Surface temperatures in the Northern Hemisphere were warmer over the last 10 years than any time during the last 1300 years, according to researchers at Penn State’s Earth System Science Center and three other U.S. universities.
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.
“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.
Results of this study without tree-ring data show that for the Northern Hemisphere, the last 10 years are likely unusually warm for not just the past 1,000 as reported in the 1990s paper and others, but for at least another 300 years going back to about A.D. 700 without using tree-ring data.
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.
– from ens-newswire