Chapter 19: Climate and Our Changing Planet

Introduction: The Changing Atmosphere (1)

The concentration of carbon dioxide in Earth’s atmosphere has increased because of: The use of fossil fuels. The clearing of forest for farmland by more than 25

percent.

Introduction: The Changing Atmosphere (2)

Earth’s climate system consists of a number of interacting subsystems: The atmosphere. The hydrosphere. The solid Earth. The biosphere.

Figure 19.1

Introduction: The Changing Atmosphere (3)

Climate can be read from: The statigraphic record. The fossil plants and animals. The nature of terrestrial and marine sediments. Polar ice cores.

The Carbon Cycle (1)

Carbon occurs in five reservoirs: In the atmosphere, it occurs in carbon dioxide. In the biosphere, it occurs in organic compounds. In the hydrosphere, it occurs as dissolved carbon

dioxide. In the crust, it occurs both in the calcium carbonate

of limestone and in decaying and buried organic matter such as peat, coal, and petroleum.

In the mantle, some carbon has been present since the Earth first formed.

Figure 19.2

The Carbon Cycle (2)

The key of the carbon cycle is the biosphere, where plants continually extract CO2 from the atmosphere and then break the CO2 down by process of photosynthesis to form organic compounds.

The passage of material through the biosphere is so rapid that all the CO2 in the atmosphere cycles every 4.5 years.

The Carbon Cycle (3)

Not all dead plant matter in the biosphere decays immediately back to CO2.

Carbon dioxide from the atmosphere also is dissolved in the water of the hydrosphere.

Aquatic animals extract calcium and carbon dioxide from the water to make shells of CaCO3.

The Carbon Cycle (4)

When compacted and cemented, the CaCO3 forms limestone.

Limestone that accompanies downgoing lithosphere into the mantle at a subduction zone eventually returns CO2 to the atmosphere in a volcano

When we mine coal and extract oil from the crust and then burn them, we convert organic matter to CO2, thereby accelerating the carbon cycle.

The Greenhouse Effect (1)

Of the radiation that reaches the planet’s surface, Some is absorbed by the land and oceans. Some is reflected into space by:

Water. Snow. Ice. Other reflective surfaces.

The Greenhouse Effect (2)

The atmospheric gases allows visible (short-wavelength) sunlight in, but prevents the escape of infrared (long-wavelength) energy. This phenomenon is the greenhouse effect.

Figure 19.3

Greenhouse Gases (1)

Dry air consists mainly of three gases: Nitrogen (79%). Oxygen (20%). Argon (1%).

Water vapor is usually present in the Earth’s atmosphere in concentrations of up to several percentage points, and accounts for about 80 percent of the natural greenhouse effect.

Greenhouse Gases (2)

The remaining 20 percent is due to other gases present in very small amounts: Carbon dioxide. Methane. Nitrous dioxide. Ozone.

Trends in Greenhouse Gas Concentrations (1)

During the growing season, CO2 is absorbed by vegetation, and its atmospheric concentration falls.

During the winter dormant period, more CO2 enters the atmosphere than is removed by vegetation, and its concentration rises.

Figure 19.4

Trends in Greenhouse Gas Concentrations (2)

Past changes in atmospheric CO2 are recorded in arctic and Antarctic ice. Ice cores obtained from the Antarctic and

Greenland ice sheets contain samples of ancient atmosphere.

Ice from Vostok Station in Antarctica contains such a record extending back 160,000 years.

Figure 19.5

Trends in Greenhouse Gas Concentrations (3)

A possible explanation for the extraordinary recent rise in the atmospheric CO2 is immediately suggested by the rate at which CO2 has been added to the atmosphere since the beginning of the Industrial Revolution.

The inescapable conclusion is that the anthropogenic (human-generated) burning of fossil fuels must be a primary factor in the observed increase in atmospheric CO2.

Trends in Greenhouse Gas Concentrations (4)

Additional contributing factors are: Widespread deforestation. Use of wood as a primary fuel.

The largest yearly increases on CO2 in the 1990s occurred in years with maximum El Niňo conditions (warmer water holds less dissolved CO2). CO2 is less likely to be absorbed by the ocean in an

El Niňo years.

Figure 19.6

Methane

Methane (CH4) absorbs infrared radiation 25 times more effectively than CO2.

Methane levels show increases that parallel the rise in the human population.

It is generated by: Biological activity related to rice cultivation. Leaks in domestic and industrial gaslines. The digestive processes of domestic livestock.

Chlorofluorocarbons (CFCs)

CFC-12, used mostly as a refrigerant, has 20,000 times the capacity of carbon dioxide to trap ultraviolet radiation.

CFC-11 is widely used in making plastic foams and as an aerosol propellant, has 17,500 times the capacity of carbon dioxide to trap ultraviolet radiation.;

These gases destroy ozone in the upper atmosphere, thereby leading to the formation of the Antarctic ozone hole.

Ozone and Nitrous Oxide (1)

Although ozone in the upper atmosphere is beneficial because it traps harmful ultra violet solar radiation when this gas builds up in the troposphere (lower atmosphere), it contributes to the greenhouse effect;

Ozone and Nitrous Oxide (2)

Nitrous oxide is produced by: The burning of fossil fuels. Microbial activity in soil. The burning of timber. The decay of agricultural residues.

Together, ozone and nitrous oxide account for about 13 percent of the anthropogenic greenhouse effect.

Global Warming (1)

Correctly assessing recent global changes in temperature is a very complicated task. Few instrumental measurements were made before

1850. Vast regions have no weather data before World

War II. The earliest records are from western Europe and

eastern North America. Records from oceanic areas (70 percent of the

globe) are sparse.

Figure 19.7

Global Warming (2)

All curves of average temperatures show a long-term rise in temperature during the past century. The total temperature increase since the late

nineteenth century is about 0.60C Some portion of the modest rise in global

temperature during the past century could fall within the natural variability of Earth’s climate system.

Global Warming (3)

Some patterns of global temperature can fluctuate with durations of 3 to 17 years and have a measurable impact on the average global temperature.

Very small fluctuations in the energy radiated by the Sun, which correlate with the varying number of sunspots observed by astronomers, may exert a significant influence on climate.

Global Warming (4)

Indirect estimates of temperature can be obtained from: The thickness of yearly growth rings in trees (on

land). Coral (at sea). Yearly ice accumulation by glaciers.

Climate Models (1)

Global climate models are three-dimensional mathematical models of Earth’s climate system. They are an outgrowth of efforts to forecast the

weather.

The most sophisticated are general circulation models (GCMs) that attempt to link processes in the atmosphere, the hydrosphere, and the biosphere.

Climate Models (2)

Despite their limitations, GCMs have been very successful in simulating the general character of present-day climates.

To run a modeling experiment that simulates the climate, scientists specify a set of boundary conditions.

Boundary conditions are mathematical expressions of the physical state of Earth’s climate system at the period of interest for the experiment.

Climate Models (3)

Present climate boundary conditions would include: The solar radiation reaching Earth. The geographic distribution of land and ocean. The position and heights of mountains and

plateaus. The concentrations of atmospheric trace gases. Sea-surface temperatures.

Climate Models (4)

The limit of sea ice. The snow and ice cover on the land. The albedo (reflectivity) of land ice and water

surfaces. The effective soil moisture.

If fossil-fuels consumption continues to grow at an increasing rate, atmospheric CO2 is projected to double by 2100, if not sooner Average global temperature will rise between 1.50

and 4.50C;

Figure 19.8

Climate Models (5)

The rate at which the projected warming will occur depends on a number of uncertainties: How rapidly will concentrations of the greenhouse

gases increase? How rapidly will the oceans respond to changing

climate? How will changing climate affect ice sheets and

cloud cover?

Figure 19.9

Environmental Effects of Global Warming (1)

Global precipitation changes because of Increased or decreased evaporation. Changes in vegetation. Increased storminess (warmer, wetter atmospheres

favor an increase in tropical storm activity). Melting or growing glaciers.

Warmer summers favor increased ablation. Warmer air in high-latitudes could evaporate and

transport more moisture causing glaciers to grow larger.

Environmental Effects of Global Warming (2)

Reduction of sea ice. Thawing of frozen ground. Rise of sea level. Changes in the hydrologic cycle. Decomposition of soil organic matter. Breakdown of gas hydrates.

Ice-like solids in which gas molecules mainly methane, are locked in the structure of solid H2O,

Found in some ocean sediments and beneath frozen ground.

The Past As A Key To The Future (1)

The most important lesson from the geologic record is that past climate has paralleled today’s climate only occasionally.

Our interglacial interval is now approximately 12,000 years old, close to the age where a return to glacial conditions can be expected.

The Past As A Key To The Future (2)

Climate in the last million years has occasionally been warmer than today’s climate. In the early Holocene, from 10,000 to about 6000

years ago, average temperatures were 5.0 to 10C warmer than today.

About 120,000 years ago, global temperatures were 1 to 20C higher than today.

The Past As A Key To The Future (3)

Because human-induced climate change is occurring over the course of decades, rather than millions of years, geologic records of rapid environmental change are also important.

Why Was the Middle Cretaceous climate So Warm? (1)

The world was inhabited by huge carnivorous dinosaurs.

Coral reefs grew 5 to 150 closer to the poles than they do now.

Vegetation zones were displaced about 15 0 poleward of their present positions.

Sea level was 100 to 200 m higher, indicating the absence of polar ice sheets.

Figure 19.10

Why Was the Middle Cretaceous climate So Warm? (2)

Several factors could be involved in producing Middle Cretaceous globally warm conditions: Geography. Ocean circulation. Atmospheric composition.

Why Was the Middle Cretaceous climate So Warm? (3)

The oceans now account for about a third of the present poleward heat transfer.

A significant cause for the Middle Cretaceous warming is an increase in CO2, the major greenhouse trace gas.

Figure 19.11

Mantle Convection and Atmospheric Greenhouse Gases (1)

The most likely source CO2 entering the atmosphere is volcanic activity.

Geologic evidence points to an unusually high rate of volcanic activity in the Middle Cretaceous.

Mantle Convection and Atmospheric Greenhouse Gases (2)

Vast outpourings of lava created a succession of great undersea plateaus across the central Pacific Ocean between 135 and 115 million years ago. The Ontong-Java plateau (twice the area of Alaska

and 40 km thick). The eruptions could have released enough CO2

to raise the atmospheric concentration to 20 times the preindustrial value.

Mantle Convection and Atmospheric Greenhouse Gases (3)

Recently, geologists have proposed that these vast lava outpouring are the result of a superplume. A superplume would originated from a substantial

overturn of mantle rock. Downgoing slabs of lithosphere can stall near 670

km depth. Hot lower mantle rises to replace the falling slabs,

inducing a superplume.

Figure 19.12

Mantle Convection and Atmospheric Greenhouse Gases (4)

If the flushing hypothesis is correct, then plate tectonics cools the mantle in two styles; In “ordinary” times, heat loss at spreading ridges

couples with the downward plunge of cool lithosphere,

In “extraordinary” times—when stalled slabs flush into the lower mantle—superplumes would allow heat to escape more efficiently from Earth’s lower mantle and core.

Mantle Convection and Atmospheric Greenhouse Gases (5)

The warm Middle Cretaceous climate was a direct consequence of the cooling of the Earth’s deep interior.

Eocene Warmth and Lithosphere Degassing (1)

By the end of the Cretaceous, world temperatures had fallen substantially below levels reached earlier in that period.

Eocene Warmth and Lithosphere Degassing (2)

In the Early Eocene, evidence points to warmer conditions, probably close to 20 C higher than today. Alligator fossils on Ellesmere Island (at about 780 N

latitude ). Tropical vegetation at up to 450 N latitude. Tropical marine surface-water organisms at about

550 N. Lateritic soils at up to 450 N or S.

Eocene Warmth and Lithosphere Degassing (3)

Two processes may have contributed to the higher Eocene temperatures: Greater poleward transfer of heat by the oceans

(preventing the formation of polar sea ice). An increased concentration of CO2 in the

atmosphere (two to six times the modern value) due to volcanism and crustal metamorphism.

Eocene Warmth and Lithosphere Degassing (4)

CO2 is released as a byproduct of the metamorphic reactions

Studies show that regional metamorphism of the Himalayas may have been contemporaneous with the Eocene warming.

At this time regional metamorphism occurred in Greece, Turkey, New Caledonia, Japan, and Western North America.

Eocene Warmth and Lithosphere Degassing (5)

Removal of atmospheric CO2 occurs during the weathering of orogenically uplifted silicate rocks. CO2 is removed from the atmosphere when

carbonic acid is produced. As the weak acid decomposes the rocks,

bicarbonate released by the weathering reactions is carried in solution by streams to the sea, and is there converted to calcium carbonate by marine organisms.

Figure 19.13

Modeling past Global Changes

Paleoclimatic reconstructions offer a means of testing the accuracy of climate models.

We can test, or “validate,” these climate simulations by comparing the model results against independent geologic evidence.

The results of the model simulation are generally consistent with the paleoclimatic data assembled by geologists.

Figure 19.14

Figure 19.15

Revisiting Plate tectonics And The Earth System (1)

The presence of extensive coal deposits in Antarctica implies a climate that supported abundant plant growth;

Antarctica drifted through more temperate latitudes during geologic history.

Revisiting Plate tectonics And The Earth System (2)

Part of the warmth of the Middle Cretaceous can be attributed to the presence of shallow seas over much of today’s dry land. Sea surface absorbs solar energy more efficiently

than land surfaces, encouraging a warmer climate. Because sea floor spreading was faster in the

Middle Cretaceous, the plate system had a large proportion of young, warm (and therefore buoyant) oceanic lithosphere.

Revisiting Plate tectonics And The Earth System (3)

Cretaceous midocean ridges were wider than at present, decreasing the average depth of the ocean.

A substantial volume of water would be displaced, sea level would rise, and low-lying continental areas would become shallow seas.

Figure B19.1

Figure B19.2