6.7.2: Human-Induced Changes
Human activities have increased atmospheric carbon dioxide by about 40% over pre-industrial levels and more than doubled the amount of nitrogen available to ecosystems. Similar trends have been observed for phosphorus and other elements, and these changes have major consequences for biogeochemical cycles and climate change.
The human mobilization of carbon, nitrogen, and phosphorus from the Earth’s crust and atmosphere into the environment has increased 36, 9, and 13 times, respectively, compared to geological sources over pre-industrial times.[3] Fossil fuel burning, land-cover change, cement production, and the extraction and production of fertilizer to support agriculture are major causes of these increases.[4] Carbon dioxide (\(CO_2\)) is the most abundant of the heat-trapping greenhouse gases that are increasing due to human activities, and its production dominates atmospheric forcing of global climate change.5 However, methane (\(CH_4\)) and nitrous oxide (\(N_{2}O\)) have higher greenhouse-warming potential per molecule than \(CO_2\), and both are also increasing in the atmosphere. In the U.S. and Europe, sulfur emissions have declined over the past three decades, especially since the mid-1990s, because of efforts to reduce air pollution.[6] Changes in biogeochemical cycles of carbon, nitrogen, phosphorus, and other elements – and the coupling of those cycles – can influence climate. In turn, this can change atmospheric composition in other ways that affect how the planet absorbs and reflects sunlight (for example, by creating small particles known as aerosols that can reflect sunlight).
State of the Carbon Cycle
The U.S. was the world’s largest producer of human-caused \(CO_2\) emissions from 1950 until 2007, when it was surpassed by China. U.S. emissions account for approximately 85% of North American emissions of \(CO_2\) [7] and 18% of global emissions. [8,9] Ecosystems represent potential “sinks” for \(CO_2\), which are places where carbon can be stored over the short or long term (see “Estimating the U.S. Carbon Sink”). At the continental scale, there has been a large and relatively consistent increase in forest carbon stocks over the last two decades,[10] due to recovery from past forest harvest, net increases in forest area, improved forest management regimes, and faster growth driven by climate or fertilization by \(CO_2\) and nitrogen.[7,11] The largest rates of disturbance and “regrowth sinks” are in southeastern, south central, and Pacific northwestern regions.[11] However, emissions of \(CO_2\) from human activities in the U.S. continue to increase and exceed ecosystem \(CO_2\) uptake by more than three times. As a result, North America remains a net source of \(CO_2\) into the atmosphere[7] by a substantial margin.
Sources and Fates of Reactive Nitrogen
The nitrogen cycle has been dramatically altered by human activity, especially by the use of nitrogen fertilizers, which have increased agricultural production over the past half century.[1,2] Although fertilizer nitrogen inputs have begun to level off in the U.S. since 1980,[12] human-caused reactive nitrogen inputs are now at least five times greater than those from natural sources.[13,14,15,16] At least some of the added nitrogen is converted to nitrous oxide (\(N_{2}O\)), which adds to the greenhouse effect in Earth’s atmosphere.
An important characteristic of reactive nitrogen is its legacy. Once created, it can, in sequence, travel throughout the environment (for example, from land to rivers to coasts, sometimes via the atmosphere), contributing to environmental problems such as the formation of coastal low-oxygen “dead zones” in marine ecosystems in summer. These problems persist until the reactive nitrogen is either captured and stored in a long-term pool, like the mineral layers of soil or deep ocean sediments, or converted back to nitrogen gas.[17,18] The nitrogen cycle affects atmospheric concentrations of the three most important human-caused greenhouse gases: carbon dioxide, methane, and nitrous oxide. Increased available nitrogen stimulates the uptake of carbon dioxide by plants, the release of methane from wetland soils, and the production of nitrous oxide by soil microbes.
Phosphorus and other elements The phosphorus cycle has been greatly transformed in the United States, [19] primarily from the use of phosphorus fertilizers in agriculture. Phosphorus has no direct effects on climate, but does have indirect effects, such as increasing carbon sinks by fertilizing plants. Emissions of sulfur, as sulfur dioxide, can reduce the growth of plants and stimulate the leaching of soil nutrients needed by plants.[20]
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