Global warming skeptics

William Nordhaus, a Yale economics professor, wrote a piece for the New York Review of Books wherein he pronounced global warming skeptics “wrong.” What prompted Nordhaus was an op-ed that appeared in the Wall Street Journal, which gainsaid the scientific consensus. I offered my two cents here.

The skeptics have focused on the apparent absence of warming over the last several years. Nordhaus includes this chart in his initial essay.

We’ll note that since roughly 1920 temperatures have risen, modulating during WWII, then resuming a steady climb to the beginning of the last decade, when temperatures appeared stable. Nordhaus, correctly I believe, judges a decade’s worth of data misleading. After all, one could take any ten-year span along the above curve and find that temperatures have stabilized or even fallen. What’s important is the long-term trend.

I’ve constructed a chart based on data obtained here.

Again, we could take any 10-year chunk along the curve to “show” that temperatures had cooled. What counts is the trend, which I’ve depicted as a dashed red line.

Consider this chart, which shows temperature anomalies from 1750 to about 2005. The base years are 1951-1980. A temperature change outside the base would be an anomaly. (source)

What strikes me about this chart is the absence of negative anomalies since roughly 1980. From that year forward all anomalies have been positive.

The climate scientists (their day jobs) at realclimate.org recently looked at a 1981 paper by James Hansen et al., published in Science, one of the top two peer-reviewed scientific journals. The scientists draw attention to the prediction of increased warming, despite the fact that temperatures had declined during the period of their research. (At the time, at least one weekly magazine had a cover article on the coming ice age.)

As it happens, actual temperatures were 30 percent higher than Hansen et al. had predicted. Nevertheless, the authors were confident that they had the science correct, which allowed them to make their prediction.

I especially like this concluding paragraph from realclimate:

To conclude, a projection from 1981 for rising temperatures in a major science journal, at a time that the temperature rise was not yet obvious in the observations, has been found to agree well with the observations since then, underestimating the observed trend by about 30%, and easily beating naive predictions of no-change or a linear continuation of trends. It is also a nice example of a statement based on theory that could be falsified and up to now has withstood the test. The “global warming hypothesis” has been developed according to the principles of sound science.

Back to the skeptics. In their reply to Nordhaus’s original essay, they write:

Professor Nordhaus presents two graphs from the IPCC 2007 report that purport to show that without anthropogenic emissions, models successfully simulate the global mean temperature until about 1970 but cannot do so thereafter. This is the basis for the IPCC’s claim that it is likely that most of the warming over the past fifty years is due to man’s emissions. Such a procedure absolutely requires that the model include correctly all other sources of variability. However, the failure of the models to predict the hiatus in warming over the past fifteen years is acknowledged to indicate that this condition has not been met. Furthermore there is the embarrassing fact that the models do not reproduce the 1910–1940 warming, which is nearly identical to the 1970–2000 warming but occurred before man’s emissions became large enough to be considered important.

I would reply by asking the skeptics to explain global warming over the long term. Why have temperatures risen? Natural cycles? Shifting of the earth’s axis? Sun spots? The real climate scientists have factored in a host of such input variables, none of which explain the warming.

It really is physics. Carbon dioxide provides a “blanket” of warmth, without which earth would be a ball of ice. But what happens if we keep adding more blankets? We would expect further warming.

The scientists of the Intergovernmental Panel on Climate Change—some 1,000 or so—acknowledge that the climate is complicated. So much so that their initial reports were heavily qualified. Over time, however, as more data have been accumulated and models further refined, the scientists have become bolder in their predictions. Their latest summary (pdf) said that global warming was “unequivocal.”

Realclimate.org says this about using arbitrary time slices to prove or disprove global warming:

Short term (15 years or less) trends in global temperature are not usefully predictable as a function of current forcings. This means you can’t use such short periods to ‘prove’ that global warming has or hasn’t stopped, or that we are really cooling despite this being the warmest decade in centuries.

So there.

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