Climate Change in the Midwest: Impacts, Risks, Vulnerability, and Adaptation

By S. C. Pryor | Go to book overview

7.
Vulnerability of Soil Carbon Reservoirs
in the Midwest to Climate Change

Z. PAN, D. ANDRADE, AND N. GOSSELIN


introduction

Rising atmospheric CO2 concentrations are the main cause of recent global warming (IPCC 2007). The atmospheric CO2 increase depends on its carbon exchange with oceans and the land that absorbs about half of anthropogenic emission into the atmosphere (Broecker et al. 1979). Soil carbon is the largest terrestrial pool, and its trends directly affect atmospheric CO2 level. Each year the earth’s terrestrial land uptakes about 60 Gt carbon through photo synthesis and at the same time it loses a similar amount of carbon by respira tion (Schlesinger and Andrews 2000). Both photosynthesis and respiration are highly sensitive to temperature, precipitation, and other climate variables. The net balance between these two large opposite terms is strongly affected by climate change and is difficult to quantify accurately. Although estimates based on different techniques differ, a range of analyses indicate that North American ecosystems are significant carbon sinks and play a disproportionate role in the global carbon budget. Using an inverse modeling technique, Fan et al. (1998) esti mated that the continental U.S. net carbon (C) sink in the early 1900s was 0.81 gigatonne-C per year (Gt-C yr−1) (1.7 Gt-C yr−1 for all North America), whereas Schimel et al. (2000) and Potter & Klooster (1999) used biogeochemical models and obtained a value of about 0.2 Gt-C yr−1 over the United States. Forest inventory data have indicated that the North American forest ecosystem sequestration rate is 0.08–0.28 Gt-C yr−1. Pacala et al. (2001) reconciled these somewhat divergent results by suggesting that the conterminous U.S. carbon sink is 0.30–0.58 Gt-C yr−1 (Bachelet et al. 2004). To provide a context for these fluxes it is worth recalling that current anthropogenic CO2 emissions are ~ 9 Gt-C yr−1, about onethird of which is absorbed by terrestrial ecosystems (IPCC 2007).

The United States has about 130–150 million hectares of croplands, 35 percent of which lie within the Midwest (Lal et al. 1999; see also chapter 2 of this volume). The U.S. soils have lost about 5 Gt-C as a

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