With all the
heat wave news coming out of the US at the moment, I have been curious about checking out the comparison with the 1930's "dust bowl" situation. For climate change skeptics, or the "lukewarmers" amongst them, it is important to claim that this present heat wave may all just be part of a natural cycle that the country last saw in the 1930's, so let's just not assume that it is something to do with CO2.
Well, I think it's important to note a few things:
a. Yes, the temperatures were very hot, and drought conditions very bad, in the 1930's in the US. The
EPA says so, and
James Hansen and colleagues wrote a 1999 NASA entry which starts:
What's happening to our climate? Was the heat wave and drought
in the Eastern United States in 1999 a sign of global warming?
Empirical evidence does not lend much support to the notion that
climate is headed precipitately toward more extreme heat and drought.
The drought of 1999 covered a smaller area than the 1988 drought,
when the Mississippi almost dried up. And 1988 was a temporary inconvenience
as compared with repeated droughts during the 1930s "Dust Bowl"
that caused an exodus from the prairies, as chronicled in
Steinbeck's Grapes of Wrath.
How can the absence of clear climate change in the United
States be reconciled with continued reports of record global
temperature?
b. The US mainland is not the globe. Continuing on with the above quote, Hansen writes:
Part of the "answer" is that U.S. climate has been
following a different course than global climate, at least so
far. Figure 1 compares the temperature history in the U.S. and
the world for the past 120 years. The U.S. has warmed during
the past century, but the warming hardly exceeds year-to-year
variability. Indeed, in the U.S. the warmest decade was the
1930s and the warmest year was 1934. Global temperature, in
contrast, had passed 1930s values by 1980 and the world has
warmed at a remarkable rate over the last 25 years.
c. A
2004 paper by Schubert and others looking at the causes of the drought side of the 1930's starts off by noting that there is long on-going cycle of drought in the mid West:
Drought in the Great Plains is not unique to the last century. A number of proxy climate records indicate that multiyear droughts comparable to those of the 1930s and 1950s are, in fact, a regular feature of the Great Plains climate, having occurred approximately once or twice a century over the last 400 years (Woodhouse and Overpeck 1998). Looking still further backin time, there is evidence for multidecadal droughts during the late thirteenth and sixteenth centuries that were of much greater severity and duration than those of the twentieth century (Woodhouse and Overpeck 1998). For example, tree-ring analyses in Nebraska suggest that the drought that began in 1276 lasted 38 years (Bark 1978)!
In retrospect, it seems a bit of a silly place to build large cities. Anyway, the Schubert paper looked at sea temperatures and circulation models, and I think paints it as a bit of natural variation. However, there are possibly more factors involved. James Hansen's recent 2011
"New Climate Dice" paper refers to the 1930's as follows:
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.
d. I think it is important to note that there is certainly no avoidance of discussing the 1930's US temperature record by Hansen at all, as this section from his paper (actually preceeding the above quote) shows:
Jun-Jul-Aug data on a longer time scale, 1900-present, including results averaged over the conterminous United States, are shown in Fig. 7. The longer time scale is useful for examining changes in the United States, because of well-known extreme heat and droughts of the 1930s. 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.
I might add more to this post later, but at the very least, I wanted to make the point that it is not being ignored, and climate researchers acknowledge how bad the 1930's were in terms of both drought and heat in the US. I think you can get the impression from climate skeptic blogs that they do.
