Hotter temperatures year-round could be here to stay, according to a new report on global temperatures by a National Research Council committee. The council said there is “sufficient evidence” from tree rings, boreholes, retreating glaciers, and other “proxies” of past surface temperatures to say with a high level of confidence that the last few decades of the 20th century were warmer than any comparable period in the last 400 years.
The committee, made up of respected climate scientists, said in its 155-page report that less confidence can be placed in proxy-based reconstructions of surface temperatures for A.D. 900 to 1600, although the available proxy evidence does indicate that many locations were warmer during the past 25 years than during any other 25-year period since 900. “Very little confidence can be placed in statements about average global surface temperatures prior to A.D. 900 because the proxy data for that time frame are sparse,” the committee added.
The committee explained that scientists rely on proxies to reconstruct paleoclimatic surface temperatures because geographically widespread records of temperatures measured with instruments date back only about 150 years. Other proxies include corals, ocean and lake sediments, ice cores, cave deposits, and documentary sources, such as historic drawings of glaciers. The globally averaged warming of about 1 degree Fahrenheit (0.6 degrees Celsius) that instruments have recorded during the last century is also reflected in proxy data for that time period, the committee noted.
The report was requested by Congress after a controversy arose last year over surface temperature reconstructions published by climatologist Michael Mann and his colleagues in the late 1990s. The researchers concluded that the warming of the Northern Hemisphere in the last decades of the 20th century was unprecedented in the past thousand years. In particular, they concluded that the 1990s were the warmest decade, and 1998 the warmest year. Their graph depicting a rise in temperatures at the end of a long era became known as the “hockey stick.”
The Research Council committee said it found the Mann team’s conclusion that warming in the last few decades of the 20th century was unprecedented over the last thousand years to be plausible, but it had less confidence that the warming was unprecedented prior to 1600; fewer proxies — in fewer locations — provide temperatures for periods before then.
“Because of larger uncertainties in temperature reconstructions for decades and individual years, and because not all proxies record temperatures for such short timescales, even less confidence can be placed in the Mann team’s conclusions about the 1990s, and 1998 in particular,” the committee said.
For more information on the report, visit the National Academies website at www.nationalacademies.org.
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