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Leading climate scientists reaffirm view that late 20th Century warming was unusual


Warming resulted in part fromfrom human activity

A group of leading climate scientists has reaffirmed the "robust consensus view" emerging from the peer reviewed literature that the warmth experienced on at least a hemispheric scale in the late 20th century was an anomaly in the previous millennium and that human activity likely played an important role in causing it. In so doing, they refuted recent claims that the warmth of recent decades was not unprecedented in the context of the past thousand years.

Writing in the 8 July issue of the American Geophysical Union publication Eos, Michael Mann of the University of Virginia and 12 colleagues in the United States and United Kingdom endorse the position on climate change and greenhouse gases taken by AGU in 1998. Specifically, they say that "there is a compelling basis for concern over future climate changes, including increases in global-mean surface temperatures, due to increased concentrations of greenhouse gases, primarily from fossil-fuel burning."

The Eos article is a response to two recent and nearly identical papers by Drs. Willie Soon and Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics, published in Climate Research and Energy & Environment (the latter paper with additional co-authors). These authors challenge the generally accepted view that natural factors cannot fully explain recent warming and must have been supplemented by significant human activity, and their papers have received attention in the media and in the U.S. Senate. Requests from reporters to top scientists in the field, seeking comment on the Soon and Baliunas position, lead to memoranda that were later expanded into the current Eos article, which was itself peer reviewed.

Paleoclimatologists (scientists who study ancient climates) generally rely on instrumental data for the past 150 years and "proxy" indicators, such as tree rings, ice cores, corals, and lake sediments to reconstruct the climate of earlier times. Most of the available data pertain to the northern hemisphere and show, according to the authors, that the warmth of the northern hemisphere over the past few decades is likely unprecedented in the last 1,000 years and quite possibly in the preceding 1,000 years as well.

Climate model simulations cannot explain the anomalous late 20th century warmth without taking into account the contributions of human activities, the authors say. They make three major points regarding Soon and Baliunas’s recent assertions challenging these findings.

First, in using proxy records to draw inferences about past climate, it is essential to assess their actual sensitivity to temperature variability. In particular, the authors say, Soon and Baliunas misuse proxy data reflective of changes in moisture or drought, rather than temperature, in their analysis.

Second, it is essential to distinguish between regional temperature anomalies and hemispheric mean temperature, which must represent an average of estimates over a sufficiently large number of distinct regions. For example, Mann and his co- authors say, the concepts of a "Little Ice Age" and "Medieval Warm Period" arose from the Eurocentric origins of historic climatology. The specific periods of coldness and warmth differed from region to region and as compared with data for the northern hemisphere as a whole.

Third, according to Mann and his colleagues, it is essential to define carefully the modern base period with which past climate is to be compared and to identify and quantify uncertainties. For example, they say, the most recent report of the Intergovernmental Panel on Climate Change (IPCC) carefully compares data for recent decades with reconstructions of past temperatures, taking into account the uncertainties in those reconstructions. IPCC concluded that late 20th century warmth in the northern hemisphere likely exceeded that of any time in the past millennium. The method used by Soon and Baliunas, they say, considers mean conditions for the entire 20th century as the base period and determines past temperatures from proxy evidence not capable of resolving trends on a decadal basis. It is therefore, they say, of limited value in determining whether recent warming in anomalous in a long term and large scale context.

The Eos article started as a memorandum that Michael Oppenheimer and Mann drafted to help inform colleagues who were being contacted by members of the media regarding the Soon and Baliunas papers and wanted an opinion from climate scientists and paleoclimatologists who were directly familiar with the underlying issues.

Mann and Oppenheimer learned that a number of other colleagues, including Tom Wigley of the University Corporation for Atmospheric Research (UCAR) in Boulder, Colorado; Philip Jones of the University of East Anglia’s Climatic Research Unit in Norwich, United Kingdom; and Raymond Bradley of the University of Massachusetts in Amherst were receiving similar media requests for their opinions on the matter. Their original memorandum evolved into a more general position paper jointly authored by a larger group of leading scientists in the field.

Mann says he sees the resulting Eos article as representing an even broader consensus of the viewpoint of the mainstream climate research community on the question of late 20th century warming and its causes. The goal of the authors, he says, is to reaffirm support for the AGU position statement on climate change and greenhouse gases and clarify what is currently known from the paleoclimate record of the past one-to-two thousand years and, in particular, what the bearing of this evidence is on the issue of the detection of human influence on recent climate change.

Harvey Leifert | AGU
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