They predicted an ice age in the 70's

The argument "they predicted an ice age in the 70's" has barnstormed into the Top Ten thanks largely to an Investor's Business Daily article claiming James Hansen believed we were heading for an ice age. This is based on the 1971 paper Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate (Rasool 1971) that speculated if aerosol levels increase 6 to 8 fold, it could trigger an ice age.

However, James Hansen wasn't an author of the Rasool paper and never made any ice age predictions. So what was his involvement? According to Investor Business Daily, "Aiding Rasool's research was a 'computer program developed by Dr. James Hansen'." [ UPDATE - James Hansen explains in more detail about his program used in Rasool's paper ] As Tim Lambert succintly puts it at Deltoid, "By their logic, if I borrow a pen from you, you must agree with everything I write with your pen."

Putting James Hansen aside, the whole logic that "climate scientists got it wrong in the 70's so they must be wrong now" is a flawed ad hominem argument that says nothing about the current science of anthropogenic global warming. Is it really appropriate to compare a single study in the 70's to the scientific consensus on anthropogenic global warming today?

National Academy of Sciences - now and then

The most comprehensive study on the subject (and the closest thing to a scientific consensus at the time) was by the US National Academy of Sciences. It's basic conclusion was "…we do not have a good quantitative understanding of our climate machine and what determines its course. Without the fundamental understanding, it does not seem possible to predict climate…"

Contrast this with the US National Academy of Science's current position: "there is now strong evidence that significant global warming is occurring... It is likely that most of the warming in recent decades can be attributed to human activities... The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action." Incidentally, this is in a joint statement with the Academies of Science from Brazil, France, Canada, China, Germany, India, Italy, Japan, Russia and the United Kingdom.

Other indications of current consensus

Other scientific bodies that have released statements endorsing anthropogenic global warming include:

None of these bodies (at least the ones that existed back then) endorsed ice age predictions in the 70's. More on scientific consensus...

So global cooling predictions in the 70's amounted to media hype over essentially a single study. Today, an avalanche of studies and overwhelming scientific consensus endorse anthropogenic global warming. To compare cooling predictions in the 70's to the current situation is both inappropriate and misleading.

Posted by John Cook on Monday, 24 September, 2007

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