Page 357 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 11 Climate Change 321
Earth’s Carbon Budget
Many climate feedbacks involve the movement of carbon through Earth systems and the balance of carbon over time within these systems. The carbon on Earth cycles through atmospheric, oceanic, terrestrial, and living systems in a biogeochemical cycle known as the carbon cycle (dis- cussed and illustrated in Chapter 19). Areas of carbon re- lease are carbon sources; areas of storage are called carbon sinks, or carbon reservoirs. The overall exchange of car- bon between the different systems on Earth is the global carbon budget, which should naturally remain balanced as carbon moves between sources and sinks. Geosystems in Action, Figure GIA 11, illustrates the components, both natural and anthropogenic, of Earth’s carbon budget.
Several areas on Earth are important carbon sinks. The ocean is a major carbon storage area, taking up CO2 by chemical processes as it dissolves in seawater and by biological processes through photosynthesis in micro- scopic marine organisms called phytoplankton. Rocks, another carbon sink, contain “ancient” carbon from dead organic matter that was solidified by heat and pressure, including the shells of ancient marine organisms that lithified to become limestone (discussed in Chapter 12). Forests and soils, where carbon is stored in both living and dead organic matter, are also important carbon sinks. Finally, the atmosphere is perhaps the most critical area of carbon storage today, as human activities cause emis- sions that lead to increasing concentrations of CO2 into the atmosphere, changing the balance of carbon on Earth.
Humans have impacted Earth’s carbon budget for thousands of years, beginning with the clearing of forests for agriculture, which reduces the areal extent of one of Earth’s natural carbon sinks (forests) and transfers carbon to the atmosphere. With the onset of the Industrial Revo- lution, around 1850, the burning of fossil fuels became a large source of atmospheric CO and began the depletion
transferred solid carbon stored in plants and rock to gas- eous carbon in the atmosphere.
Given the large concentrations of CO2 currently being released by human activities, scientists have for several decades wondered why the amount of CO2 in Earth’s at- mosphere is not higher. Where is the missing carbon? Studies suggest that uptake of carbon by the oceans is off- setting some of the atmospheric increase. When dissolved CO2 mixes with seawater, carbonic acid (H2CO3) forms, in a process of ocean acidification. The increased acidity af- fects seawater chemistry and harms marine organisms, such as corals and some types of plankton, that build shells and other external structures from calcium carbonate (dis- cussed in Chapter 16). Scientists estimate that the oceans have absorbed some 28% of the rising concentrations of at- mospheric carbon, slowing the warming of the atmosphere. However, as the oceans increase in temperature, their ability to dissolve CO2 is lessened. Thus, as global air and ocean temperatures warm, more CO2 will likely remain in the atmosphere, with related impacts on Earth’s climate.
Uptake of excess carbon is also occurring in Earth’s terrestrial environment, as increased CO2 levels in the atmosphere enhance photosynthesis in plants. Research suggests that this produces a “greening” effect as plants produce more leaves in some regions of the world (see Chapter 19, Figure HD 19c).
Water Vapour Feedback
Water vapour is the most abundant natural greenhouse gas in the Earth–atmosphere system. Water vapour feed- back is a function of the effect of air temperature on the amount of water vapour that air can absorb, a subject dis- cussed in Chapters 7 and 8. As air temperature rises, evap- oration increases, because the capacity to absorb water vapour is greater for warm air than for cooler air. Thus, more water enters the atmosphere from land and ocean surfaces, humidity increases, and greenhouse warming accelerates. As temperatures increase further, more water vapour enters the atmosphere, greenhouse warming fur- ther increases, and the positive feedback continues.
The water vapour climate feedback is still not well understood, in large part because measurements of global water vapour are limited, especially when compared to the relatively strong data sets for other greenhouse gases, such as CO2 and methane. Another complicating factor is the role of clouds in Earth’s energy budget. As atmospheric water vapour increases, higher rates of condensation will lead to more cloud formation. Remember from Chapter 4 (Figure 4.8) that low, thick cloud cover increases the albedo of the atmosphere and has a cooling effect on Earth (cloud albedo forcing). In contrast, the effect of high, thin clouds can cause warming (called cloud greenhouse forcing).
Carbon–Climate Feedbacks
We saw earlier that over long time periods, CO2 and methane concentrations track temperature trends, with a slight lag time (Figure 11.11). One hypothesis for this rela- tionship is that warming temperatures caused by changes in Earth’s orbital configuration may trigger the release of greenhouse gases (both CO and methane), which then
leads to increases in gas concentrations; elevated gas con- centrations then amplify warming; and so on.
In this chapter’s Geosystems Now, we discuss this carbon–climate feedback as it occurs in permafrost areas. Figure GIA 11.4 illustrates the permafrost–carbon feed- back now underway in the Arctic. This process occurs as warming temperatures lead to permafrost thaw. Rising atmospheric CO2 leads to increased plant growth and mi- crobial activity, and eventually leads to a greater amount of carbon emitted to the atmosphere rather than stored in the ground—a positive feedback that accelerates warming.
CO2–Weathering Feedback
Not all climate feedback loops act on short time scales and not all have a positive, or amplifying, effect on
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of fossilized carbon stored in rock. These activities have
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act as a positive feedback mechanism: Initial warming
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