PATTERNS OF GLOBAL CLIMATE CHANGE OVER RECENT CENTURIES REPORTED BY NOAA AND UNIVERSITY SCIENTISTS
November 21, 2000 For the first time, year-by-year patterns of global temperature over the past few centuries have now been revealed. Evidence from ice cores, tree rings, corals, historical records and sediments in lakes was used by a team of university and NOAA scientists to obtain the yearly maps, extending the history of global climate to a time before people began taking measurements with weather instruments.
Writing in the online journal Earth Interactions, the scientists note that their work highlights periods of unusual climatic conditions, such as the period of "dry fogs" that were reported by Benjamin Franklin in the 1780s. The new study shows that the hazy conditions Franklin described from his home (at that time, in Paris) were related to a cold episode that affected all of Europe for several years following the eruption of a volcano in Iceland (Laki) in 1784. Other major eruptions have had similar climatic effects. After the eruption of Tambora in Indonesia in 1815, temperatures in North America and Europe fell sharply, and cool conditions prevailed for several years.
The current study provides an interactive database that can be used to examine maps of global temperature patterns for any year since 1730. These are based on natural archives such as ice cores, tree-rings, lake sediments, and corals, which record seasonal or annual climate conditions. Data from the natural archives were calibrated by the instrumental surface temperature data available during the 20th century.
The study also provided a longer-term view of temperatures across the northern hemisphere. "The best evidence, based on the extension of hemispheric climate reconstructions back a full millennium is that late 20th century conditions are probably warmer than those which prevailed any time this millennium," the scientists wrote. They noted that conditions during the 11th through 14th centuries appear warmer than those which prevailed during the 15th through 19th centuries in general.
The scientists also studied the statistical relationship between variations in the northern hemisphere mean temperatures and estimates of the histories of solar, greenhouse gas, and volcanic factors. "While the naturalsolar and volcanicforcings appear to be important factors governing the natural variations of temperatures in past centuries, only human greenhouse gas forcing alone....can statistically explain the unusual warmth of the past few decades," they wrote.
The scientists are: lead author Michael E. Mann, from the University of Virginia; Ed Gille and Wendy Gross, NOAA's National Geophysical Data Center; Raymond S. Bradley and Frank Keimig, University of Massachusetts; Jonathan Overpeck, formerly of NOAA, now with the University of Arizona, and Malcolm K. Hughes, University of Arizona.
The publication, titled Global Temperature Patterns in Past Centuries, contains the latest look at temperatures over the last 600 years, and includes data and animations of global temperatures from paleoclimatic records starting in 1730, up through instrumental data through 1993.
Relevant Web Sites