A groundbreaking study that traces Arctic temperatures further back than ever before has shown the region is now warmer than at any time in the past 2,000 years.
The international study, published Thursday in the journal Science, also provides real-world evidence to back mathematical climate models that suggest greenhouse gases are behind global warming.
"There are no other forcing factors at work other than the greenhouse gas composition of the atmosphere that could explain the dramatic warming that took place," said Darrell Kaufman, the paper's lead author, from Northern Arizona University.
Prof. Kaufman and nearly three dozen scientists used data from tree rings, lake sediments and glacial ice deposits from 23 sites around the circumpolar world to track average summer temperatures for every decade of the past two millennia - 1,600 years longer than had ever been done before.
All three methods are well-accepted ways of estimating weather. The warmer the summer, the thicker a tree ring or layer of organic sediment.
The measurements showed that from Year 1 to about 1900, summer Arctic temperatures were gradually decreasing by about 0.2 C every 1,000 years. That corresponds with other studies that have calculated the result of tiny decreases in the amount of solar energy the North gets as a result of wobbles in the Earth's orbit.
But things began to change at the beginning of the 20th century. Data from all three sources showed a dramatic upward jog that, in a graph, looks like the blade of a hockey stick.
The research suggests that four of the Arctic's five warmest decades occurred after 1950. And the warmest decade was 1999-2008, which saw average temperatures about 1.4 C higher than they would have been if the cooling trend had continued.
"The warming that we detected, particularly during the second half of the 20th century, is especially dramatic considered against the background of the previous nineteen," Prof. Kaufman said.
The study says that the fact no other major variables changed about that time - there were no large volcanic eruptions, for example - suggests there could only be one culprit for the warming Arctic: carbon dioxide emissions that began increasing rapidly during the Industrial Revolution.
"When we consider the 20th century, there are no other forcing factors at work," Prof. Kaufman said.
Just as important was the comparison between Prof. Kaufman's data-based temperatures and those generated by computer climate models. The latter are often used by scientists to predict the effects of global warming and are just as often criticized by skeptics as mere speculation. The correlation between the two suggests the models work, said Prof. Kaufman.
"In my mind, that shows that we have a fundamentally correct understanding of the climate system."
Prof. Kaufman acknowledges that the Earth's climate still holds many mysteries. But he believes the match between his results and those from computer models adds more weight to the scientific consensus that greenhouse gases are the main cause of global warming.
He hopes that's a conclusion leaders meeting in Copenhagen next December to discuss solutions to climate change will take to heart.
"Our study is among many others that have documented the dramatic change in climate that has taken place in the last 50 years," Prof. Kaufman said.
"The evidence that humans play a role in climate change is now overwhelming. It's time for policy makers to act decisively on regulating carbon emissions."