Research Links Extreme Summer Heat Events to Global Warming
Aug. 6, 2012
You may have already seen this reported.
"A new statistical analysis by NASA scientists has found that Earth's land areas have become much more likely to experience an extreme summer heat wave than they were in the middle of the 20th century."
The journal Proceedings of the National Academy of Sciences.
Earth's Northern Hemisphere over the past 30 years has seen more "hot" (orange), "very hot" (red) and "extremely hot" (brown) summers, compared to a base period defined in this study from 1951 to 1980. This visualization shows how the area experiencing "extremely hot" summers grows from nearly nonexistent during the base period to cover 12 percent of land in the Northern Hemisphere by 2011. Watch for the 2010 heat waves in the Middle East, Western Asia and Eastern Europe, or the 2011 heat waves in Texas, Oklahoma and Mexico. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio
Recent years of extremely warm summers, including the Midwest this year, are quite likely due to global warming, according to lead author James Hansen of NASA's Goddard Institute for Space Studies (GISS) in New York.
"This summer people are seeing extreme heat and agricultural impacts," Hansen says. "We're asserting that this is causally connected to global warming, and in this paper we present the scientific evidence for that."
Summer temperatures since 1951 were studied. The odds have increased in recent decades for what they define as "hot," "very hot" and "extremely hot" summers.
The "extremely hot" summers occur more often. They define "extremely hot" as a mean summer temperature experienced by less than 1% of Earth's land area between 1951 and 1980. This 30 year span is the base period for the study. Since 2006, about 10% of land area across the Northern Hemisphere has experienced these "extremely hot" temperatures each summer.
The day after Hansen's publication, a follow-up analysis was posted August 7 in the Earth Observatory Blog called Earth Matters. Written by Adam Voiland, the blog post discusses the significance of the temperature extremes data profile when compared to the standard bell curve so many of us are familiar with from statistics studies. There is discussion of the standard deviation notation (σ) and how it is interpreted in this case of summer temperatures in the northern hemisphere.
Come below the fold for more of those details.
What is a Bell Curve and Standard Deviation?
Sigma (σ) is a letter of the Greek alphabet. It is used in statistics to represent standard deviation
. It essentially describes how much data is spread out from the average, or mean. A plot of a normal distribution of data typically yields the familiar bell curve. Here are six examples of normal distributions with different standard deviations. Low standard deviation, in blue, says the data points are bunched up and close to the mean. High standard deviation, in yellow, says the data is more spread over a wider range.
Click image for source.
In a standard bell curve, 68% of the data points fall within one standard deviation (1σ) of the mean. While 95% are within two standard deviations (2σ). Three standard deviations (3σ) of the mean will contain 99.7% of the data. Very few points will lie beyond 3σ. Those are the characters that make up the extreme right wing and left wing fringe. The scientists reporting the finding of the Higgs Boson spoke of data within 3σ and how certain they were of it.
How is Temperature Related?
Now, substitute temperature variations from the mean, or anomalies, into a plot. The graph on the left shows how frequently summer temperature anomalies occurred in the 30-year base period 1951-1980, a time of stable global climate. The standard deviation σ was 0.6°C (1.1°F) in 1951-1980. Or, 68% of the variations measured were within 0.6˚C of the mean. By the next decade of 1981-1991, the peak of values shifted toward the right indicating warmer temperature anomalies. Each successive decade has shifted the curve more to the warmer end of the curve. In addition, there are many more temperature anomalies beyond the 3σ tail at the right end of the graph. And, the curve is more spread out indicating larger standard deviation of temperature anomalies.
The surface temperatures have increase over the recent 3 decades. And, the number of extreme high heat events beyond 3σ has increased.
As stated by Hansen...
We have shown that these “3-sigma” (3σ) events, where σ is the standard deviation — seasons more than three standard deviations removed from “normal” climate — are a consequence of the rapid global warming of the past 30 years. Combined with the well-established fact that the global warming is a result of increasing atmospheric CO2 and other greenhouse gases, it follows that the increasingly extreme climate anomalies are human-made.
Below is an animation of the data presented by Hansen and his colleagues.
Q&A with James Hansen
Hansen supplied a number of answers and explanations which you can access at this link
. The link is to a 4 page pdf titled Q&A of The New Climate Dice
. He addresses several questions, in particular, how the coming years are going to be like rolling a loaded dice for extremes of high temperature anomalies. They are going to be more frequent and more severe. There will still be the occasional colder than normal year. But, hotter than normal will be more frequent and worse. Not a good prospect.
Some questions he considers are...
1. What is the most important finding of the paper?
2. Why is such an anomaly important? Isn't it just a few degrees warmer than average?
3. Didn't 3-sigma events occur in the past?
4. So you can use your old metaphor of "loaded" climate dice to describe the situation?
5. Why are you also introducing the "bell curve?" Isn't that too esoteric for the public?
6. How is the "bell curve" related to "loaded climate dice?"
7. You note that the bell curve has become "squashed". Is that important?
8. How do you know that the bell curve will continue to shift to the right?
9. What are consequences of the increasing extremes?
10. Are we necessarily going to see more and more extreme climate? Gloom and doom?
11. Could we just redefine what is normal climate, obtaining a new symmetric bell curve?
12. Did you write this paper and your 1988 paper because of the extreme droughts?
13. Are there other effects that should be noticeable, besides the climate extremes?