I am reading The Book of Why: The New Science of Cause and Effect by Judea Pearl and Dana Mackenzie. Pearl’s work on cause and effect enables us to know whether one thing causes another, or not. A chapter discusses the origin of heat waves. Pearl looks at the causes of regional heat waves and considers the possibility that climate change is responsible. The target for analysis was the 2003 heat wave in France that killed thousands of people, the highest temperatures recorded was 101.5 F. My question was this a one-off event caused by anthropogenic climate change or more of a natural phenomenon.
Wikipedia has this insight:
A very significant heat wave occurred in Europe in July 1757. The heat wave may have been the second hottest summer in Europe in the past 500 years, according to reporting after the 2003 European heat wave.
July 1757 was the hottest month in the history of Paris with an average temperature of 25 °C (77 °F) (compared to 24.8°C during the 2006 European heat wave), and it reached a high of 37.5 °C (99.5 °F) on July 14th.
1906 – During the 1906 United Kingdom heat wave which began in August and lasted into September broke numerous records. On the 2nd temperatures reached 36 °C (96 °F) which still holds the September record, however, some places beat their local record during September 1911 and September 2016.
1911 – The 1911 United Kingdom heat wave was one of the most severe periods of heat to hit the country with temperatures around 36 °C (97 °F). The heat began in early July and didn’t let up until mid-September where even in September temperatures were still up to 33 °C (92 °F). It took 79 years for temperature higher to be recorded in the United Kingdom during 1990 United Kingdom heat wave.
All these 1700 and 1900 heat waves were before climate change was an issue.
I was looking for some sign of California heat waves in the history files and discovered many Historical Climate Network sites with higher summer temperatures in 1930, spanning summer months with 100-degree temperatures. It turned out that Nevada City has one of the longest temperature records from 1895 to 2017. Below is a graph of the Max temperatures recorded for each year.
What caught my attention was the slow decline in Max temperatures overtimes, especially over the time when CO2 emissions were increasing.
If increased CO2 emissions are causing climate change, the change in Nevada County is a drop in max daily temperatures, not an increase. However, this is only one data point. Looking at the summer temperatures of all Historical Climate Network in the US it is possible to see a similar decline in summer temperatures.
The summer temperature decline starts in the 1940s and continued to 2017, while global CO2 has been increasing. In looking at the cause and effect, is it possible to say increasing CO2 has reduced the US summer temperatures? About as easy as saying that rising levels of anthropogenic generated CO2 are responsible for heat waves. And in either case, we do not know, and neither does Judea Pearl. He would have been much wiser to have avoided the climate change cause question.
Update: I have added the Bakersfield Max Temperatures from 1914 to 2016 and you can see the decline in max temps over time.
Note all of these temperatures were higher than the 2003 maximum in Paris, all the folks living in Bakersfield should be dead from the heat.