AMHERST, Mass. - Climatologists at the University of Massachusetts have reconstructed the global temperature over the past 600 years, determining that three recent years, 1997, 1995, and 1990, were the warmest years since at least AD 1400. The study, which was conducted by Michael Mann and Raymond Bradley of the geosciences department, along with University of Arizona colleague Malcolm Hughes, is detailed in the April 23 issue of the journal Nature.
The researchers were able to estimate temperatures over more than half the surface of the globe, pinpointing average yearly temperatures in the northern hemisphere to within a fraction of a degree, going back to AD 1400. The study places in a new context long-standing controversy over the relative roles of human and natural changes in the climate of past centuries, according to Mann. Scientists were particularly interested in natural "forcings," that is, factors that can affect the climate significantly, but which are not part of the climate system itself. Based on statistical comparisons of reconstructed northern hemisphere temperatures, the best estimates indicate that natural changes in the brightness of the sun and volcanic emissions both played an important role in governing climate variations over the period studied.
However, over the past few decades, greenhouse gases produced by human activities appear to have had an increasing influence on temperatures. "The anomalous warmth of several recent years appears likely to be related to human influences on climate," said Mann.
The study bears out concerns voiced by scientists in recent years regarding global warming, Bradley said. It is known that industrialization during the past century has increased levels of carbon dioxide in the Earth’s atmosphere by more than 25 percent over its pre-industrial level. Several so-called greenhouse gases have the potential to heat the atmosphere, Mann said, "but the one we’re most concerned about is carbon dioxide, because carbon dioxide is the primary greenhouse gas resulting from industrialization." These greenhouse gases form a sort of blanket around the Earth, trapping in heat that would otherwise be radiated back to space, Mann explained. This causes the Earth’s atmosphere to heat up.
If the amount of carbon dioxide in the atmosphere were to continue to increase at its current rate, it could rise to double its pre-industrial level during the next century, leading to a magnification of the already observed warming, according to Mann. For example, melting ice caps could raise sea levels, threatening coastal regions with more frequent flooding. The planet as a whole might expect to see frequent extreme weather events, Mann said. "Heat waves and droughts could become more common, and more intense," he said.
Climatologists are also concerned about the degrees of uncertainty surrounding increased or accelerated global warming, Mann said. "We have a sense of what might happen to the planet as a whole, but the fact is, we don’t really know what the regional impacts might be."
Weather instruments were introduced only in the mid-1800s, so to go back 600 years, scientists reconstructed the climate records by relying on the small number of very long historical records, along with annually recorded "proxies" – natural archives that actually chronicle climate variations, said Bradley. Among these archives are: the density and width of tree rings, samples of centuries-old layered ice, and corals, which incorporate chemicals into their skeletons depending on water temperature and salinity, both of which are affected by climate.
The researchers relied on proxies from more than 100 sites across the globe, ranging from Arctic regions, to South American mountaintops, to locations throughout North America and Europe. In certain cases they used historical temperature estimates, which had previously been translated into an estimate of seasonal climate conditions. These were based on diary accounts of events such as crop yields, dates of first frosts, wine harvests, and famines. Several documents were lengthy weather station records which relied on conventional, though cruder, versions of meteorological instruments, such as early thermometers. These were used to supplement the network of proxy climate records. Advanced statistical techniques were used to translate the proxy information into surface temperature patterns, so that past centuries could be compared with the 20th century.
Researchers found certain individual years particularly intriguing. For example, historical documents from 1791 suggested conditions consistent with a strong El Nino event that year; the proxy-reconstructed temperature pattern bore out these suspicions. The weather was much cooler than usual over most of the globe in 1816 following the eruption of the Indonesian volcano, Tambora, the year before. Warming observed in certain regions, however, was consistent with changes in atmospheric circulation also expected to result from a strong volcanic eruption.