Our Changing Climate: The Underlying Role of Biogeochemistry

Elisabeth A. Holland
Senior Scientist and Program Head
Biogeosciences Program
National Center for Atmospheric Research, Boulder, CO


Climate Change 2007: The Physical Science Basis, Working Group 1 (WG1) the Fourth Assessment Report of Intergovernmental Panel on Climate Change was released earlier this year and has generated world-wide attention. I will review key conclusions of this report, focusing on Chapter 7, Couplings Between Changes in the Climate System and Biogeochemistry. This was the first IPCC report to take an explicit look at the global nitrogen cycle and addresses the relevance of atmospheric deposition measurements to the climate system. I will address the interactions between the carbon and nitrogen cycles, including atmospheric N deposition, including some exciting recent results produced with the NCAR Community Climate System Model run with interactive Biogeochemistry.

The WG1 Summary for Policy Makers (SPM) includes the following excerpts: “The understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR), leading to very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 [+0.6 to +2.4] W m-2 Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land-use change, while those of methane and nitrous oxide are primarily due to agriculture. Carbon dioxide is the most important anthropogenic greenhouse gas. Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. Eleven of the last twelve years (1995 -2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). Paleoclimate information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 meters of sea level rise. Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.”