April 28, 2005

James Hansen Increasingly Insensitive

Filed under: Climate Models

It seems that the longer NASA scientist Jim Hansen studies the climate, the more insensitive he, or should we say, his interpretation of the climate, becomes.

Climate “sensitivity” is the change in surface temperature expected for each additional Watt of energy that is re-radiated onto the earth’s surface and lower atmosphere by slight changes in the greenhouse effect. The main cause of these changes in the greenhouse effect, of course, is the increasing levels of atmospheric carbon dioxide caused by the combustion of fossil fuels.

You would think that it would be big news when Hansen—the guy who started all this mess with his incendiary 1988 congressional testimony—lowers his estimate for the sensitivity to two-thirds of the value he used back then.

After all, he does get a lot of ink. That’s what happened in October, 2004, when he traveled to hotly contested and environmentally sensitive Iowa the weekend before the election, and publicly berated his Boss’ global warming policy. Talk about insensitive!

Hansen’s most recent figure, just published in Sciencexpress, is that the surface temperature ultimately changes 0.67˚C per Watt per square meter (W/m2). In 1988 he said it was a full degree, and in 2001 he lowered it to 0.75.

The lower the climate sensitivity, the less that the global temperature will rise in the future (given the same amount atmospheric carbon dioxide) and the lower the threat of catastrophic climate change.

But the greenhouse emissions are also much lower than people expected. The standard modeling technique raises the carbon dioxide concentration of the atmosphere 1% per year, but the actual rate of increase for the last three decades has been around 0.45% per year. And, despite scary news stories, there’s little evidence for any sharp upward change. There was a lot of press when the 2003 concentration of atmospheric carbon dioxide rose considerably, and virtually no coverage when it was balanced out by much smaller changes in 2004.

The United Nations Intergovernmental Panel on Climate Change (IPCC) assumes an average change of roughly 0.65% per year in carbon dioxide, or 41% greater than the observed, very constant rate.

Hansen has written repeatedly a variant of his 2001 statement that “much of the warming of the next 50 years” will result from emissions already in the atmosphere, and that’s also in his latest work. Because is characteristic of climate models to warm at a constant (not an increasing rate), Hansen is really saying that the recent (very modest) rate of warming is likely to continue.

He arrives at the conclusion by using a combination of climate model output and observations of heat build-up in the global oceans. Hansen calculates that since the 1880s, there has been, in net, an added +1.8W/m2 of radiation reaching the surface, (resulting from positive additions from greenhouse gases, solar changes, black carbon aerosols, and negative changes from sulfate aerosols and land-use changes). His Goddard Institute for Space Studies (GISS) climate model produces a total change in temperature as a result of the 1.8W/m2 of added energy to the earth’s climate system of about 1.2ºC (indicating a climate sensitivity of about 0.67ºC/Wm2). Since the planet has warmed up about 0.6-0.7ºC between 1880 and now, that leaves another 0.5-0.6ºC of warming yet to occur. By “yet to occur” we mean that it is not yet being measured by thermometers at the earth’s surface. Using the “old” sensitivity of 1 degree would give a remaining warming of 1.1˚C, or nearly double what is now expected.

These are big changes and should be big news, but it is apparent that those who report on these matters may be far from a hand calculator.

Hint to taxpayers, who fund over $4 billion per year in climate change research (at least that’s what’s in the proposed budget): all of these calculations are pretty much unnecessary. It is already become well established over the course of the past 35 years or so that the rate of global average temperature rise is a remarkably constant 0.17ºC/dec. That would give right around 0.8ºC additional warming to 2050.

In his new work, Hansen is really just doing what any rational scientist would do: adjusting the sensitivity for what has been observed in the real world.

Of course, there could be a more cynical explanation for the reduced climate sensitivity, namely the rational tendency to cover ones back. Here’s what Hansen wrote in 2003 in the Journal Natural Science.

Emphasis on extreme scenarios may have appropriate at one time, when the public and decision-makers were relatively unaware of the global warming issue. Now, however, the need is for demonstrably objective climate…scenarios consistent with what is realistic under current conditions.

(The logical parsing of that paragraph was that it was OK for scientists to lie to gain policies they wanted, and that they weren’t being objective).

In climate science, we really have only two tools: computer models and observations. And it is clear, when the two are combined, that future warming is going to be at the low end of the wild projections that have been made by the IPCC. What Hansen has done is really nothing more than this, lending more evidence to what we already pretty much know. The rate of future temperature rise will be modest, as will be the accompanying climate impacts. Some will be positive, some will be negative, but they will all be at the low end. How insensitive!


Hansen, J.E., et al., 1988. Global climate changes as forecast by Goddard Institute for Space Studies three-dimensional model. Journal of Geophysical Research, 93, 9341-9364.

Hansen, J.E., and M. Sato, 2001. Trends of measured climate forcing agents. Proceedings of the National Academy of Sciences, 26, 14778-14783.

Hansen, J.E., 2003. The global warming time bomb? Natural Science, http://www.naturalscience.com/ns/articles/01-16/ns_jeh.html

Hansen, J.E., et al., 2005. Earth’s energy imbalance: confirmation and implications. Sciencexpress, April 28, 2005.

Michaels, P.J., et al. 2002. Revised 21st century temperature projections. Climate Research, 23, 1-9.

Michaels, P.J., 2004. Meltdown: The Predictable Distortion of Global Warming by Scientists, Politicians, and the Media. Cato Books, Washington DC. 272pp.

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