finding 15.1 : human-increased-element-levels

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.



This finding is from chapter 15 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: The key messages and supporting text summarize extensive evidence documented in two technical input reports submitted to the NCA: 1) a foundational report supported by the Departments of Energy and Agriculture: Biogeochemical Cycles and Biogenic Greenhouse Gases from North American Terrestrial Ecosystems: A Technical Input Report for the National Climate Assessment,6b1b7945-4773-4923-8a45-3dc034dff5f8 and 2) an external report: The Role of Nitrogen in Climate Change and the Impacts of Nitrogen-Climate Interactions on Terrestrial and Aquatic Ecosystems, Agriculture, and Human Health in the United States: A Technical Report Submitted to the U.S. National Climate Assessment.4b55a6d6-94ab-4ca9-abaf-e17bf9d86bc4 The latter report was supported by the International Nitrogen Initiative, a National Science Foundation grant, and the David and Lucille Packard Foundation. Author meetings and workshops were held regularly for the foundational report,6b1b7945-4773-4923-8a45-3dc034dff5f8 including a workshop at the 2011 Soil Science Society of America meeting. A workshop held in July 2011 at the USGS John Wesley Powell Center for Analysis and Synthesis in Fort Collins, CO, focused on climate-nitrogen actions and was summarized in the second primary source.4b55a6d6-94ab-4ca9-abaf-e17bf9d86bc4 An additional 15 technical input reports on various topics were also received and reviewed as part of the Federal Register Notice solicitation for public input. The entire author team for this chapter conducted its deliberations by teleconference from April to June 2012, with three major meetings resulting in an outline and a set of key messages. The team came to expert consensus on all of the key messages based on their reading of the technical inputs, other published literature, and professional judgment. Several original key messages were later combined into a broader set of statements while retaining most of the original content of the chapter. Major revisions to the key messages, chapter, and traceable accounts were approved by authors; further minor revisions were consistent with the messages intended by the authors.

Description of evidence base: The author team evaluated technical input reports (17) on biogeochemical cycles, including the two primary sources.4b55a6d6-94ab-4ca9-abaf-e17bf9d86bc4 a65e5260-d143-49dd-b20e-c0fefddbef70 In particular, one report4b55a6d6-94ab-4ca9-abaf-e17bf9d86bc4 focused on changes in the nitrogen cycle and was comprehensive. Original literature was consulted for changes in other biogeochemical cycles. The foundational report6b1b7945-4773-4923-8a45-3dc034dff5f8 updated several aspects of our understanding of the carbon balance in the United States. Publications have shown that human activities have altered biogeochemical cycles. A seminal paper comparing increases in the global fluxes of carbon (C), nitrogen (N), sulfur (S), and phosphorous (P) was published in 20005fdca82e-89fd-42db-925f-b4e78642a1ec and was recently updated.727f55a4-21f7-4f97-bd1f-3aa19ddfefa1 Changes observed in the nitrogen cycleb541ff53-3814-4f33-8ca1-0cd900282ce2 c713eeb5-4e17-41e2-b65d-da453bbf9f97 7bb29bb1-926d-4367-aa10-d15808b296b3 show anthropogenic sources to be far greater than natural ones.54c2f4e5-ecf8-4a38-9230-f7782d9a1ff8 d1fa458e-26e1-489b-82b7-1e12b02a730e eaa34c7b-6f78-41cb-8dcd-b1abece4cfb0 For phosphorus, the effect of added phosphorus on plants and microbes is well understood.f05d1b2b-ceb2-491b-a6b0-33b062e7ac4f 3c440da6-7ee4-4406-aa92-1e779947552c 2caad53b-a039-47bb-a1fc-391522eb0949 eaa34c7b-6f78-41cb-8dcd-b1abece4cfb0 Extensive research shows that increases in CO2 are the strongest human impact forcing climate change, mainly because the concentration of CO2 is so much greater than that of other greenhouse gases.c54b9473-cdc3-4f22-97a8-4df5253f9682 b130f588-b962-4133-9aa9-46f1f8c8ffdb 5fdca82e-89fd-42db-925f-b4e78642a1ec

New information and remaining uncertainties: The sources of C, N, and P are from well-documented processes, such as fossil fuel burning and fertilizer production and application. The flux from some processes is well known, while others have significant remaining uncertainties. Some new work has synthesized the assessment of global and national CO2 emissionsb130f588-b962-4133-9aa9-46f1f8c8ffdb and categorized the major CO2 sources and sinks.4b55a6d6-94ab-4ca9-abaf-e17bf9d86bc4 6b1b7945-4773-4923-8a45-3dc034dff5f8 Annual updates of CO2 emissions and sink inventories are done by EPA (for example, EPA 201313e4d075-f7bd-4fd4-a41f-a1f0cd93356d). Advances in the knowledge of the nitrogen cycle have quantified that human-caused reactive nitrogen inputs are now at least five times greater than natural inputs.4b55a6d6-94ab-4ca9-abaf-e17bf9d86bc4 346e77a5-93b9-4c37-bd95-d7db44091a4c 54c2f4e5-ecf8-4a38-9230-f7782d9a1ff8

Assessment of confidence based on evidence: High confidence. Evidence for human inputs of C, N, and P come from academic, government, and industry sources. The data show substantial agreement. The likelihood of continued dominance of CO2 over other greenhouse gases as a driver of global climate change is also judged to be high, because its concentration is an order of magnitude higher and its rate of change is well known.

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