According to a 2007 report by the U.N. Intergovernmental Panel on Climate Change (IPCC) state-of-the-art climate models indicate the odds are about 90% that manmade influences are and will continue to be the principal cause of global warming.
Model estimates of global average temperature rise over the 21st century range from 3.2°F to 7.2°F. To what extent should we accept these projections at face value? How certain is the stated range of uncertainty? Can today's climate models provide credible predictions of the regional impacts of climate change (e.g., on the scale of U.S. states or most European countries)?
Lenny Smith, a statistics professor at the London School of Economics and Political Science (and someone who I've worked with in the past and regard highly), believes human activities are changing the global climate, but that climate scientists are "overselling" their results. "…we must stop pretending that we know the details of how it will all play out," comments Lenny, who points out that the estimates of uncertainty — based on the distribution of results from as many as 300 runs of global climate models — are themselves uncertain. This is especially true, he says, in the extremes (or "tails") of the distribution, which are often particularly important for decision-makers. Thus, it's quite possible that future warming could be significantly more or less than the range indicated by the models.
In questions and comments responding to Lenny's remarks, the obvious issue raised is whether we should believe the reports of the IPCC and, by inference, any statements by climate scientists based on climate models. Lenny and others say broadly yes — as long as the qualifiers are acknowledged and carefully read. To that end, they point to one item in particular — buried deep in the report's first section (chapter 10) — that has likewise bothered me. Namely, the report explicitly acknowledges that the range of uncertainty in warming is too narrow. But, to quote one commenter, "It's good that the qualifier is in there, but it is a hell of a qualifier to find on page 797."
So while climate models should not be ignored, the majority of climate scientists who believe that humans are contributing to global warming should be especially forthright in not overselling their case. To which I must add that global warming skeptics should be equally concerned about not overstating their position by exaggerating justifiable questions about the credibility of climate models.