E-mail furor doesn't alter evidence for climate change

The hacked e-mails have been mined for words and phrases that can be distorted to misrepresent what the scientists were discussing. In a Dec. 9 op-ed, former Alaska governor Sarah Palin argued that "The e-mails reveal that leading climate 'experts' . . . manipulated data to 'hide the decline' in global temperatures." Yet the e-mail she cites was written in 1999, just after the warmest year ever recorded (1998) to that date. It could not possibly have referred to the claim that global temperatures have declined over this decade -- a claim that is false (the current decade, as has been recently reported, will go down as the warmest on record).

In one case, professor Phil Jones of the University of East Anglia refers to a "trick" regarding temperature data that he attributes to an article that co-authors and I published in the journal Nature in 1998. We showed one up-to-date temperature data set from thermometer measurements along with a longer data set, based on calculations from natural "proxy" records such as ice cores, corals and tree rings, that ended in 1980. The "trick" (by which scientists generally mean a clever solution, i.e., a "trick of the trade") was that the longer-term record could be viewed in the context of recent temperature measurements.

There was nothing secret about this. Both temperature curves were clearly labeled in our Nature article, and anyone could download the data we plotted. The National Academy of Sciences reviewed this work in 2006 in a study reported by this newspaper ["Past Few Decades Warmest on Record, Study Confirms," news story, June 23, 2006]. Members of the peer-review panel said that they "saw nothing that spoke . . . of any manipulation" and that the study was "an honest attempt to construct a data analysis procedure."

In the same e-mail, Jones uses the phrase "hide the decline" in reference to work by tree-ring expert Keith Briffa. Because tree-ring information has been found to correlate well with temperature readings, it is used to plot temperatures going back hundreds of years or more. Briffa described a phenomenon in which the density of wood exhibits an enigmatic decline in response to temperature after about 1960. This decline was the focus of Briffa's original article, and Briffa was clear that these data should not be used to represent temperatures after 1960. By saying "hide the decline," Jones meant that a diagram he was producing was not to show those data during the unreliable post-1960 period.

The conspiracy theories about the e-mails are fueled in part by their criticisms of the quality of two papers regarding global warming and a suggestion that at least one of the papers be kept out of an Intergovernmental Panel on Climate Change report. As Nature noted in a recent editorial, neither the e-mail writers nor the IPCC suppressed any findings. Both papers were included in the IPCC's report. Some statements in the stolen e-mails reflect poor judgment -- for example, a colleague referring to deleting e-mails that might be subject to a Freedom of Information Act request -- but there is no evidence that this happened.

Palin wrote that Alaska's climate is changing but referred to "thawing permafrost and retreating sea ice" as "natural, cyclical environmental trends." In fact, such changes are among the effects scientists predicted would occur as greenhouse gas levels increase. Scientific evidence for the reality of human-caused climate change includes independently replicated data documenting the extent of warming; unprecedented melting of glaciers; rises in global sea levels; increasingly widespread continental drought; and models that predict all of these things but only when human impacts are included. Those same models project far more profound and potentially damaging impacts of climate change if we do not take action to stabilize greenhouse gas levels.

The scientific consensus regarding human-caused climate change is based on decades of work by thousands of scientists around the world. The National Academy of Sciences has concluded that the scientific case is clear. As world leaders work in Copenhagen to try to combat this problem, some critics are seeking to cloud the debate and confuse the public.

Michael E. Mann, the author of "Dire Predictions: Understanding Global Warming," is a professor in the meteorology department at Penn State University and director of the Penn State Earth System Science Center.