By David L. Chandler, Globe Staff, 04/23/98
Boston Globe : April 23, 1998
Last year was the warmest the world has experienced, going back at least to the century before Christopher Columbus sailed, scientists reported yesterday. The new findings provide the clearest and most dramatic evidence that the world is experiencing global warming caused by human activity.
By using a combination of written records and information gleaned from tree rings, ice cores and coral reefs, researchers at the University of Massachusetts have been able to build a record of the world's climate that extends back 600 years. The record revealed that the warmest years in that span were 1997, 1995, and 1990.
Previous records had shown that these recent years were the warmest since consistent records of temperature have been kept. But those records go back only about 100 years - a span many scientists have considered too short to be a reliable indicator of long-term trends.
Michael Mann and Raymond Bradley, climate scientists at U Mass-Amherst, and Malcolm Hughes of the University of Arizona presented their new results in a paper published today in the journal Nature. Mann said yesterday their study, which used data from more than 100 sites around the world, provides strong evidence that the extraordinary warming of recent years is a direct result of the burning of fossil fuels and other human activities that add carbon dioxide to the atmosphere.
''There were warm times, there were cold times,'' Mann said, ''but nothing in the past like this dramatic increase beginning at the dawn of the 20th century.''
Other researchers praised the new work, but cautioned that more analysis must be done to assess the reliability of the different kinds of data used in this study. Phil Jones, a climate researcher at the School of Environmental Sciences in Norwich, England, said in an analysis to be published this week in Science that by looking at the relationships between different kinds of records, such as tree rings and ice cores, specialists in different fields ''can learn from one another and potentially make the whole greater than the sum of the parts.''
But Jones, while praising the basic method, cautioned that such studies ''are just a start.''
Mann said that he and his colleagues examined closely the correlation between temperature changes and other factors that some scientists have suggested might help explain those changes: volcanic activity and variations in the sun's brightness. Those factors showed a strong relationship with temperature deviations in earlier centuries, he said, ''but during the 20th century, with its abrupt warming, there is little relationship between any of the natural factors we looked at'' and the rising temperatures.
In the past century, he said, ''all of a sudden, we see a remarkable correlation with carbon dioxide emissions, which swamps these natural factors.''
These data, he said, are especially significant because they provide ''an independent line of evidence for a human influence in the 20th century.'' Previous evidence for global warming, he said, has come from comparisons of records from the past century with computer models of the effects of greenhouse gases.
Carbon dioxide and other gases in the air act like the glass in a greenhouse, allowing the sun's heat to enter but then blocking it from radiating back out into space, and thus raising Earth's temperature.
Mann and his coauthors ''use a quite original and promising method,'' wrote Gabriele Hegerl, a climate scientist at the University of Washington, in a commentary in Nature. She added that ''given the novelty of this approach, it is not surprising that the uncertainties need more investigation. As the authors acknowledge, climate reconstructions can only be as good as the underlying data.''
Nevertheless, she wrote, ''their results support, independently of climate models, the conclusion that anthropogenic influences have dominated the evolution of temperature in the 20th century.''
Richard Lindzen of the Massachusetts Institute of Technology, an outspoken skeptic of the global warming phenomenon, said yesterday he had not yet seen the Nature paper, but suggested that the inherent uncertainties in such temperature determination methods might make it impossible to discern the true trends.
Even with more recent temperature data derived from actual measurements, he said, it is difficult to obtain accurate global averages because of large gaps in coverage, especially over the oceans. Also, by using ''surrogate data that is itself not that accurate, you probably can get whatever you want,'' he said, questioning the relevance of the team's indirect methods.
But Mann said that known climatic anomalies during the last 600 years, and the detailed records of the past century, provide useful tests of the accuracy of the global analysis. By comparing the results obtained from the indirect measures with actual temperature records during the last century, they were able to tell just how accurate the other indicators were in revealing the same trends. And their data clearly show known events such as the ''year without a summer'' in 1816, caused by the eruption of the volcano Tambora in Indonesia, and a strong El Nino year in 1791, confirming that their indirect means do reveal even single-year climatic variations.
As the analysis goes further back in time, Mann said, the records get sparser and the uncertainties are greater. But even at the earliest period encompassed by their study, back to AD 1400, he said, ''there is still only uncertainty of just a fraction of a degree'' in their estimates of global temperature.
That uncertainty in the data, he said, is ''small compared to the trends we see,'' and thus should not affect the conclusions he and his colleagues draw. And Mann said that by continuing this work, it should be possible to extend an accurate reconstruction of global climate back to at least 1,000 years ago.
By correlating their reconstructions of past temperatures to the results of various computer climate models, he said, ''it does tie into our ability to predict the future.''
Hegerl, in her Nature commentary, said the new technique ''brings us closer to glimpsing what the future may hold for a world with increasing levels of greenhouse gases.''
This story ran on page A01 of the Boston Globe on 04/23/98.
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