6.7.3: Sinks and Cycles
In total, land in the United States absorbs and stores an amount of carbon equivalent to about 17% of annual U.S. fossil fuel emissions. U.S. forests and associated wood products account for most of this land sink. The effect of this carbon storage is to partially offset warming from emissions of \(CO_2\) and other greenhouse gases.
Considering the entire atmospheric \(CO_2\) budget, the temporary net storage on land is small compared to the sources: more \(CO_2\) is emitted than can be taken up (see “Estimating the U.S. Carbon Sink”).[7,21,22,23] Other elements and compounds affect that balance by direct and indirect means (for example, nitrogen stimulates carbon uptake [direct] and nitrogen decreases the soil methane sink [indirect]). The net effect on Earth’s energy balance from changes in major biogeochemical cycles (carbon, nitrogen, sulfur, and phosphorus) depends upon processes that directly affect how the planet absorbs or reflects sunlight, as well as those that indirectly affect concentrations of greenhouse gases in the atmosphere.
Carbon
In addition to the \(CO_2\) effects described above, other carbon-containing compounds affect climate change, such as methane and volatile organic compounds (VOCs). As the most abundant non-\(CO_2\) greenhouse gas, methane is 20 to 30 times more potent than \(CO_2\) over a century timescale. It accounted for 9% of all human-caused greenhouse gas emissions in the United States in 2011,[8] and its atmospheric concentration today is more than twice that of pre-industrial times.[24,25] Methane has an atmospheric lifetime of about 10 years before it is oxidized to \(CO_2\), but it has about 25 times the global warming potential of \(CO_2\). An increase in methane concentration in the industrial era has contributed to warming in many ways.26 Methane also has direct and indirect effects on climate because of its influences on atmospheric chemistry. Increases in atmospheric methane and VOCs are expected to deplete concentrations of hydroxyl radicals, causing methane to persist in the atmosphere and exert its warming effect for longer periods.[25,27] The hydroxyl radical is the most important “cleaning agent” of the troposphere (the active weather layer extending up to about 5 to 10 miles above the ground), where it is formed by a complex series of reactions involving ozone and ultraviolet light.[3]
Nitrogen and Phosphorus
The climate effects of an altered nitrogen cycle are substantial and complex.[4,28,29,30,31] Carbon dioxide, methane, and nitrous oxide contribute most of the human-caused increase in climate forcing, and the nitrogen cycle affects atmospheric concentrations of all three gases. Nitrogen cycling processes regulate ozone (O3) concentrations in the troposphere and stratosphere, and produce atmospheric aerosols, all of which have additional direct effects on climate. Excess reactive nitrogen also has multiple indirect effects that simultaneously amplify and mitigate changes in climate. Changes in ozone and organic aerosols are short-lived, whereas changes in carbon dioxide and nitrous oxide have persistent impacts on the atmosphere.
The strongest direct effect of an altered nitrogen cycle is through emissions of nitrous oxide (\(N_{2}O\)), a long-lived and potent greenhouse gas that is increasing steadily in the atmosphere.[25,26] Globally, agriculture has accounted for most of the atmospheric rise in \(N_{2}O\).[32,33] Roughly 60% of agricultural \(N_{2}O\) derives from elevated soil emissions resulting from the use of nitrogen fertilizer. Animal waste treatment accounts for about 30%, and the remaining 10% comes from crop-residue burning.[34] The U.S. reflects this global trend: around 75% to 80% of U.S. human-caused \(N_{2}O\) emissions are due to agricultural activities, with the majority being emissions from fertilized soil. The remaining 20% is derived from a variety of industrial and energy sectors.[35,36] While \(N_{2}O\) currently accounts for about 6% of human-caused warming,26 its long lifetime in the atmosphere and rising concentrations will increase \(N_{2}O\)-based climate forcing over a 100-year time scale.[33,37,38]
Excess reactive nitrogen indirectly exacerbates changes in climate by several mechanisms. Emissions of nitrogen oxides (NOx) increase the production of tropospheric ozone, which is a greenhouse gas.[39] Elevated tropospheric ozone may reduce \(CO_2\) uptake by plants and thereby reduce the terrestrial \(CO_2\) sink.[40] Nitrogen deposition to ecosystems can also stimulate the release of nitrous oxide and methane and decrease methane uptake by soil microbes.[41]
However, excess reactive nitrogen also mitigates changes in greenhouse gas concentrations and climate through several intersecting pathways. Over short time scales, NOx and ammonia emissions lead to the formation of atmospheric aerosols, which cool the climate by scattering or absorbing incoming radiation and by affecting cloud cover.[26,42] In addition, the presence of NOx in the lower atmosphere increases the formation of sulfate and organic aerosols.43 At longer time scales, NOx can increase rates of methane oxidation, thereby reducing the lifetime of this important greenhouse gas.
One of the dominant effects of reactive nitrogen on climate stems from how it interacts with ecosystem carbon capture and storage, and thus, the carbon sink. As mentioned previously, addition of reactive nitrogen to natural ecosystems can increase carbon storage as long as other factors are not limiting plant growth, such as water and nutrient availability.[44] Nitrogen deposition from human sources is estimated to contribute to a global net carbon sink in land ecosystems of 917 to 1,830 million metric tons (1,010 to 2,020 million tons) of \(CO_2\) per year. These are model-based estimates, as comprehensive, observationally-based estimates at large spatial scales are hindered by the limited number of field experiments. This net land sink represents two components: 1) an increase in vegetation growth as nitrogen limitation is alleviated by human-caused nitrogen deposition, and 2) a contribution from the influence of increased reactive nitrogen availability on decomposition. While the former generally increases with increased reactive nitrogen, the net effect on decomposition in soils is not clear. The net effect on total ecosystem carbon storage was an average of 37 metric tons (41 tons) of carbon stored per metric ton of nitrogen added in forests in the U.S. and Europe.[45]
When all direct and indirect links between reactive nitrogen and climate in the U.S. are added up, a recent estimate suggests a modest reduction in the rate of warming in the near term (next several decades), but a progressive switch to greater net warming over a 100-year timescale.[28,29] That switch is due to a reduction in nitrogen oxide (NOx) emissions, which provide modest cooling effects, a reduction in the nitrogen-stimulated \(CO_2\) storage in forests, and a rising importance of agricultural nitrous oxide emissions. Current policies tend to reinforce this switch. For example, policies that reduce nitrogen oxide and sulfur oxide emissions have large public health benefits, but also reduce the indirect climate mitigation co-benefits by reducing carbon storage and aerosol formation.
Changes in the phosphorus cycle have no direct effects on climate, but phosphorus availability constrains plant and microbial activity in a wide variety of land- and water-based ecosystems.[46,47] Changes in phosphorus availability due to human activity can therefore have indirect impacts on climate and the emissions of greenhouse gases in a variety of ways. For example, in land-based ecosystems, phosphorus availability can limit both \(CO_2\) storage and decomposition46,48 as well as the rate of nitrogen accumulation.[49] In turn, higher nitrogen inputs can alter phosphorus cycling via changes in the production and activity of enzymes that release phosphorus from decaying organic matter, [50] creating another mechanism by which rising nitrogen inputs can stimulate carbon uptake.
Other Effects: Sulfate Aerosols
In addition to the aerosol effects from nitrogen mentioned above, there are both direct and indirect effects on climate from other aerosol sources. Components of the sulfur cycle exert a cooling effect through the formation of sulfate aerosols created from the oxidation of sulfur dioxide (\(SO_2\)) emissions.[26] In the United States, the dominant source of sulfur dioxide is coal combustion. Sulfur dioxide emissions rose until 1980, but have since decreased by more than 50% following a series of air-quality regulations and incentives focused on improving human health and the environment, as well as reductions in the delivered price of low-sulfur coal.[51] That decrease in emissions has had a marked effect on U.S. climate forcing: between 1970 and 1990, sulfate aerosols caused cooling, primarily over the eastern U.S., but since 1990, further reductions in sulfur dioxide emissions have reduced the cooling effect of sulfate aerosols by half or more.[42] Continued declines in sulfate aerosol cooling are projected for the future, [42] particularly if coal continues to be replaced by natural gas (which contains far fewer sulfur impurities) for electricity generation. Here, as with nitrogen oxide emissions, the environmental and socioeconomic tradeoffs are important to recognize: lower sulfur dioxide and nitrogen oxide emissions remove some climate cooling agents, but improve ecosystem health and save lives.[16,31,52]
Three low-concentration industrial gases are particularly potent for trapping heat: nitrogen trifluoride (\(NF_{3}\)), sulfur hexafluoride (\(SF_{6}\)), and trifluoromethyl sulfur pentafluoride (\(SF_{5}CF_{3]\)). None currently makes a major contribution to climate forcing, but since their emissions are increasing and their effects last for millennia, continued monitoring is important.). None currently makes a major contribution to climate forcing, but since their emissions are increasing and their effects last for millennia, continued monitoring is important.
Source:
Galloway, J. N., W. H. Schlesinger, C. M. Clark, N. B. Grimm, R. B. Jackson, B. E. Law, P. E. Thornton, A. R. Townsend, and R. Martin, 2014: Ch. 15: Biogeochemical Cycles. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 350-368. doi:10.7930/J0X63JT0. : http://nca2014.globalchange.gov/report/sectors/biogeochemical-cycles