Bell Curves & New Climate Normals

Posted on August 21, 2012


We all know what a bell curve is, a graph in the shape of a bell.  A lot of things distribute that way, with the bulk of things happening in the middle, tailing away towards either extreme.

Climate stacks up like a bell curve.  Even with climate change it still stacks up that way.  Thus, most manifestations of climate stack up in the middle, with examples of extreme cold and extreme heat at either side.  What climate change has altered is not the distribution pattern, but where the middle of the curve is, what the normal is.

If we compare the middle of the curve in decades from 1950 to 1980, to the most recent decade, we find that in the most recent decade, the middle has shifted significantly towards the hot.  This has contributed greatly to the number of extreme heat events worldwide.

On the following diagram, James Hansen has plotted climate distribution curves for all the decades from 1951 to 2011.  Note that the distribution curves for the first three decades sit almost on top of each other.  The black line histogram averages the first three decades.

Starting in 1981-1991 (in light blue) the curve starts to shift over to the right.  The curve for 1991-2001 (in dark blue) shifts even further.  The curve for 2001-2011 (in violet) shifts the farthest of all.

The second diagram is the same as the first, except that I have drawn in a line meant to separate off extreme heat events as those events would have been defined in 1951-1981.  I have also highlighted an area to the far right of the diagram representing extreme heat events that, compared to the 1951-81 base period, are fundamentally off-the-charts.

Notice that the area “off-the-charts” in the 2001-2011 period has essentially replaced the extreme event category as defined in 1951-1981.  And that “extreme events” as those events were classified in 1951-81 have migrated to the fat part of the curve this century.  They have simply become the warmer part of normal.

That’s what normal means in a diagram like this.  When an event becomes so common that it joins the fat part of the curve, when it is no longer an unusual or low probability event, then it has become normal.

As scientists have been saying for some time now, in terms of climate, we have entered a “new normal.”

Unfortunately, the process is only beginning.  Unless humanity makes some effort to stop climate change, there is no part of our climate which will strike us as normal again for a long, long time.

I have taken these diagrams from a recent study by James Hansen, Head of NASA’s Goddard Institute of Space Studies.  In a recent op-ed in the Washington Post (Climate change is here–and worse than we thought – Washington Post) he wrote:

  • [It] is no longer enough to say that global warming will increase the likelihood of extreme weather and to repeat the caveat that no individual weather event can be directly linked to climate change. To the contrary, our analysis shows that, for the extreme hot weather of the recent past, there is virtually no explanation other than climate change.
  •  The odds that natural variability created these extremes are minuscule, vanishingly small. To count on those odds would be like quitting your job and playing the lottery every morning to pay the bills.
  •  Extremely hot temperatures covered about 0.1 percent to 0.2 percent of the globe in the base period of our study, from 1951 to 1980. In the last three decades, while the average temperature has slowly risen, the extremes have soared and now cover about 10 percent of the globe.

James Hansen’s study, published in the prestigious Proceedings of the National Academy of Sciences, may be found here: