt o car on io i e emitte per nit o the glo al econom eca se o shi s in the econom an increasing costs o ossil els

it s e pecte that the car on intensit o the econom ill ecrease even itho t signi cant mitigation e orts Ho ever eca se the

orl econom ill contin e to gro act al car on emissions are e pecte to increase or e ample i the car on intensit o the

orl econom ere to ecrease hal hile the econom gre to o r times its present si e total emissions o l o le

There are speci c scenarios sho n (CI ) the lo est-emission scenario o the three in hich car on intensit ecreases

(CI ) in hich car on intensit ecreases an (CI C ee ac ) hich is the same as the

secon scenario e cept that it also acco nts or ee ac s s ch as a ecrease in car on io i e pta e soils as temperat res increase

meaning that more car on io i e o l sta in the atmosphere

amanathan et al ell nder egrees elsius ast ction Policies to Protect People and the Planet from treme limate hange Image rom ig re

http igs org p-content ploa s ell- n er- - egrees-Celsi s- eport- p

warming. We also saw that one of the largest sources for the tempera-ture range projected by climate models is di ering projections of how the amount and distribution of clouds will change in a warming world. Because of this, scientists express projections of future temperature in terms of a range of probabilities rather than a single temperature.

The curves in Box . . show the probability of various levels of warming for di erent scenarios of emissions growth. The three curves in red and brown to the right side of the curve are for scenarios in which emissions growth is essentially unchecked.

The key point is that continued growth in emissions would result in

Bo Impacts of C Warming or Greater

• arming o C o l li el e pose over o the pop lation (this o l e a o t illion people ) to lethal heat

aves More than illion people co l e e pose to iseases carrie mos itoes an other pests

• arming o C o l li el e pose a o t o nat ral species to e tinction This is in a ition to the ro ghl or more o species that ill e e pose to e tinction thro gh ha itat estr c-tion the illion h mans pop lating the planet n e tinction rate o or more is consi ere to e a mass ex-tinction similar to hat happene ring the Cretaceo s perio

hen inosa rs isappeare rom the planet

• Over several cent ries arming greater than C co l res lt in an ice- ree arth ith a rise in sea level o more than meters

i esprea ro ghts are li el the most serio s o tcome threat-ening oo an ater or most o the illion people e pecte to e on the planet ( ig re )

• These impacts ill e in a ition to orsening ro ghts oo s res storms h rricanes an ing orests i esprea ro ghts

are li el the most serio s o tcome threatening oo an ater sec rit or most o the illion people e pecte to e on the planet

• These eather e tremes sea level rise an the sprea o vector- orne viral iseases ill li el lea to the mass migration o millions o h man eings

temperatures that expose human society and natural ecosystems to very severe threats. In these scenarios, the likely warming by ranges from less than C to more than C. There is less than probability that the warming will be less than C, and less than probability that it will be greater than C.

Warming in excess of C would produce catastrophic changes, while warming of C or more would have impacts so severe that it could pose an existential threat to society, as illustrated by the ndings in Box . . .

One type of high-impact conse uence that is of major concern is the possibility of runaway feedbacks. For example, large temperature increases could result in methane release by warming permafrost and wetlands and the disappearance of sea ice and glaciers. This could start a feedback loop in which higher temperatures cause more methane to be released, in turn causing further warming. Such a feedback loop would be outside human control and could undo much of the bene t of any reductions in anthropogenic emissions.

. . oi moisture conditions in with unchec ed emissions as simu ated by the rinceton ni ersity c imate mode . Adapted from Coo et a . .

Extreme temperature increases of C or greater are an example of low-probability, high-impact events. While the chance that these events will occur may be comparatively low, their conse uences would be so severe that signi cant e orts to avoid them are warranted. One way to think about this is to consider the uestion, Would you get on a plane if you knew there was a chance it would crash While there is a high probability ( ) that you would survive the ight, the severe conse uences of the low-probability crash might make you rethink your plans.

However, the green curve in Box . . , labeled Solutions, rep-resents the warming probability if the ten climate solutions presented in Chapter are implemented. Note that this curve gives a high probability of remaining below the C threshold for dangerous climate change. The green curve shows us there is real hope that if we act now, we will be able to avoid the most serious negative conse uences of climate change.

Summary: What Have We Learned So Far?

Many scientists propose that the current era be called the Anthro-pocene in view of the fact that human beings have emerged as a major force transforming the planet, comparable to major geolog-ical events.

Human impacts on climate began gradually with the Agricultural Revolution that started , years ago; the rate of transformation picked up with the Industrial Revolution that started around . Two hundred years later, there was a uantum jump in the pace of transformation with the Great Acceleration beginning in .

The post- period witnessed massive changes in the composition of the atmosphere due primarily to the use of coal and petroleum for power generation, transportation, and industries.

Climate change caused by emissions of greenhouse gases and black carbon has emerged as one of the iconic impacts of the Anthropocene.

The primary sources for anthropogenic CO emissions are fossil fuel combustion, biomass burning, cement manufacturing, and de-forestation and other land use changes. Methane sources include natural gas leaks during production, processing, and transmission; wood burning; cattle and other livestock; rice paddy agriculture; and land lls and sewage water treatment plants. Sources for CFCs and HFCs are refrigeration and air conditioning. Ozone is not directly emitted by human activities, but the emissions of ozone precursor gases (methane, nitrogen oxides, volatile organics) produce ozone in the lower atmosphere. Nitrous oxide is released by agriculture as a result of fertilization. Black carbon is produced by diesel combus-tion; burning of solid coal; and burning of rewood, crop residues, and dung for cooking.

The emitted greenhouse gases cover the planet like a blanket and trap the heat energy (infrared radiation) emitted by the surface. This trapped heat energy warms the planet. Carbon dioxide is the most important warming pollutant, contributing as much as of the present-day warming. Other greenhouse gases and black car-bon particles contribute the remaining . The planet has already warmed by about C because of this added heat. All parts of the Earth system, including the atmosphere, land, oceans, glaciers, and sea ice, are warming. The warming has extended down to me-ters below the ocean surface and up to about kilometers in the atmosphere—just as predicted by climate science and climate mod-els. The last time the planet was this warm was during the Eemian interglacial period of , to , years ago. Impacts of this warming include heat waves, severe storms, droughts, and sea level rise.

Climate change science is intensely data driven. The changes in the planetary climate have been documented by thousands of in-struments at the surface and aboard ships, aircra , balloons, and satellites. These data have been integrated into sophisticated cli-mate models run by the world s fastest computers to determine the causes and impacts of climate change.

The validity and veracity of models have been assessed by simu-lating climate changes during the twentieth century and so far in the twenty- rst century, and then comparing the models predic-tions against observations. Predictions that have been veri ed in-clude when human-induced warming would be detected above the background natural variations; amplifying feedbacks involving water vapor, sea ice, and sea level rise; and the depth of penetration of the warming in the oceans and the atmosphere.

The observed C warming has already led to a substantial retreat of sea ice, an increase in hurricane intensity, an increase in the intensity of precipitation worldwide, large-scale droughts and more fre uent res, and a tenfold increase in extreme temperatures and lethal heat

waves. Major health impacts have also been documented

The planet is currently on a path to warm to . C (from preindustrial

levels) within years, most likely by the year . If we keep add-ing climate pollutants at the present rate, it will continue to warm to C by , and to a catastrophic C to C ( con dence range) by the end of this century. The potential impacts on human health, ecosystem health, and species extinction lead to the conclu-sion that warming in excess of C would pose existential threats to Homo sapiens (all billion of us) and numerous other species.

As we have seen in this chapter, the science of global warming is clear, and the potential impacts of continuing emissions for human soci-ety and natural ecosystems are severe. There is still time to act, but we have only about years to bring all of the solutions described in this book up to full speed.

Fortunately, a range of climate solutions that o er real hope are available and will be explored in detail in the remainder of this book. These solutions will help provide you, climate warriors, with the tools to avoid such a catastrophic future.

Supplementary eadings

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uploads SPM. rev - .png.

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Figure . . : . Ramanathan. Bending the Curve: Lecture , Module . Anthropocene and Planetary Stewardship.

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Figure . . : Ramanathan, . Bending the Curve: Lecture , Module . Icons designed by Freepik from http: www. aticon.com packsecology-enviroment- .

Figure . . : Earth image from Apollo in . https: nssdc.gsfc.nasa.govimgcat html object page a h .html.

Figure . . : Photograph by Matthew Henry on Unsplash.

Figure . . : All images from NASA. enus image: https: nssdc.gsfc.nasa.govimage planetary venus pvo uv .jpg. Earth image: https: www.nasa.gov image-feature nasa-captures-epic-earth-image. Mars image: https: www.jpl.nasa.gov news news.php feature .

Figure . . : Image from NOAA. https: sos.noaa.gov datasetsearth-our-goldilocks-planet .

Figure . . : Image from the Scripps CO Program from the Scripps Institution of Oceanography. http: scrippsCO .ucsd.edu graphics gallerymauna loa and south pole mauna loa and south pole.

Figure . . : limate in Peril Popular uide to the atest P eports. UNEPGRID-Arendal. Page . https: www.uncclearn.org sites default lesinventory unep .pdf.

Figure . . : Top le photograph by Alan Kiniry, reproduced from https:www.pexels.com . Top right image reproduced from https: pixabay.com . Bottom le photograph by . Ramanathan. Bottom right image reproduced from https: pixabay.com .

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