The Northern Hemisphere's three warmest years in the past 600 came this decade - and rising levels of greenhouse gases are probably to blame, according to research published this week.
Scientists reconstructed average annual temperatures back to the year 1400 and found that 1990, 1995 and 1997 were the warmest.Either 1995 or 1997 could be considered the record-setter, depending on whether one considers temperatures over land, at the ocean surface or both, researcher Michael Mann said.
When land and ocean temperatures are combined, 1995 and 1997 ran about nine-tenths of a degree Fahrenheit above the average for the 20th century.
He and colleagues present the work in Wednesday's issue of the journal Nature. Mann is an adjunct professor of geosciences at the University of Massachusetts and has a postdoctoral fellowship financed by the federal Department of Energy.
Climate experts called the study an early step and said more work is needed to improve such long-term reconstructions, but they said the finding about the warmest years fits in with previous studies.
Last January, the federal government announced that 1997 was the warmest year globally in about 100 years, which was as far back as they 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 proxy indicators to actual temperature measurements from 1902 to 1980 to find how to use the ancient data to estimate average annual temperatures.
They also compared estimated temperatures since 1610 to trends in three influences in climate: variations in sun 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 federal government's National Climatic Data Center, who didn't participate in the study, called it "another indication that it seems quite likely that the increase in greenhouse gases is contributing to the very warm conditions we see now."