NEW YORK -- The years 1997, 1995 and 1990 were the warmest in the Northern Hemisphere since the days of Christopher Columbus, a study says.
And the researchers say they found evidence that rising levels of greenhouse gases are probably responsible.
Scientists reconstructed annual average temperatures back to the year 1400 and found no year warmer than those three. Either 1997 or 1995 could be considered the warmest, depending on whether one considers temperatures over land or at the ocean surface or both, researcher Michael Mann said.
When land and ocean temperatures are combined, 1997 and 1995 ran about nine-tenths of a degree Fahrenheit above the average for the 20th century, he said.
He and colleagues present the work in Thursday's issue of the journal Nature. Mann is an adjunct professor of geosciences at the University of Masssachusetts at Amherst, and has a postdoctoral fellowship financed by the Department of Energy.
Climate experts said that the study is an early step in such long-term reconstructions and that more work is needed to improve them, but that the finding about the warmest years fits in with previous studies.
Last January, the government announced that 1997 was the warmest year globally in about 100 years, which was as far back as the researchers checked.
To reconstruct temperatures over the past 600 years, Mann and colleagues used a network of indirect indicators, including ancient tree rings, coral and ice as well as historical records. They compared these indicators to actual temperature measurements from 1902 to 1980 to find how to use the ancient data to estimate average annual temperatures.
They also looked at trends in three influences on climate: variations in the sun's brightness, volcanic activity and the atmosphere's supply of carbon dioxide, a heat-trapping greenhouse gas. They looked for statistical relationships that would suggest which was influencing the hemisphere's climate.
While the sun's brightness and volcanoes appeared important in the past, greenhouse gases appear to dominate over the past few decades, Mann said.
Tom Karl of the government's National Climatic Data Center said the study is "another indication that it seems quite likely that the increase in greenhouse gases is contributing to the very warm conditions we see now."