Chapter 14: Climate Chapter 14: Climate ChangeChange

The earth’s changing climateThe earth’s changing climate Possible causes of climatic changePossible causes of climatic change Global warmingGlobal warming

The Earth’s The Earth’s Changing Changing ClimateClimate

• The earth’s climate is always changing• 18,000 years ago the 18,000 years ago the sea level was so low sea level was so low that the English that the English Channel didn’t Channel didn’t even exist, and even exist, and Bering land bridge did Bering land bridge did existexist

18,000 years ago

Determining Past Determining Past ClimatesClimates1) fossil evidence1) fossil evidence

Q1: if fossil pollen of a tundra plant is collected in a layer of Q1: if fossil pollen of a tundra plant is collected in a layer of sediments in New England and dated to be 12,000 years old, sediments in New England and dated to be 12,000 years old, it would suggest that compared with today, the climate then isit would suggest that compared with today, the climate then is

a) much colder, b) much warmer, c) similar, d) unknowna) much colder, b) much warmer, c) similar, d) unknown

2) Ocean floor sediment oxygen isotope ratio (of oxygen 18 over 2) Ocean floor sediment oxygen isotope ratio (of oxygen 18 over oxygen 16)oxygen 16)

• Isotopes are atoms whose nuclei have the same number of Isotopes are atoms whose nuclei have the same number of protons but different numbers of neutrons.protons but different numbers of neutrons.

• This ratio is higher during a colder climateThis ratio is higher during a colder climate 3) dendrochronology- tree ring growth depends on 3) dendrochronology- tree ring growth depends on

temperature and precipitation over certain regions temperature and precipitation over certain regions Q2: Which university established the dendrochronology Q2: Which university established the dendrochronology

field first?field first?a) U. of Arizona; b) Harvard; c) Stanford; d) Yale a) U. of Arizona; b) Harvard; c) Stanford; d) Yale

Determining Past Determining Past ClimatesClimates sediment cores: ocean, lakesediment cores: ocean, lake

ice cores: Antarctic, Greenlandice cores: Antarctic, Greenland borehole temperature profilesborehole temperature profiles Dating calcium carbonate layers of stalactites in cavesDating calcium carbonate layers of stalactites in caves

• As the global warming debate has intensified in recent As the global warming debate has intensified in recent years, many methods of reconstructing past climates years, many methods of reconstructing past climates have undergone close scrutinyhave undergone close scrutiny

Q3: What is the main uncertainty of these proxy data in Q3: What is the main uncertainty of these proxy data in representing temperature?representing temperature?

a) relation is not lineara) relation is not linearb) non-temperature effects are not easy to separateb) non-temperature effects are not easy to separatec) correlation is not causalityc) correlation is not causalityd) all of themd) all of them

Climate During Climate During Past 1000 Years Past 1000 Years and since 1860and since 1860

“ “Hockey Stick” graphHockey Stick” graph

The global warming of the The global warming of the past 100 years has notpast 100 years has notbeen constantbeen constant

Q4: is the temperature decrease from 1940-1970 contradictory with global warming?a) yes; b) no

Climate Change and Climate Change and Feedback MechanismsFeedback Mechanisms

Q5: The water vapor-greenhouse feedback is?Q5: The water vapor-greenhouse feedback is?a) positive feedback; b) negative feedback a) positive feedback; b) negative feedback c) unknownc) unknownQ6: Why is the snow-albedo feedback positive?Q6: Why is the snow-albedo feedback positive?A: increasing T decreases snow cover; this decreases surface A: increasing T decreases snow cover; this decreases surface

albedo and hence increases surface solar absorption; leading to albedo and hence increases surface solar absorption; leading to further increasing Tfurther increasing T

Q7: why is cloud feedback overall negative?Q7: why is cloud feedback overall negative?A: low clouds are negative by reflecting solar radiation; high A: low clouds are negative by reflecting solar radiation; high

clouds are positive just like greenhouse gases; overall low clouds are positive just like greenhouse gases; overall low clouds are more dominant, leading to overall negative feedbackclouds are more dominant, leading to overall negative feedback

• Feedbacks cause climate changes to be either amplified Feedbacks cause climate changes to be either amplified (positive feedback) or reduced (negative feedback).(positive feedback) or reduced (negative feedback).

Possible Causes of Climate Possible Causes of Climate ChangeChange

external: changes in incoming solar radiationexternal: changes in incoming solar radiation external (e.g., volcano) or human (e.g., CO2): changes in the external (e.g., volcano) or human (e.g., CO2): changes in the

composition of the atmospherecomposition of the atmosphere external (e.g., mountain uplifting) or human (e.g., land use in external (e.g., mountain uplifting) or human (e.g., land use in

urban and agricultural areas): changes in the earth’s surfaceurban and agricultural areas): changes in the earth’s surface

• Emissions of COEmissions of CO22 and other greenhouse gases are and other greenhouse gases are

by no means the only way to change the climate.by no means the only way to change the climate.

Climate Change and Atmospheric Climate Change and Atmospheric Particles (decadal-century)Particles (decadal-century)

• Sulfate aerosols cool the climate and therefore Sulfate aerosols cool the climate and therefore counteract global warming to some extent.counteract global warming to some extent.

Q8: why does volcano eruptions, rich in sulfur, cool the atmosphere in general?A: sulfur aerosols reflect solar radiation; they also modify clouds by increasing the cloud’s reflectivity

Climate Change and Variations Climate Change and Variations in Solar Output (decadal-in Solar Output (decadal-

century)century) Sunspots: are huge magnetic storms on the sun that show up Sunspots: are huge magnetic storms on the sun that show up

as cooler (darker) regions on the sun’s surfaceas cooler (darker) regions on the sun’s surface

Q9: why does solar radiation increases with the increase of sunspot numbers?A: sunspots are cooler; but the surrounding margins of sunspots are brighter (i.e., hotter); overall effect is to increase solar radiation

Climate Change and Variations in the Earth’s Climate Change and Variations in the Earth’s Orbit (10K-100K years)Orbit (10K-100K years)

Milankovitch theory:Milankovitch theory:- eccentricity: geometry of orbit - eccentricity: geometry of orbit (100K yr)(100K yr)- obliquity: tilt angle of axis (41K yr)- obliquity: tilt angle of axis (41K yr)

- precession: direction of axis (23K yr)- precession: direction of axis (23K yr)

Min: 22o

Max: 24.5o

Climate Change, Plate Climate Change, Plate Tectonics, and Mountain-Tectonics, and Mountain-

Building (100M yr)Building (100M yr) theory of plate tectonicstheory of plate tectonics

Landmasses 150M years ago Today

Earth’s land mass in 650 million years:http://boingboing.net/2009/07/16/time-lapse-sim-of-ea.html

Q10: what are the external causes of climate change?a) solar radiation change; b) air composition change; c) earth’s land mass change; d) all of them

Q11: with the change of earth’s axis in 11K years (due to precession), current July in northern hemisphere would be

a) spring; b) summer; c) fall; d) winter

Q12: How do major volcanoes change surface temperature?a) decrease; b) no change; c) increase

Q13: Since the sunspot number is below average in 1986, the solar radiation that year is

a) below average; b) normal; c) above average

Rising atmospheric temperature

Rising sea level

Reductions in NH snow cover

And oceans..

And upper atmosphere….

Warming is Unequivocal

Human and Natural Drivers of Human and Natural Drivers of Climate ChangeClimate Change

CO2 is causing the bulk of the forcing

On average, it lives > 100 years in the atmosphere and therefore affects climate over long time scales.

without air, the without air, the earth surface earth surface temperature would temperature would be much colderbe much colder

Human and Natural Drivers of Human and Natural Drivers of Climate ChangeClimate Change

Q14: Since the radiative forcing due to CO2 is nearly the same as the total forcing in the figure, can we claim that only CO2 is important?

a) yes; b) no

Natural versus human-Natural versus human-caused temperature caused temperature

increaseincrease

Climate models are needed:Climate models are needed:

aerosols;aerosols;

cloud;cloud;

precipitation;precipitation;

land processes;land processes;

… …..

Understanding and Attributing Understanding and Attributing Climate ChangeClimate Change

Anthropogenic warming is

likely discernible on

all inhabited continents

Observed

Expected for all forcings

Natural forcing only

Q15: Does the figure prove that the global warming is man-made?a) yes; b) no

What’s in the pipeline and what could What’s in the pipeline and what could comecome

Warming will increase if GHG increase. If GHG were kept fixed Warming will increase if GHG increase. If GHG were kept fixed at current levels, a committed 0.6°C of further warming would be at current levels, a committed 0.6°C of further warming would be expected by 2100. More warming would accompany more expected by 2100. More warming would accompany more emission.emission.

1.8oC = 3.2oF

2.8oC = 5.0oF

3.4oC = 6.1oF

CO2 Eq

850

600

4000.6oC = 1.0oF

A1B is a typical “business as usual” (2090-2099) scenario: Global mean warming 2.8oC;

Much of land area warms by ~3.5oCArctic warms by ~7oC; would be less for less emission

Q16: will the world actually warm this much?a) yes; b) no

Q17: What is a possible reason?a) volcano, b) earthquake, c) orbital change, d) axis change

What else happens in a hotter world?

Observations of sea level rise from satellites, 1993-2003.

Q18: what are the reasons for the sea level rise? a) expansion of warmer ocean, b) glacier melt; c) both

Future changes just from these processes could be up to 1.5 feet (0.5 m) by 2100, and up to 3 feet (1 meter) within about 2-3 centuries, depending on how much GHGs are emitted.

Other related issuesOther related issues

DroughtDrought Tropical forest dieoffTropical forest dieoff Hurricane activitiesHurricane activities Kyoto ProtocolKyoto Protocol