Climate Change and New Odds on the 'Climate Dice'
Frederik Lichtenberg and Barb Gottlieb
January 30, 2012
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
“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
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
Tomorrow: Extreme heat events.