Open Letter from James Hansen

by William Yeatman on October 31, 2000

in Science

James Hansen, of NASAs Goddard Institute for Space Studies, recently published a study in the Proceedings of the National Academy of Sciences which argued that CO2 may not have contributed as much to global warming as previously thought, but that the warming could be explained by other emissions, such as black carbon aerosols, methane, and ozone.

Hansens paper made a big splash in the media and received criticism from environmentalists who believed that it gave too much ammo to global warming skeptics who argue that theres no need to reduce CO2 emissions. Hansen has recently written an open letter (which appears to be nearly as long as the paper itself) to clarify what his paper said.

Hansen argues that his paper is merely an “alternative scenario” of how to reduce anthropogenic forcing to 1 Watt per meter squared (Wm-2) over the next 50 years. He derives his alternative scenario by noting that CO2 produced by burning fossil fuels induces a positive 1.4 Wm-2 climate forcing. This also produces a sulfate aerosol climate forcing of negative 1.4 Wm-2, which cancels out the CO2 forcing. Hansen then claims that what is left is a 1.4 Wm-2 climate forcing from other greenhouse gases.

Hansens scenario is to reduce forcing to 1 Wm-2 over the next fifty years, which requires, he is eager to point out, some reduction in CO2. But this begs the question, If CO2 forcing is completely cancelled by sulfate aerosol forcing, what then causes global warming? Hansens protestations aside, his paper clearly relegates CO2 to a non-factor in climate change. Only his arbitrary choice of a forcing target of 1 Wm-2 makes its reduction necessary.


The Greening Earth Society has just released a new book, The Greening of the American West, which features before and after photos of different locations in the American West showing changes in vegetation over the last 125 years. The book, authored by Craig and Keith Idso of the Center for the Study of Carbon Dioxide and Global Change, shows that vegetation has increased in several areas of the Western United States, which is attributed partly to the fertilizing effects of rising concentrations of CO2. Details on how to obtain the book can be found at

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