"A Paleoperspective on Global Warming - the Polar View Could be the Most Important View"

Jonathan Overpeck, University of Arizona
Institute for the Study of Planet Earth And Department of Geosciences

All recently published multi-century paleoclimatic temperature reconstructions indicate that 20th century hemispheric and global warmth is unprecedented in the last 400, and perhaps 1000 years. These same studies, as well as those based on the analyses of instrumental ocean and atmospheric data, also indicate that a significant portion of 20th century warming was likely due to human-induced increases in atmospheric greenhouse gas concentrations. However, there is still significant debate regarding the exact magnitude of the human- versus naturally-forced temperature change, and thus the sensitivity of the climate system to a set amount (e.g., doubling of atmospheric CO2) of anthropogenic forcing. Using a newly published compilation of hypothesized climate forcing data (i.e., solar, volcanic, trace gas and aerosol) and a NASA GISS global climate model, we have produced the first 3-D climate model simulations of climate change over the last 500 years. Our results contrast with a similar set of simulations made with a simpler model (Crowley, 2000), and suggest that the sensitivity of the climate system may be closer to 4°C for a CO2 doubling, rather than 2°C - the implications are large. Our results also indicate that some proxy sources of paleoclimatic variability may underestimate the full range of interdecadal to century-scale variability, and that climatic cooling associated with volcanic eruptions larger than Pinatubo (1991) may be self-limiting due to the aerosol dynamics. Although ocean circulation could be a large "wild card" in anticipating future climate change, the biggest threat to humans and natural systems alike, could be non-linear and difficult to reverse warming and melting in polar regions.

See the news piece on Dr. Overpeck's talk at the conference