The Arctic had been cooling for almost 2,000 years before a recent, sudden warming in the last 50 years, a new study on climate change suggests.
The five-year study by an international team of scientists from the U.S., Canada, Iceland, Norway and Finland provides more evidence that human activity and greenhouse gases are affecting the Earth's climate.
The researchers, led by Darrell Kaufman, a geologist and environmental scientist at Northern Arizona University, reconstructed 2,000 years of summer temperature data from Arctic lake sediments, tree rings and glacier ice.
Prior to this study, the record of temperature data with this level of detail went back only 400 years.
"Our reconstruction shows that the last half-century was the warmest of the last 2,000 years," said Kaufman in a release.
The warming in the last 50 years was preceded by a steady cooling trend that should have continued through the 20th century, the researchers said.
The cooling trend lasting at least 1,900 years was caused by a wobble in the Earth's rotational axis that slowly increased the distance between the Earth and the Sun during the summer in the Arctic, reducing the amount of summer sunlight there, the scientists said.
The wobble, or precession, in the Earth's rotation continued into the 20th century, so the cooling should have continued, too, but by the 1950s summer temperatures in the Arctic were about 0.7 degrees higher than expected.
As well, the 1990s were the warmest decade of the last two millennia, the researchers said, with summer temperatures in the Arctic averaging about 1.4 degrees higher than would have been expected if the cooling trend had continued.
The study also agreed with previous studies that found that warming in the Arctic is occurring at a higher rate than in the rest of the world because of changes in albedo, or how much light the ground reflects back into space.
"The Arctic amplifies climate change as reflective snow and ice are replaced by dark, heat-absorbing water and vegetated surfaces," said Kaufmann. As well, melting glacial ice contributes to sea-level rise and thawing permafrost releases methane gas, an important contributor to the greenhouse effect.
"Arctic warming will continue to exceed temperature increases in the rest of the Northern Hemisphere, resulting in accelerated loss of land ice and an increased rate of sea-level rise, with global consequences," said research team member Gifford Miller of the University of Colorado, Boulder.
The study, published this week in Science, was based on previously published data by Kaufman's team from studies of sediment cores taken from 14 lakes. It also included a computer simulation of climate change over the last 2,000 years, which agrees with the climate model proposed by the National Center for Atmospheric Research used to predict future climate change.