Hansen et al.: “Extreme Heat Waves … in Texas and Oklahoma in 2011 and Moscow in 2010 Were ‘Caused’ by Global Warming”

by Joe Romm, Climate Progress, January 6, 2012

“Climate dice,” describing the chance of unusually warm or cool seasons relative to climatology, have become progressively “loaded” in the past 30 years, coincident with rapid global warming.   The distribution of seasonal mean temperature anomalies has shifted toward higher temperatures and the range of anomalies has increased.  An important change is the emergence of a category of summertime extremely hot outliers, more than three standard deviations (σ) warmer than climatology.

This hot extreme, which covered much less than 1% of Earth’s surface in the period of climatology [1951-1980], now typically covers about 10% of the land area.  We conclude that extreme heat waves, such as that in Texas and Oklahoma in 2011 and Moscow in 2010, were “caused” by global warming, because their likelihood was negligible prior to the recent rapid global warming.  We discuss practical implications of this substantial, growing climate change.

That’s the finding of a detailed climatological analysis by NASA’s James Hansen along with Makiko Sato and Reto Ruedy in which they attribute some of the uber-extreme heat waves to global warming.

Here’s a key figure from “Perceptions of Climate Change: The New Climate Dice“:

Percent area covered by temperature anomalies in categories defined as hot (> 0.43σ), very hot (> 2σ), and extremely hot (> 3σ).  Anomalies are relative to 1951-1980.  A normal distribution of variability has 68% of the anomalies falling within one standard deviation (σ) of the mean value.  The tails decrease quite rapidly so there is only a 2.3% chance of the temperature exceeding +2σ.  The chance of exceeding +3σ is only 0.13% for a normal distribution of variability.

This analysis builds on some of the recent new papers on the subject, such as “Study Finds 80% Chance Russia’s 2010 July Heat Record Would Not Have Occurred Without Climate Warming” [see figure below]

The entire Hansen et al paper is a must-read.  The authors explain why they focus on summer:

Summer, when most biological productivity occurs, is the most important season for humanity and thus the season when climate change may have its biggest impact.  Global warming causes spring warmth to come earlier and it causes cooler conditions that initiate fall to be delayed.  Thus global warming not only increases summer warmth, it also protracts summer-like conditions, stealing from both spring and fall.  Our study therefore places emphasis on study of how summer temperature anomalies have been changing.

The paper also explains the ‘dice’ metaphor and why they are not fans of using a new climatological period, such as 1981-2010 in place of 1951-1980.  I will excerpt some key parts and post some key figures.

First, you may be wondering why the top chart of summer hot area percentage doesn’t have as clear a trend for the United States as it does for  North America or the globe.  As the authors explain:

The small area of the contiguous 48 states (less than 1.6% of the globe) causes temperature anomalies for the United States to be very “noisy.”  Nevertheless, it is apparent that the long-term trend toward hot summers is not as pronounced in the United States as it is in hemispheric land as a whole.  Also note that the extreme summer heat of the 1930s,  especially 1934 and 1936, is comparable to the most extreme recent years.

Year-to-year variability, which is mainly unforced weather variability, is so large for an area the size of the United States that it is perhaps unessential to find an “explanation” for either the large 1930s anomalies or the relatively slow upturn in hot anomalies during the past few decades.  However, this matter warrants discussion, because, if the absence of a stronger warming in recent years is a statistical fluke, the United States may have in store a relatively rapid trend toward more extreme anomalies.

Some researchers have suggested that the high summer temperatures and drought in the United States in the 1930s can be accounted for by sea surface temperature patterns plus natural variability (10, 11).  Other researchers (12-14), have presented evidence that agricultural changes and crop failure in the 1930s contributed to changed surface albedo, aerosol (dust) production, high temperatures, and drying conditions.  Furthermore, both empirical evidence and climate simulations (14, 15) indicate that agricultural irrigation has a significant regional cooling effect. Thus increasing amounts of irrigation over the second half of the 20th century may have contributed a summer cooling tendency in the United States that partially offset greenhouse  warming.  Such regionally-varying effects may be partly responsible for differences between observed regional temperature trends and the global trend.

They explain the “loaded climate dice” metaphor:

“Loading” of the “climate dice” describes the systematic shift of the frequency distribution of temperature anomalies.  Hansen et al. (2) represented the climate of  1951-1980 by colored dice with two sides colored red for “hot,” two sides blue for “cold,” and two sides white for near average temperatures.  With a normal distribution of temperatures the dividing point would be at 0.43σ to achieve equal (one third) chances of being in each of these three categories in the period of climatology (1951-1980).

A climate model was used (2) to project how the odds would change due to global warming for alternative greenhouse gas scenarios.  Scenario B, which had climate forcing that turned out  to be very close to reality, led to four of the six dice sides being red early in the 21st century based on global climate model simulations.

Fig. 5 confirms that the global occurrence of “hot” anomalies (seasonal mean temperature anomaly exceeding +0.43σ) has approximately reached the level of 67% required to make four sides of the dice red, with the odds of either an unusually “cool” season or an “average” season now each approximately corresponding to one side of the six-sided dice.  However, the loading of the dice over land area in summer is even stronger (Fig. 5, lower row).

Fig. 5. Area of the world covered by temperature anomalies in the categories defined as hot (> 0.43σ), very hot (> 2σ), and extremely hot (> 3σ), with analogous divisions for cold anomalies.

Probably the most important change is the emergence of a new category of “extremely hot” summers, more than 3σ warmer than climatology.  For practical purposes it is important to look at the changes over land areas, where most people live, rather than the global mean for which anomalies are more constrained by the ocean’s thermal inertia.  Fig. 6 illustrates that +3σ anomalies practically did not exist in the period of climatology (1951-1980), but in the past several years these extreme anomalies have covered of the order of 10% of the land area.

… Warming is larger in winter than in summer, but this tends to be more than offset by the much larger natural variability in winter (Fig. 2), which makes it harder for the public to notice climate change in winter.  Another factor affecting the public’s perception of winter warming is the fact that snowfall amounts increase with global warming (in regions remaining cold enough for snow), and there is a tendency of the public to equate heavy snowfall and harsh winter conditions, even if temperatures are not extremely low.

The increase, by more than a factor 10, of area covered by extreme hot anomalies (> +3σ ) in summer reflects the shift of the anomaly distribution in the past 30 years of global warming, as shown succinctly in Fig. 4.   One implication of this shift is that the extreme summer climate anomalies in Texas in 2011, in Moscow in 2010, and in France in 2003 almost certainly would not have occurred in the absence of global warming with its resulting shift of the anomaly distribution.  In other words, we can say with a high degree of confidence that these extreme anomalies were a consequence of global warming…

It is not uncommon for meteorologists to reject global warming as a cause of these extreme events, offering instead a meteorological explanation.  For example, it is said that the Moscow heat wave was caused by an atmospheric “blocking” situation, or the Texas heat wave was caused by La Nina ocean temperature patterns.  Certainly the locations of the extreme anomalies in any given case are related to specific weather patterns.  However, blocking patterns and La Ninas have always been common, yet the large areas of extreme warming have come into existence only with large global warming. Today’s extreme anomalies occur because of simultaneous contributions of specific weather patterns and global warming.

The paper notes that warming leads to drying (and heavy precipitation):

Changes of global temperature are likely to have their greatest practical impact via effects on the hydrologic cycle.  Amplification of hot, dry conditions by global warming is expected, based on qualitative considerations.  For example, places experiencing an extended period of high atmospheric pressure develop dry conditions, which we would expect to be amplified by global warming and by ubiquitous surface heating due to elevated greenhouse gas amounts.

See “Nature Publishes My Piece on Dust-Bowlification and the Grave Threat It Poses to Food Security” for  some of the recent literature on drying.  See also NOAA Bombshell: Human-Caused Climate Change Already a Major Factor in More Frequent Mediterranean Droughts; “The magnitude and frequency of the drying that has occurred is too great to be explained by natural variability alone,” said lead author Martin Hoerling, Ph.D. of NOAA’s Earth System Research Laboratory [see figure]:

Reds and oranges highlight lands around the Mediterranean that experienced significantly drier winters during 1971-2010 than the comparison period of 1902-2010.  [Click to enlarge.]

And, of course, Hansen et al. note that warming leads wet areas to get wetter

The other extreme of the hydrologic cycle, unusually heavy rainfall and floods, is also expected to be amplified by global warming.  The amount of water vapor that the atmosphere holds increases rapidly with atmospheric temperature, and thus a warmer world is expected to  have more rainfall occurring in more extreme events.  What were “100-year” or “500-year”  events are expected to occur more frequently with increased global warming.  Rainfall data reveal significant increases of heavy precipitation over much of Northern Hemisphere land and in the tropics (3) and attribution studies link this intensification of rainfall and floods to humanmade global warming.

See “Two seminal Nature papers join growing body of evidence that human emissions fuel extreme weather, flooding that harm humans and the environment.”

Their bottom line:

If global warming approaches 3°C by the end of the century, it is estimated that 21-52% of the species on Earth will be committed to extinction (3).  Fortunately, scenarios are also possible in which such large warming is avoided by placing a rising price on carbon emissions that moves the world to a clean energy future fast enough to limit further global warming to several tenths of a degree Celsius (29).  Such a scenario is needed if we are to preserve life as we know it.

They don’t even contemplate the 4C to 5C+  warming we are projected to see if we stay anywhere near our current emissions path.

The time to act is now.

http://thinkprogress.org/romm/2012/01/06/399350/hansen-extreme-heat-waves-texas-oklahoma-moscow-were-caused-by-global-warming/