According to Harvard researchers, recent summer temperatures in certain northern regions of the world have been the hottest in more than 600 years, significantly higher than the mean temperatures predicted for a stationary climate.

The new results combined with previous studies linking carbon dioxide emissions to global warming provide further evidence to support the presence of anthropogenic, or man-made, climate change, according to Martin P. Tingley, a research associate in the department of Earth and Planetary Sciences and co-author of the study,

“I would hope that there’s already an urgency to act,” Tingley said. “I think we’ve done a very good job at formalizing some of these notions that recent years are unprecedented in a longer-term context.”

The paper, published last week in the journal Nature, showed not only that warm temperatures are increasing, but also that recent levels are unprecedented in history. By analyzing tree-rings, ice cores, and lake sediment records, Tingley and Earth and Planetary Sciences professor Peter J. Huybers, the other author of the study, reconstructed hypothetical climate histories for particular regions of the world and compared them to recent temperature extremes.

“Bayesian inference allows us to come up with many thousand possible realizations of the temperature history,” Tingley said. “And once you have four or five thousand equally likely temperature fields, you can say, ‘In how many of these realizations was 2011 the warmest year?’ And if that’s a very high number, we can say with confidence that these recent years are unprecedented in their warmth.”

Tingley and Huybers’ study provides a strong statistical foundation for the notion that recent temperature extremes exceed natural variability and stem from a dramatic change in the larger climate. Specifically, the researchers found that the 2010 summer was almost certainly the warmest in 600 years in western Russia, western Greenland, and the Canadian Arctic.

“To provide context with these recent extremes, it’s necessary to think about variability in both space and time, and to compare these recent extremes that occur in particular places to the overall variability of the system rather than focusing, say, on a time series for Moscow,” Tingley said.

While the study did support the notion that there has been a dramatic shift in the mean temperatures of these regions, the research showed no evidence that temperatures are becoming more variable over time. Recent temperature extremes can be explained by only an increase in the mean of the temperature distribution, which dramatically increases the probability of very hot temperatures.

“A scientist is always skeptical but we’re all open to reason,” Tingley said. “In my view, the evidence for an anthropogenic impact on the climate is overwhelming. This is not an issue that rational people doubt.”