American Geological Institute American Geological InstituteDr. Mike Mann, Alexander Hollaender Distinguished Postdoctoral Fellow of the Department of Energy and Adjunct Associate Professor of Geosciences at the University of Massachusetts, began the discussion by addressing factors which influence the interpretation of climate data. According to Dr. Mann, the instrumental temperature record contains available temperature measurements and can be used to plot global and hemispheric trends over the past century. To determine temperatures of past centuries, geologists utilize proxy climate data from ice cores, tree rings, ocean sediments, and lake sediments. By incorporating modern statistical techniques, scientists can then correlate these proxy-based data to present temperature measurements in order to gain knowledge about long term climate variations. The reconstruction of climate data was examined by a battery of statistical "verification" experiments in order to give geologists a broader perspective to determine the existence of global warming. Data suggest that the decade of the 1990's can be considered the warmest for the Northern Hemisphere as a whole since the 1400's. In addition, El Nino events have intensified compared to past centuries.
Such changes in temperature over an extended time period can be attributed to factors like variations in solar brightness, volcanic eruptions, and carbon dioxide emissions. Tests show that the dominant influence of past temperature variations is the change in the sun's brightness. Yet, Mann argued, the intense warming during the 1990's cannot be explained by natural sources alone. In fact, he maintained that the warming corresponds with the increase of greenhouse gases over the past couple of decades and may explain why the global climate has warmed approximately 1 degrees Fahrenheit. Based on the past couple of decades, Dr. Mann concluded that the situation of greenhouse gases must be taken seriously.
Dr. Mark Meier, Professor Emeritus of the Department of Geological Sciences and Fellow of the Institute of Arctic and Alpine Research at the University of Colorado, addressed the issue of glacier research because glaciers record seasonal differences in precipitation and temperature in unobserved parts of the world. These differences supplemented the proxy-based climate studies since glaciers play a part in local and regional hydrology and global sea-level changes. To determine information on glacier change, Dr. Meier examined the mass balance (snow accumulation minus melting and runoff) of glaciers. Studies have shown that the rate of snow accumulation has increased at higher altitudes while the rate of snow melting has also increased at lower altitudes. Such data implied a rise in summer air temperatures. Overall, present glaciers seemed to be equal or smaller in size than during the 16th through 19th centuries. Specifically, glaciers have lost 0.18 m of thickness per year and have supplied about 20% of the documented increase in sea level. Dr. Meier remarked that "the present long-term average rate of global sea-level rise is 1.8 mm/year." Besides signaling the possibility of climate change, glacier lost influences river flow since glacier wastage contributes to river flow. Therefore, glacier recession will affect the environment and the local ecology of various regions as well as mark the trend of current warming that is unprecedented for many centuries.
In the question/answer session held after the presentations, most of the questions focused on the research methods of the presenters and the accuracy of the data in the studies. However, when asked to predicate possible future scenarios if current trends continued, Dr. Mann suggested a 1.5-4.5 degree Celsius increase in temperature with an additional intensification of storms and El Nino events.
Sources: U.S. Global Change Research Program Seminar Series
Contributed by Shannon Clark, AGI Government Affairs Intern.
Posted August 10, 1998
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