CL-Crowley-Lowery composites compared to P.D. Jones (UEA) and M. Mann (UVA) smoothed data

INTRODUCTION:

Dr. Margaret Leinen
Assistant Director, Directorate for Geosciences, National Science Foundation, Arlington, VA

SPEAKER:

Dr. Thomas J. Crowley
Department of Oceanography, Texas A & M University, College Station, TX

Recent progress in synthesizing various proxy records of past climates (e.g., as seen from ice cores, tree rings, etc.) enable placement of the 20th century climate warming within a longer-term perspective. For example, reconstructions of past climates in the Northern hemisphere indicate that the Middle Ages were relatively mild and that a significantly colder period occurred during the peak of the Little Ice Age (approximately 1580-1880). There has been considerable question however, about the relative magnitude of Medieval warmth relative to the observed global warming of the 20th Century. Despite clear evidence from individual records, of occasional episodes of regional-scale climate warming having been greater than the present globally-averaged warming, three different composite reconstructions of Northern hemisphere climate indicate that, at most, peak Medieval warmth was comparable to the mid-20th Century warm period. This result occurs because the timing of Medieval warmth is not the same in all places. In terms of magnitude and duration, the late 20th century warming is unique in at least the last 1000 years.

Further work has enabled testing of various mechanisms of climate change operating on decadal to centennial time scales. Recent results indicate that much of the variability over the last 1000 years, prior to the rise in anthropogenic emissions of greenhouse gases (beginning in about 1850), can be attributed to pulses of volcanism or changes in the output of the sun's energy. Neither of these mechanisms, nor natural climate variability in the ocean-atmosphere system, can explain the late 20th century rise in the globally-averaged surface temperature. The latter increase is however, consistent with projections of climate warming resulting from the buildup of greenhouse gases.

The increase in global temperatures resulting from greenhouse-induced warming in the next century will likely exceed considerably, any climate changes over the last 1000 years. In fact, the global warming estimated from "Business As Usual" projections (i.e., from the Intergovernmental Panel on Climate Change reports) will exceed globally-averaged surface temperatures from the last interglacial period -- the warmest time in the last 400,000 years.

Dr. Thomas Crowley is a Professor of Oceanography at Texas A&M University. Prior to this, he held a number of positions in government, universities, and the private sector. In particular, he was an assistant professor at the University of Missouri-St. Louis, program director in Climate Dynamics at the National Science Foundation, National Research Council Fellow at NASA Goddard Space Flight Center, and Senior Scientist with the Applied Research Corporation.

Dr. Crowley's principal interest and research expertise lies in the study of past climates (paleoclimatology), using both observations and climate models. More specifically, his special interest lies in framing recent and future projected climate change within the perspective of past climate variations. He has also co-authored approximately 100 peer-reviewed, scientific papers and a widely read textbook on paleoclimatology.

Dr. Thomas Crowley received his Ph.D. in Geology from Brown University, Rhode Island, in 1976.