Below is a shortened version of the excellent article by Roy W. Spencer, Ph. D. from July 02 which lays out the 3 biggest pitfalls when trying to link heatwaves with Global Warming.
For the article in full, click here.
1. Record High Temperatures Occur Even Without Global Warming
The question is, are the number of record high temperatures increasing over time? At least in the U.S., the answer is ‘no’, as the number of days over 100 and 105 deg. F have not increased (see Fig. 5 here).
And even if they are increasing, one needs to determine the cause. Most of the warming since the Little Ice Age (up to about 1900) occurred before greenhouse gases could be blamed. We have no temperature measurements during the Medieval Warm Period of 1,000 years ago. How hot were some of the summer days back then? No one knows. Weather changes, which leads me to my next point…
2. Summer Heat Waves are Weather-Related, and Unusual Cold is Usually Nearby
The recent excessive heat in Europe wasn’t caused by summer air sitting there and cooking in a bath of increased human-emitted carbon dioxide. It was caused by a Saharan Air Layer (SAL) flowing in from that gigantic desert to the south.
The SAL event flowed north from the Sahara Desert to cover western Europe while a cold air mass flowed south over eastern Europe. As evidence of just how large natural weather variations can be, the full range of temperature departures from normal just over this small section of the world spanned 25C (45F).
Meanwhile, the global average temperature anomaly for June (from NOAA’s Climate Forecast System, CFSv2 model) was only +0.3C (0.5F).
Do you see the disparity between those two numbers? Weather-related temperature variations of 45F versus a climate-related global average “warmth” of only 0.5F. Thus, when we talk of new temperature records, we should be looking at normal weather variations first.
3. Most Thermometer Measurements Have Been Spuriously Warmed by the Urban Heat Island Effect
I am thoroughly convinced that the global thermometer record has exaggerated warming trends due to the Urban Heat Island (UHI) effect. When natural vegetation is replaced with buildings, pavement, and we add spurious heat sources like air conditioning units, cars, and ice cream trucks, the microclimate around thermometer sites changes.
Many of us experience this on a daily basis as we commute from more rural surroundings to our jobs in more urban settings.
For example, Miami International Airport recently set a new high temperature record of 98F for the month of May. The thermometer in question is at the west end of the south runway at the airport, at the center of the Miami-Ft. Lauderdale metroplex. Only 120 years ago, virtually no one lived in Miami; in 1896 it had a population of 300.
The UHI effect is so strong and pervasive that it is now included in the GFS weather forecast model, and in the case of Miami’s recent hot spell, we see the metroplex at midnight was nearly 10F warmer than the rural surroundings:
When a thermometer site has that kind of spurious warming at night, it’s going to produce spuriously warm temperatures during the day (and vice versa).
The most thorough analysis of the UHI effect on U.S. temperature was by Anthony Watts and co-authors, who analyzed the siting of hundreds of thermometers around the U.S. and showed that if only the best (most rural) sited thermometers are used, U.S. warming trends are roughly cut in half.
Curiously, they found that the official NOAA-adjusted temperature data (which uses both urban and rural data) has even more warming than if no UHI adjustments were made, leading many of us to conclude that the NOAA UHI adjustment procedure has made the rural data look like urban, rather than the other way around as it should be.
How does this impact the recent record high temperatures in France?
There is no question that temperatures were unusually hot, I’m only addressing the reasons why high temperature records are set. I’ve already established that (1) record high temperatures will occur without global warming; (2) weather variations are the primary cause (in this case, an intrusion of Saharan air), and now (3) many thermometer sites have experienced spurious warming.
On this third point, this MeteoFrance page lists the temperature records from the event, and one location (Mont Aigoua) caught my eye because it is a high altitude observatory with little development, on a peak that would be well-ventilated. The previous high temperature record there from 1923 [solar minimum of cycle 15] was beat by only 0.5C.
Some of the other records listed on that page are also from the early 20th Century, which naturally begs the question of how it could have been so hot back then with no anthropogenic greenhouse effect and little urban development.
The bottom line is that record high temperatures occur naturally, with or without climate change, and our ability to identify them has been compromised by spurious warming in most thermometer data which has yet to be properly removed.
By ROY W. SPENCER, Ph. D. — http://www.drroyspencer.com
Roy W. Spencer (Ph.D. Meteorology) is a weather and climate researcher at the University of Alabama in Huntsville.