Lord Kelvin, Out of Context?
Robert Zubrin (“The Human Factor,” September 21) may well be right to deny that global warming poses a risk of catastrophic problems. Part of his case for that denial, however, rests on statistical sleight-of-hand.
Zubrin says that, over the past 50 years, the result of unconstrained emission of carbon dioxide into the atmosphere has been an increase in the global mean temperature of only 0.2 percent. The temperature went from 287 K to 288 K. He is right — if you measure global temperatures on the Kelvin scale. But why does he choose a scale that starts at absolute zero?
In astrobiology, “habitable zone” refers to the region in which a planet orbiting its sun can have liquid water. Planets outside the habitable zone cannot support life. The freezing point of water is 273 K. Thus, 273 K marks the lower bound of any plausibly relevant scale measuring changes in mean global surface temperature. (Indeed, it sets the lower bound conservatively.)
Here, then, is the statistical sleight-of-hand. Zubrin employs a scale that has the current global mean temperature at 288 K. But on that very scale, every possible mean temperature from 0 K to 273 K is irrelevant. If instead we measure changes in Celsius — a scale on which 0 marks the freezing point of water — we see that the average global mean temperature has increased from 14 to 15 degrees. That is an increase of about 7 percent. Maybe that is a catastrophic increase, maybe it is not. But putting the temperature numbers that way at least gives the public a real sense of the scale of the problem. Measuring global temperatures in kelvins does not.
Neil A. Manson
Robert Zubrin responds: The average global temperature has increased 0.6 degrees Celsius over the past 60 years. This is 0.2 percent of the absolute temperature, or 1 percent of the range (–25 to +35 degrees Celsius) experienced in my home state of Colorado, among many other places, in the course of a single year. In either case, it is trivial in comparison with the 15 percent increase in the rate of plant growth and the 400 percent increase in average global GDP per capita caused by the unconstrained use of carbon-based fuels over the same period. Moreover, there is no evidence to support the contention that the climate of the 1950s was in any way preferable to that of today, let alone the idea that the barely detectable temperature increase represents some sort of catastrophe — which it would certainly have to be to outweigh the enormous increase in living standards made possible by fossil fuels.