AMHERST, Mass. - Researchers at the University of Massachusetts who study global warming have released a report strongly suggesting that the 1990s were the warmest decade of the millennium, with 1998 the warmest year so far. Researchers have also found that the warming in the 20th century counters a 1,000-year-long cooling trend. The study, by Michael Mann and Raymond Bradley of UMass, along with Malcolm Hughes of the University of Arizona, appears in the March 15 issue of Geophysical Research Letters, published by the American Geophysical Union. The research was supported by the U.S. Department of Energy and the National Science Foundation.
"Temperatures in the latter half of the 20th century were unprecedented," said Bradley.
The UMass/University of Arizona study involved a close examination of natural archives, such as tree rings and ice cores, which record climate variations each year. These natural archives are called "proxy indicators" by scientists, and allow researchers to consider the short instrumental record of climate in a longer-term perspective. Using proxy information gathered by scientists around the world during the past few decades, the team used sophisticated computer analysis and statistics to reconstruct yearly temperatures and their statistical uncertainties, going back to the year AD 1000.
Specifically, the team relied on three sets of 1,000-year-long tree-ring records from North America, plus tree rings from northern Scandinavia, northern Russia, Tasmania, Argentina, Morocco, and France. Additionally, the team studied ice cores from Greenland and the Andes mountains.
"As you go back farther in time, the data becomes sketchier. One can’t quite pin things down as well," noted Mann, "but, our results do reveal that significant changes have occurred, and temperatures in the latter 20th century have been exceptionally warm compared to the preceding 900 years. Though substantial uncertainties exist in the estimates, these are nonetheless startling revelations."
Research published by the same team last year reconstructed yearly global surface temperature patterns going back 600 years. That study and the current report both determined that human-induced greenhouse gases were a major factor in 20th century global warming.
The year 1998 was also found to be the warmest year on record in separate reports released by National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA). The reports were issued in January 1999, reviewing climatic conditions for the previous year, but those studies examined records only going back 120 years. The UMass/University of Arizona report puts the conclusions from NASA and NOAA in a much longer perspective.
According to the UMass researchers, the 1,000-year reconstruction reveals that temperatures dropped an average of 0.02 degrees Celsius per century prior to the 20th century. This trend is consistent with the "astronomical theory" of climate change, which considers the effects of long-term changes in the nature of the Earth’s orbit relative to the sun, which influence the distribution of solar energy at the Earth’s surface over many millennia.
"If temperatures change slowly, society and the environment have time to adjust," said Mann. "The slow, moderate, long-term cooling trend that we found makes the abrupt warming of the late 20th century even more dramatic. The cooling trend of over 900 years was dramatically reversed in less than a century. The abruptness of the recent warming is key, and it is a potential cause for concern."
The latest reconstruction supports earlier theories that temperatures in medieval times were relatively warm, but "even the warmer intervals in the reconstruction pale in comparison with mid-to-late 20th-century temperatures," said Hughes.