In his March 28 column for The Wall Street Journal's OpinionJournal.com, former Delaware governor and one-time Republican presidential candidate Pete du Pont quoted Washington Post columnist David Ignatius's claim that “human activity is accelerating dangerous changes in the world's climate,” and responded to Ignatius by claiming that “it is not clear that human activity is wholly responsible” for global warming. Ignatius, however, did not assert that humans are “wholly responsible” for global warming -- he claimed that humans are “accelerating” global warming, as the quote du Pont provided clearly indicated. In fact, the overwhelming weight of scientific evidence supports Ignatius's contention: global climate change is a natural process that is being abnormally affected by anthropogenic carbon dioxide emissions.

Moreover, the evidence du Pont provided to rebut the argument Ignatius never made does little to challenge the findings of the scientific community. Du Pont pointed to a report from the Washington Policy Center spanning 44 years of temperature readings from Mount Rainier in Washington and reports of shrinking ice caps on Mars. Du Pont also noted that Duke University scientists concluded that “at least 10 to 30 percent of global warming measured during the past two decades may be due to increased solar output,” but failed to mention that the scientists also “emphasized that their findings do not argue against the basic theory that significant global warming is occurring because of carbon dioxide and other 'greenhouse' gases.” According to a September 30, 2005, Duke news release: “This study does not discount that human-linked greenhouse gases contribute to global warming, they stressed. 'Those gases would still give a contribution, but not so strong as was thought,” [Duke associate research scientist Nicola] Scafetta said."

From du Pont's March 28 column:

So are we now at the beginning of a global warming catastrophe? Again, scientists and the press think so: the same NASA data indicates a 0.7-degree warming trend from 1970 to 2000. The Washington Post's David Ignatius reflects the media view in saying that “human activity is accelerating dangerous changes in the world's climate.”

But it is not clear that human activity is wholly responsible. The Washington Policy Center reports that Mount Rainier in Washington state grew cooler each year from 1960 to 2003, warming only in 2004. And Mars is warming significantly. NASA reported last September that the red planet's south polar ice cap has been shrinking for six years. As far as we know few Martians drive SUVs or heat their homes with coal, so its ice caps are being melted by the sun--just as our Earth's are. Duke University scientists have concluded that “at least 10 to 30 percent of global warming measured during the past two decades may be due to increased solar output.”

Du Pont quoted Ignatius's March 8 Post column, titled "The Planet Can't Wait." Nothing Ignatius wrote, however, indicated that he believes humans are solely responsible for global warming -- as du Pont suggested -- or that the global climate does not warm through natural processes. The quote du Pont provided indicated as much: Ignatius claimed that humans are “accelerating” changes in the global climate.

Nevertheless, du Pont went on to offer a false choice between the suspected causes of global warming:

So what is causing these cooling and warming increases? Normal temperature trends? Solar radiation changes? Or human-caused global warming? There is little we can do about historical temperature or solar heat cycles, but if human actions are in fact causing global warming, what could be done to reduce it?

Contrary to du Pont's suggestion, global warming may be caused by a variety of factors. According to the Third Assessment Report of the United Nation's International Panel on Climate Change (IPCC): “Changes in climate occur as a result of both internal variability within the climate system and external factors (both natural and anthropogenic).” The IPCC report, released in 2001, also concluded that "[e]missions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate." Citing multiple studies that demonstrated “evidence for an anthropogenic signal in the climate record of the last 35 to 50 years,” the IPCC stated: “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.”

The National Academy of Sciences came to a similar conclusion in a 2001 report on climate change commissioned by the Bush administration:

Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.