And, if the climate scientists include the somewhat controversial data derived from tree-ring records, they say that warming is greater from 1998 to the present than at any time for at least 1700 years.
Icebreaking ship easily navigates the melting Northwest Passage across the Arctic. (Photo courtesy Maritime Connector)
"Some have argued that tree-ring data is unacceptable for this type of study," says Michael Mann, associate professor of meteorology and geosciences and director of the Earth System Science Center. "Now we can eliminate tree rings and still have enough data from other so-called proxies to derive a long-term Northern Hemisphere temperature record."
The proxies used by the researchers included information from marine and lake sediment cores, ice cores, coral cores and tree rings.
"We looked at a much expanded database and our methods are more sophisticated than those used previously," says Mann, who served as lead author of the 2001 Third Assessment Report for the Intergovernmental Panel on Climate Change and an expert reviewer for the Fourth Assessment report issued in 2007.
Mann also is the co-author of "Dire Predictions, Understanding Global Warming: The Illustrated Guide to the Findings of the IPCC."
The research reported today responds to a suggestion from the National Research Council that Mann and his colleagues revisit the surface temperatures in their "Surface Temperature Reconstructions for the Last 2,000 Years," to include newer data and techniques and confirm results presented in a 1990s paper.
Dr. Michael Mann is director of the Earth System Science Center at Penn State University. (Photo courtesy Penn State)
The same conclusion holds back to A.D. 300 if the researchers include tree-ring data.
One of the reasons that including tree-ring data in these studies raises possible concerns is something called the "segment length curse."
This "curse" occurs because trees put on rings every year, but older trees put on narrower rings. When tree ring researchers piece together tree-ring series from two trees, they must account for this factor in how they combine the later rings on one tree with the earlier rings on a younger tree. In the process, some information regarding long-term trends can be lost.
"Ten years ago, we could not simply eliminate all the tree-ring data from our network because we did not have enough other proxy climate records to piece together a reliable global record," says Mann. "With the considerably expanded networks of data now available, we can indeed obtain a reliable long-term record without using tree rings."
The new study shows that even without including tree ring data, it is clear that the unusual nature of recent warmth, which most scientists believe to be a result of human impacts on climate, is a reality.
In today's online edition of the "Proceedings of the National Academy of Sciences," the researchers note, "Conclusions are less definitive for the Southern Hemisphere and globe, which we attribute to larger uncertainties arising from the sparser available proxy data in the Southern Hemisphere."
Dr. Mann says he is hopeful about the future despite the past 10 years of rapid warming. He believes that the world will rally behind the cause of global warming and start to mitigate the effects of climate change.
The research team included Ray Bradley, professor of geosciences and director of the Climate System Research Center, University of Massachusetts; Malcolm Hughes, regents' professor with the Laboratory of Tree Ring Research, University of Arizona; and Scott Rutherford, assistant professor, environmental sciences, Roger Williams University in Rhode Island.
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