Dangerous Scientific Reticence - by Jim Hansen et al.

Several years ago I wrote a paper[1] on scientific reticence, naively thinking that drawing attention to the phenomenon might ameliorate its incidence.  Specific reference then was to likelihood of large sea level rise, which also is a central topic in our current paper[2]. However, here I address a broader issue of scientific reticence, because, I believe, the affliction is widespread and severe.  Unless recognized, it may severely diminish our chances of averting dangerous climate change.
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It seems to me that scientific reticence is now greater than when I first wrote on the topic.[5],[6],1 Do the public, policymakers, and courts understand that scientists use the word “dangerous” with a different meaning than that in the dictionary?  Did the Framework Convention not mean the common dictionary definition of the word?  In the “dark street” analogy, a person does not need to conclude with certainty that he would be killed on that street to conclude that it is dangerous.

Inertia of the climate system reduces present climate impacts, but it also makes it difficult to stop larger ones in decades ahead.  Inertia in our energy systems implies that it takes decades to make major changes in emissions and atmospheric composition.  Because of the combination of these two slow systems, we are in danger of passing points of no return, such that we hand young people a climate system with great consequences, including the potential for large sea level rise and shutdown of the ocean’s overturning circulations, consequences that could be locked in soon if we do not reduce global emissions rapidly.

Scientific reticence is dangerous, and wrong in my opinion.  I will return to that subject soon.

One final comment, closely related to scientific reticence.  A criticism of our paper that may warrant response is that the ice melt rates that we assumed were “unrealistic”.  In fact it is certain that multi-meter per century melt rates have occurred many times in Earth’s history, spurred by much weaker forcings than the human-made forcing.  We presented evidence in our paper that rapid sea level rise even occurred in late-Eemian, when there was less ice available for melt than today.  Just this week a paper was published showing that the fastest natural increase of greenhouse gas climate forcing in the past 66 million years was at least 10 times slower than the human-made change[7].  Unfortunately, the melt rates we talk about for the next several decades are very realistic, and we are already seeing expected response to current melt rates.

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