How has Extreme Heat been Affected by Climate Change?

Back in 2012, Hansen published a paper[1] showing the impact of global warming on temperature extremes. He plotted the summer climatology of the NH land temperatures for the 1951-1980 mean as a probability distribution, then showed how the the bell curve has changed with global warming. He showed a similar analysis of winter temperatures in the SH. What it demonstrated was that what was considered "extreme" summer heat in the base period covered far more of the globe in 2000-2010 than it did during the base period. In effect, what would count as a 3σ summer covered ~0.1% of land surface in 1951-1980, but by 2001-2010 this same weather would cover ~10% of the globe. Hansen concludes,

It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small.

In effect, Hansen is saying that since with AGW, extreme heat (3σ summers) is 100x more likely, so it makes it virtually certain that extreme summers are caused in large part by AGW. It should be acknowledged this does not translate immediately into other forms of extreme weather, like hurricanes, tornadoes, flooding, etc. Each type of extreme weather will be affected by AGW in different ways. Some may become more frequent, some may become less frequent, and some may be unaffected. But there's an obvious correlation between the increase in GMST in summer and extreme heat that can be quantified, and I think Hansen's paper demonstrates this relationship pretty well.

What Hansen did is summarize the climatology of 1951-1980 by binning the Z-Score (basically anomaly/standard deviation) of summer temperatures in each gridded land cell in 0.05 increments and count the number of cells in each bin. "The data curves were obtained by binning the local anomalies divided by local standard deviation into intervals of 0.05 (i.e., by counting the number of grid boxes having a ratio within each successive 0.05 interval)." This produced a probability distribution what we could call "normal" climatology (from here on "normal" is the climatology of the 1951-1980 mean). Then for each successive decade, the probability distribution is plotted against the mean temperatures and standard deviation of 1951-1980. With each successive decade, the mean shifts to to the right as land area warms, and the probability distribution becomes shorter and fatter, with the hot side becoming fatter than the cold side of the distribution. This prompts Hansen to say,

The hot tail of the temperature anomaly distribution shifted by more than +1σ in response to the global warming of about 0.5 °C over the past three decades. Additional global warming in the next 50 y, if business-as-usual fossil fuel emissions continue, is expected to be at least 1 °C. In that case, the further shifting of the anomaly distribution will make +3σ anomalies the norm and +5σ anomalies will be common.

We're only 10 years into the 50 year time frame Hansen describes, but it's worth asking if we see progress towards 3σ hot summers becoming "normal." In a recent post[2], Hansen updated his 2012 paper to include the decade from 2010-2020. The results show just how much climate is actually changing for those living in the NH. Hansen showed the distribution for the 1951-1980 mean summer temperatures with the coolest third of temperatures in blue, the middle third in white, and the warmest third in red, with extreme heat (>3σ) occurring about 0.1% of the time. However, during the last decade, what was the coolest 33.4% of base period temperatures was only 2.9% of 2010-2020. The "normal" 33.3% of the base period was only 9.4% of 2010-2020 climatology. The warmer 33.2% of the base period became 65.5% of 2010-2020 climatology. And the extreme 0.1% in the base period was 22.1% of the 2010-2020.

% Land Area Covered by 3σ Summer Weather
(1980s figure is a guess, since I couldn't find the precise value)

Stated another way, for NH summer land temperatures, what was the coolest 66.7% of 1951-1980 makes up only 13.3% of 2010-2020. What was the warmest third is now the new normal, with what was extreme (>3σ) summers is now occurring over 1/5 of the time. The jump from ~10%  for the 2000s to ~22% in the 2010s means that extreme heat is progressing just as expected. As climate warms, the % land area covered by 3σ summers relative to "normal" will continue to increase. With 1σ more warming, we can expect roughly 50% of the summer NH land surface to be covered in 3σ weather. With another 2σ warming after that, then perhaps 95% of summer NH land surface will be covered in 3σ weather, relative to normal.

As % land area of 3σ summers increases, the return rate of 3σ summer weather also increases. That is, if 3σ summer weather now covers 220x more land area than it did in 1951-1980, then this weather is 220x more likely, and what was a 1 in 1000 year summer becomes roughly a 1 in 5 year summer. This has significant implications for how risk of death from extreme heat increases with GMST. If climatology was unchanged since the 1951-1980 mean, then a 3σ summer would occur ~1x every 1000 summers, and most of us would live our entire lives never experiencing a 3σ summer. But with climate change, these summers now occur ~220x every 1000 summers - that is, roughly 1 in 5 of our summers is extreme heat, on average. Of course some places will experience less of an increase in extreme heat and some more. But our risk of death from extreme heat must increase with the increased frequency of our exposure to extreme heat. Whatever the likelihood of death from a 3σ summer in 1951-1980, we're experiencing those conditions 220x more frequently now than we were 70 years ago. And while I fully acknowledge that with our increased standard of living and better technology, it may well be that deaths are not 220x more frequent, the increase in risk from a 220x more frequent exposure to extreme heat is virtually undeniable.

References:

 
[1] James Hansen, Makiko Sato, Reto Ruedy. Perception of climate change. Proceedings of the National Academy of Sciences Sep 2012, 109 (37) E2415-E2423; DOI: 10.1073/pnas.1205276109
https://www.pnas.org/content/109/37/E2415

[2] "Dr. James Hansen on Climate."
https://redgreenandblue.org/2021/07/16/dr-james-hansen-climate-june-2021-global-temperature-update/