Climate Change and New Odds on the ‘Climate Dice’

This week, PSR is pleased to present a series of postings by Frederik Lichtenberg, PSR’s climate intern and a student of geography at the University of Bonn in Germany.  Today’s installment is coauthored with Barbara Gottlieb, PSR director of environment & health.

National Aeronautics and Space Administration’s James Hansen is one of the world’s leading experts on climate change. In 1988 he spoke before Congress and, as one of the first scientists ever to do so, brought the issue of climate change to the attention of a broad audience.

Hansen and coauthors Makiko Sato and Reto Ruedy recently released a new article entitled “Perceptions of Climate Change: The New Climate Dice.”  The term “climate dice” refers to a measure of the likelihood that we will experience temperature extremes – uncommonly warm or cool seasons. The article notes that “the climate dice have become progressively ‘loaded’ in the past 30 years.” 

The climate dice

Hansen et al. represent the climate by colored dice, and explain that climate over roughly the past 10,000 years, the Holocene period, was relatively stable.  The climate dice during that stable period have two red sides, indicating ‘hot’ seasons; two blue sides for ‘cold,’ and two sides for ‘average’ (white). The chances of red, blue and white were equal, with a one-third chance for each type of weather event to occur.

Dice have shifted

That was the case, at least, until roughly 30 ago.  Fast-forward now to the present day.  Today, Hansen argues, if this imagery is adapted to our current situation, four of the six sides of the dice would be red, reflecting a greatly increased likelihood of hot conditions. Of the two remaining sides, only one is blue and one white.  That is, the chance for ‘cold’ and ‘average’ conditions has greatly decreased. This change in the face of the dice indicates how dramatically the climate has shifted over the course of just three short decades.

Not only is it hot more frequently; it’s very hot.  In fact, Hansen felt the need to cite the emergence of a category he calls summertime extremely hot “outliers,” more than three standard deviations warmer than climatology.  During to a baseline period that Hansen says fairly represents the Holocene (he uses 1951-1980), such hot extremes affected only 1% of the Earth’s land surface.  Now, it’s about 10%. 

“Caused” by global warming

This shift in the likelihood of extreme heat leads Hansen to make a startling statement:  that extreme heat waves, such as those in Texas and Oklahoma in 2011 and Moscow in 2010, were "caused" by global warming.

“Caused” - !  This statement of causation is a bold departure from prior ways of speaking about climate and weather.  Earlier, speakers would say that extreme weather events were “associated with” climate change, that they were “in accordance with an increased likelihood” of occurrence.  Hansen justifies his shift in language by noting that the likelihood of such outlier events – a likelihood that was “negligible prior to the recent rapid global warming” – has grown greatly and in very little time.

While a cold, harsh winter may still occur from time to time – a shake of the dice that does not disprove global warming – the chances of a summer hot enough to fall into the extreme category have risen dramatically and are now at 80%. In effect, even with four of the six sides of the dice painted red, it under-represents how severe the changes to our climate have actually been.

Tomorrow:  Extreme heat events.

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