James Hansen has updated his “Bell Curve” paper to include the last decade (2009-2019). To me, these bell curves — that show the distribution of average summer (July to August) temperatures — are some of the most powerful climate change visualizations. These curves are not predictions and are not based on models… they show, just based on temperature readings, what has already happened. And they show that “Extremely Hot Summers” in the Northern Hemisphere (defined as an average summer temperature in a particular place that is more than 3 standard deviations (“3-sigma”) above its 1951-1980 mean) have gone from happening 0.1% of the time in 1951-1980 to 22.1% of the time in 2009-2019. That is over a 200X (20,000%) increase! Extremely Hot Summers went from happening once every thousand years to once every 5 years.
And this is not just a mathematical trick. It is the climate extremes that cause infrastructure to fail, droughts to be devastating, and life to be more challenging. It is an indication of what has already happened due to climate change.
You can also see that Average Summers (where the average temperature is within 0.5 standard deviations from the 1951-1980 mean) have gone from 33.5% in the 1951-1980 period to 10.4% in the 2009-2019 period. And Colder Than Average “Cold” Summers (-0.5 to -3 sigma) have gone from 33.3% to 3.7% in those respective periods and Hot Summers (+0.5 to +3 sigma) have gone from 33.1% to 63.8%. So, compared to the 1951-1980 baseline period, we have Cold Summers happening about 90% less often, Average Summers occur about 1/3rd as often, Hot Summers occur about twice as often, and Extremely Hot Summers occur about 200 times as often.
The global average temperature has gone up about +1.3ºC since the industrial revolution but that does not tell the whole story. The increase in the global average temperature is the shifting to the right of the bell curve. But the increase in “3-sigma events” goes up a staggering amount when the curve is shifted and it is these 3-sigma events that cause death and destruction more than does the shift in the average temperature. And 5-sigma events that should only occur once every 3.5 million years and now occurring a few percent of the time!
As an interesting side note, there is a corresponding bell curve for the Southern Hemisphere and the globe as a whole. As Jim Hansen pointed out in his original bell curve paper, the shift of the global bell curve to the right IS global warming (QED). It’s like saying when you take your temperature and it reads 103ºF, we can say “you have a fever”. Your temperature reading doesn’t say why you have a fever and, correspondingly, the shift of the bell curve doesn't say why we have global warming. But it does prove we have global warming and other studies do show that the warming is mostly cause by the burning of fossil fuels.
Monday, Jul 13, 2020 · 12:02:30 AM +00:00 · dannym999
I should mention an important implication of the dramatic increase in 3-sigma events. Since the “natural” rate for these events was 0.1% and the recent rate is 22.1%, when an Extremely Hot Summer in a particular place occurs now, we can say there is a 22 to 0.1 (99.5%) chance that the Extremely Hot Summer was caused by global warming rather than natural causes! In other words, we can “attribute” the Extremely Hot Summer to climate change and we don't need to use weasel words about how it is impossible to attribute any specific event to climate change. This doesn’t work for all climate changes, but it does work for those formerly extremely rare “long tail” events that are getting common now.