Unit 2: Atmosphere // Section 8: The Global Carbon Cycle
As this unit shows, temperature and moisture levels are major variables shaping Earth's weather patterns. By increasing atmospheric concentrations of greenhouse gases through activities such as burning fossil fuels, humans are changing the planet's radiative balance. This process is altering global temperatures and moisture levels, so we can expect that it will change Earth's weather patterns. One of the key issues in current atmospheric science research is understanding how GHG emissions affect natural cycling of carbon between the atmosphere, oceans, and land. The rate at which land and ocean sinks take up carbon will determine what fraction of man-made CO2 emissions remain in the atmosphere and alter Earth's radiative balance.
Atmospheric levels of CO2, the most important anthropogenic greenhouse gas, are controlled by a dynamic balance among biological and inorganic processes that make up the carbon cycle. These processes operate on very diverse time scales ranging from months to geological epochs. Today, human intervention in the carbon cycle is disturbing this natural balance. As a result, atmospheric CO2 concentrations are rising rapidly and are already significantly higher than any levels that have existed for at least the past 650,000 years.
In recent decades, only about half of the CO2 added to the atmosphere by human activities has stayed in the atmosphere. The rest has been taken up and stored in the oceans and in terrestrial ecosystems. The basic processes through which land and ocean sinks (storage reservoirs) take up carbon are well understood, but there are many questions about how much anthropogenic carbon these sinks can absorb, which sinks are taking up the largest shares, and how sensitive these sinks are to various changes in the environment. These issues are concerns for atmospheric scientists because carbon that cannot be taken up by land and ocean sinks will ultimately end up in the atmosphere. By monitoring atmospheric concentrations of CO2 and other greenhouse gases, scientists are working to understand the operation of natural carbon sinks more accurately (Fig. 16).
"We use the atmosphere as a diagnostic to get a handle on these processes—to quantify where they take place and how long they are. If we can get an understanding of what the Earth itself is doing with these excess gases, we can make better prognoses of what future climate change might be like."
Dr. Pieter Tans, National Oceanographic and Atmospheric Administration
Figure 16. Tall tower monitoring station for atmospheric sampling
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The carbon cycle can be viewed as a set of reservoirs or compartments, each of which holds a form of carbon (such as calcium carbonate in rocks or CO2 and methane in the atmosphere), with carbon moving at various natural rates of transfer between these reservoirs (Fig. 17). The total amount of carbon in the system is fixed by very long-term geophysical processes such as the weathering of rock. Human actions that affect the carbon cycle, such as fossil fuel combustion and deforestation, change the rate at which carbon moves between important reservoirs. Burning fossil fuels speeds up the "weathering" of buried hydrocarbons, and deforestation accelerates the natural pace at which forests die and decompose, releasing carbon back to the atmosphere.
Figure 17. Global carbon cycle
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Source: © Climate Change 2007: The Physical Scientific Basis, Intergovernmental Panel on Climate Change.
The residence time of carbon varies widely among different reservoirs. On average a carbon atom spends about 5 years in the atmosphere, 10 years in terrestrial vegetation, and 380 years in intermediate and deep ocean waters. Carbon can remain locked up in ocean sediments or fossil fuel deposits for millions of years. Fast cycling processes that take place in months or a few years have rapid effects but only influence small CO2 reservoirs, so they do not change long-term CO2 levels significantly. Slow processes that take place over centuries, millennia, or geologic epochs have greater influence on CO2 concentrations over the long term.
Two processes remove CO2 from the atmosphere: photosynthesis by land plants and marine organisms, and dissolution in the oceans. There is an important distinction between these processes in terms of permanence. CO2 taken up through photosynthesis is converted into organic plant material, whereas CO2 dissolved in the oceans is transferred to a new carbon reservoir but remains in inorganic form. Organic carbon in plant tissues can remain sequestered for thousands or millions of years if it is buried in soils or deep ocean sediments, but it returns to the atmosphere quickly from material such as leaf litter. Similarly, CO2 dissolved in the oceans will stay a long time if sequestered in deep water, but will escape more readily back into the atmosphere if ocean mixing brings it to the surface.
Oceans and land ecosystems thus serve as both sources and sinks for carbon. Until recently these processes were in rough equilibrium, but the balance is being disrupted today as human activities add more carbon to the atmosphere and a large fraction of that anthropogenic carbon is transferred to the oceans. Therefore, it is important to understand the chemical and biological processes through which the oceans take up CO2.
Atmospheric CO2 dissolves into surface waters, where it reacts with liquid water to form carbonic acid, carbonate, and bicarbonate. This process makes the oceans an important buffer against global climate change, but there are limits to how much CO2 the oceans can absorb. Seawater is slightly basic, with a pH value of 8.2. Adding CO2 acidifies the water. Dissolved CO2 gas reacts with carbonate (CO3 2-) ions in the water, increasing concentrations of H+ and other hydrogen ions, which drives pH values lower (Fig. 18).
Figure 18. Relative proportions of inorganic forms of CO2 dissolved in seawater
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Source: © 2005. British Royal Society Report, Ocean Acidification Due to Increasing Atmospheric Carbon Dioxide, p. vi.
Over the long term, reducing the concentration of carbonate ions will slow the rate at which oceans take up CO2. However, this process could significantly alter ocean chemistry. The British Royal Society estimated in a 2005 report that uptake of anthropogenic CO2 emissions had already reduced the pH of the oceans by 0.1 units, and that the average pH of the oceans could fall by 0.5 units by 2100 if CO2 emissions from human activities continued to rise at their current pace (footnote 1).
Theoretically the oceans could absorb nearly all of the CO2 that human activities are adding to the atmosphere. However, only a very small portion of the ocean (the mixed layer, discussed further in Unit 3, "Oceans") comes into close contact with the atmosphere in a year. It would take about 500 years for all ocean water to come into contact with the atmosphere. As we will see in Unit 12, "Earth's Changing Climate," solutions to climate change are needed on a faster scale.
As noted above, biological uptake in the oceans occurs when phytoplankton in surface waters use CO2 during photosynthesis to make organic matter. The organic carbon stored in these organisms is then transferred up the food chain, where most is turned back into CO2. However, some ultimately falls to lower depths and is stored in deep ocean waters or in ocean sediments, a mechanism called the "biological pump" (for more details, see Unit 3, "Oceans").
Forests take up CO2 through photosynthesis and store carbon in plant tissue, forest litter, and soils. Forests took up a rising share of CO2 from fossil fuel combustion in the 1980s and 1990s. Scientists believe that this occurred mainly because forests in the Northeastern United States and similar areas in Europe, many of which were clear-cut or used for agriculture in the 1700s and 1800s, have been growing back with the decline of agriculture in the region (Fig. 19).
Figure 19. Farm abandonment (1850) and hardwood forest regrowth (1930) in central New England
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Source: © Harvard Forest Dioramas, Fisher Museum, Harvard Forest, Petersham, MA. Photography is by John Green and David Foster.
Can forests solve the problem of rising atmospheric CO2 levels? If lands are managed to optimize CO2 uptake through sustainable forestry practices, forests can continue to sequester a significant fraction of the carbon that human activities are adding to the atmosphere. However, this share is unlikely to grow much beyond its current level (about 10% of anthropogenic emissions) because the rate of carbon uptake levels off as forests mature. Forests can help, but are not a total solution. (For more details, see Unit 13, "Looking Forward: Our Global Experiment.")