NASA’s Hansen admits sulfates are a ‘wild card’

Published November 1, 2000

When NASA’s James Hansen talks, people listen. Ditto for his writing, which explains his recent bombshell paper and the naive response it has engendered.

Hansen recently published another paper in the Proceedings of the National Academy of Sciences (PNAS), this time arguing we can effectively limit global warming during much of the next century without stringent reductions in carbon dioxide output. Instead, he reasons, the cooling of sulfate aerosols will do our regulating for us, all but balancing out whatever effect added carbon dioxide has.

Carbon dioxide has traditionally been thought of as the “main” human greenhouse gas, and the one therefore most in need of reduction. But when all of the numbers work out (as Environment & Climate News‘ devoted readership is all too aware), it doesn’t look like there’s any chance that carbon dioxide (CO2) emissions will drop by much (if at all) within the lifetime of almost anyone on the planet.

In that light, the very expensive Kyoto Protocol, in which the United States would have to agree to reduce its emissions to 7 percent below 1990 levels by the averaging period 2008–2012, seems silly.

After all, we are currently at about 14 percent above 1990 levels, and we have reduced our CO2 emissions during only one year (1973, the year after the Arab oil embargo). The economic damage then was so great it brought us Jimmy Carter.

Perhaps when federal climatologist Jerry Mahlman told Science magazine in 1997 that “it might take another 30 Kyotos over the next century” to reduce global warming to an acceptable level, he had overlooked that reality.

So Hansen’s latest argument would seem to be tremendously attractive—except to the legion that wants to use global warming to make the United States miserable. But his argument is almost certainly faulty.

Hansen bases his theory upon the notion that cooling aerosol emissions, which go into the air along with carbon dioxide from the burning of fossil fuels, almost exactly cancel the warming effect from the CO2. Those sulfates are the particulates we often see in smoke, and they are assumed to grow into a haze that reflects so much of the sun’s radiation that they completely counter carbon dioxide warming.

That old saw is, well, old. It has long been the logical redoubt used to explain the lack of correspondence between predictions (big) and observations (small) of global warming for decades. In fact, Hansen says as much in his PNAS paper: “Two empirical pieces of information are consistent with our estimated net climate forcing [the first one being] global warming of the past century. . . .”

Translation: The only way we can explain why so much warming was forecast and so little was observed is to somehow make the assumption that aerosol cooling cancels carbon dioxide warming.

How sure is Hansen of that? Not at all. In the end, we believe he reads Environment & Climate News, too. Why else would his “Summary” section include this statement: “Climate forcing due to aerosol changes is a wild card. Current trends, even the sign of the effect [emphasis added], are uncertain.”

In other words, everything in his paper is based upon an assumption whose sign, much less magnitude, is in doubt.

That isn’t science. It’s speculation, and it’s probably incorrect. The aerosol effect should cause much more cooling in the Northern Hemisphere than it has. Yet when we compare the behavior of the Northern and Southern Hemisphere data (Figure 1), it leans in the opposite direction of what is predicted by strong aerosol cooling. That is to say, if aerosols are indeed even cooling things (which Hansen says is in doubt), then they can’t be doing it very much.

Climate may not be predictable, but the reaction to Hansen’s new paper sure has been.

Greenies are mad, because they feel Hansen has betrayed them. After all, he started this whole mess with his inflammatory Congressional testimony on June 23, 1988.

According to Nature magazine, University of Virginia environmental sciences professor Patrick J. Michaels is probably the nation’s most popular lecturer on the subject of climate change. Michaels is coauthor of The Satanic Gases: Clearing the Air About Global Warming.


Hansen, J., et al., 2000. Global warming in the twenty-first century: An alternate scenario. Proceedings of the National Academy of Sciences, 97, 9875-9880.