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Seminar: Natural and Anthropogenic Influences on Earth’s Surface Temperature

Date(s): January 28, 2009, 10:00-11:30am

Presenter(s): Judith Lean (U.S. Naval Research Laboratory)

Description: There are many sources of climate variability, including anthropogenic gases, the El Nino Southern Oscillation (ENSO), volcanic aerosols and solar activity. Deciphering their concurrent impacts on Earth’s surface temperature is difficult because the solar activity cycle and volcanic cooling project onto each other, and both project onto ENSO. Longer-term solar changes may also project onto anthropogenic influences in the past century. A multivariate analysis is one approach for quantifying the natural and anthropogenic components of the surface temperature record simultaneously. Such an analysis, using the best available estimates of each together with the observed temperature, enables comparisons of the geographical distributions of surface temperature responses to the individual influences consistent with their global impacts from 1889 to 2006. The combined natural and anthropogenic influences (at appropriate lags) capture 76% of the variance in the monthly global surface temperature record, suggesting that much of the variability arises from processes that can be identified and their impact on the global surface temperature quantified by direct linear association with the observations. The response to solar forcing is quite different from that reported in several papers published recently: solar forcing produces a detectable 11-year cycle of amplitude 0.1K but contributed negligible long-term warming in the past 25 years and 10% of the warming in the past 100 years, not 69% as claimed elsewhere. Zonally averaged responses to both natural and anthropogenic forcings do not increase rapidly from mid to high latitudes, and therefore differ distinctly from those indicated by the IPCC, whose conclusions depended on model simulations. A recent paper co-authored by Dr. Lean on this subject can be found at Judith L. Lean and David H. Rind, "How Natural and Anthropogenic Influences Alter Global and Regional Surface Temperatures: 1889 to 2006, Geophysical Research Letters, Vol. 35, L18701, September 16, 2008. Dr. Lean is a Senior Scientist for Sun-Earth System Research in the Space Science Division of NRL. She is a member of the National Academy of Sciences and the author or co-author of over 100 papers in professional journals.

Presentation Document: Natural and Anthropogenic Influences on Earth's Surface Temperature

Seminar Category: Climate Science