climate

[David]

What is the most compelling argument that humans are indeed affecting climate?

[E. Kirsten]

David,

I’m a geologist by training and your question really relates to modern climate science – so my answer may not be as authoritative as you’d assume. That said, I know what has impressed me recently. It’s a three-part argument that can be described fairly simply.

First, the evidence is pretty clear that average global temperature has been getting warmer since the end of the Little Ice Age around 1850 or even a bit before that. We know a fair amount about temperature during that time because we have written records from weather stations across North America and indeed around much of the globe. The records aren’t perfect – nothing that humans do is going to be that! But they are the best data set we have and they seem to show warming trends in many places.

Do you know of a physicist at U.C. Berkeley named Richard Muller? You can Google him to get to an essay he wrote in The New York Times last summer about work he and his group have done about temperature records. The piece is titled “The Conversion of a Climate-Change Skeptic.” Muller set himself the task of looking at historical records of weather. There had been some question if warming trends in the data are “real” or if they might be caused by such things as the encroachment of urban areas on what had previously been rural weather stations. (Urban areas trap heat and are warmer compared to bucolic regions.) But Muller and his group looked at all the data and he became convinced that warming is real in most places. That fits with conclusions of climate scientists such as those published in the IPCC reports.

The second part of the argument is the simple fact that we are producing a lot of carbon dioxide (and other greenhouse gases) and that we’ve really changed the concentration of such gases in the atmosphere. That observation I believe is beyond dispute: we know of past concentrations from the tiny air bubbles trapped in glacial ice and we know of recent dramatic changes in carbon dioxide by on-going measurements of the atmosphere at such places as Hawaii. We have boosted carbon dioxide beyond values known anytime previously in the Holocene or what we know of the Pleistocene Epoch. It’s not a comforting picture – you can see it in figures in Chapter 11 of The Whole Story of Climate.

Next, and here’s the third part of the argument, Muller looked at what could be the cause of the warming trend. Some have suggested that things like solar cycles might be at work, driving recent warming as much or more than anything people are doing. Muller’s group tried “fitting the data” about temperature to a number of variables. The best statistical match they found is between temperature and carbon dioxide concentrations in the air. Muller and associates tried to look at such things as solar cycles and compare them to the temperature record, but the best fit comes not from other factors but between the record of greenhouse gas concentration and temperature.

Muller’s argument isn’t a firm “proof” that carbon dioxide is driving the recent temperature climb. But it does mean, as Muller says, that we should treat the hypothesis with real respect. To put it another way, it’s the best explanation we have of recent warming. It makes sense, then, to believe that greenhouse gases are the culprit until and unless we have a better explanation.

Kirsten