Klimageschichte der USA: Die Jahrtausend-Dürre von 1934

Extremwetter ist ein wiederkehrendes Problem auf der Erde. Keine Region ist dagegen immun. Das hält Klimaaktivisten nicht davon ab, die Ereignisse zu instrumentalisieren. Bei jeder Dürre, Starkregen, Hitzewelle oder Kältewelle stürmen die interessierten Klimaapokalyptiker zu den Mikrofonen und warnen mit besorgter Miene vor einer kommenden Steigerung des Klimaübels. Dagegen kann man wenig machen, denn die Damen und Herren Journalisten dürsten nach Sensationen. Eine fruchtbare Zusammenarbeit. Dazu kommt dann noch die Politik, die mit Hinweis auf die vermeintliche Klimabedrohung unpopuläre Einschnitte durchboxt. Angesichts dieser eng verbandelten Interessensgemeinschaft bleibt uns nichts weiter übrig, uns an dieser Stelle auf die wissenschaftlichen Fakten zu beschränken. Mehr können wir nicht tun.

Heute blicken wir auf das Dürregeschehen in den USA. Was gibt es Neues aus diesem Teil der Welt? Aus dem Februar 2017 stammt eine Arbeit von McCabe et al. im International Journal of Climatology. Fazit: Es gibt große regionale Unterschiede. Insgesamt sind Dürren jedoch seltener geworden:

…for most of the [conterminous United States], drought frequency appears to have decreased during the 1901 through 2014 period.

Zudem konnte über die letzten 100 Jahre ein langfristiger Rückgang bei den sogenannten Flash-Hitzwellen in den USA festgestellt werden, wie Mo & Lettenmaier 2015 berichteten:

Heat wave flash droughts in decline
Flash drought is a term that was popularized during rapidly evolving droughts in the Central U.S. in 2012 that were associated with heat waves. We posit that there are two kinds of flash droughts, and we will focus on heat wave flash droughts, of which the 2012 events were typical. We find, based on an analysis of temperature observations and model-reconstructed soil moisture (SM) and evapotranspiration from 1916 to 2013, that heat wave flash droughts in the conterminous U.S. (CONUS) are most likely to occur over the Midwest and the Pacific Northwest during the growing season. We also find that the number of such events across the CONUS has been decreasing over the last century but rebounded after 2011. The long-term downward trends appear to be associated with generally increasing trends in SM resulting from increasing trends in precipitation over the areas where heat wave flash droughts are most likely to occur.

Richard Seager and Martin Hoerling wiesen bereits im Juni 2014 im Journal of Climate darauf hin, dass ein großer Teil der nordamerikanischen Dürreentwicklung durch Ozeanzyklen gesteuert wird:

Atmosphere and Ocean Origins of North American Drought
The atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada. Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. Oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of internal atmospheric variability. Up to half of the soil moisture deficits during severe droughts in the southeast United States in 2000, Texas in 2011, and the central Great Plains in 2012 were related to SST forcing, although SST forcing was an insignificant factor for northern Great Plains drought in 1988. During the early twenty-first century, natural decadal swings in tropical Pacific and North Atlantic SSTs have contributed to a dry regime for the United States. Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts.

Auch Kam et al. 2014 sehen in den Ozeanzyklen einen wichtigen Steuerungmechanismus für die USA:

Changes in drought risk over the contiguous United States (1901–2012): The influence of the Pacific and Atlantic Oceans
We assess uncertainties in the influence of sea surface temperatures on annual meteorological droughts over the contiguous U.S. within a Bayesian approach. Observational data for 1901–2012 indicate that a negative phase of the Pacific Decadal Oscillation (PDO) and the El Niño–Southern Oscillation (ENSO) elevated annual drought risk over the southern U.S., such that the 4 year return period event becomes a 3 year event, while a positive phase of the Atlantic Multidecadal Oscillation has a weak influence. In recent decades, the impacts of the negative phases of the PDO and ENSO on U.S. drought have weakened and shifted toward the southwestern U.S. These changes indicate an increasing of role of atmospheric variability on the U.S. drought overall with implications for long-term changes in drought and the potential for seasonal forecasting.

Abschließend noch ein Paper von Cook et al. 2014 über die schlimmste Dürre Nordamerikas des letzten Jahrtausends. Sie stammt nicht etwas aus den letzten Jahren sondern ereignete sich bereits 1934:

The worst North American drought year of the last millennium: 1934
During the summer of 1934, over 70% of western North America experienced extreme drought, placing this summer far outside the normal range of drought variability and making 1934 the single worst drought year of the last millennium. Strong atmospheric ridging along the West Coast suppressed cold season precipitation across the Northwest, Southwest, and California, a circulation pattern similar to the winters of 1976–1977 and 2013–2014. In the spring and summer, the drying spread to the Midwest and Central Plains, driven by severe precipitation deficits downwind from regions of major dust storm activity, consistent with previous work linking drying during the Dust Bowl to anthropogenic dust aerosol forcing. Despite a moderate La Niña, contributions from sea surface temperature forcing were small, suggesting that the anomalous 1934 drought was primarily a consequence of atmospheric variability, possibly amplified by dust forcing that intensified and spread the drought across nearly all of western North America.

Hier noch die dazugehörige Pressemitteilung der NASA aus dem Oktober 2014:

NASA Study Finds 1934 Had Worst Drought of Last Thousand Years

A new study using a reconstruction of North American drought history over the last 1,000 years found that the drought of 1934 was the driest and most widespread of the last millennium.

Using a tree-ring-based drought record from the years 1000 to 2005 and modern records, scientists from NASA and Lamont-Doherty Earth Observatory found the 1934 drought was 30 percent more severe than the runner-up drought (in 1580) and extended across 71.6 percent of western North America. For comparison, the average extent of the 2012 drought was 59.7 percent. „It was the worst by a large margin, falling pretty far outside the normal range of variability that we see in the record,“ said climate scientist Ben Cook at NASA’s Goddard Institute for Space Studies in New York. Cook is lead author of the study, published in Geophysical Research Letters.

Two sets of conditions led to the severity and extent of the 1934 drought. First, a high-pressure system in winter sat over the west coast of the United States and turned away wet weather – a pattern similar to that which occurred in the winter of 2013-14. Second, the spring of 1934 saw dust storms, caused by poor land management practices, suppress rainfall. „In combination then, these two different phenomena managed to bring almost the entire nation into a drought at that time,“ said co-author Richard Seager, professor at the Lamont-Doherty Earth Observatory of Columbia University in New York. „The fact that it was the worst of the millennium was probably in part because of the human role.“

According to the recent Fifth Assessment Report of the Intergovernmental Panel on Climate Change, or IPCC, climate change is likely to make droughts in North America worse, and the southwest in particular is expected to become significantly drier as are summers in the central plains. Looking back one thousand years in time is one way to get a handle on the natural variability of droughts so that scientists can tease out anthropogenic effects – such as the dust storms of 1934. “We want to understand droughts of the past to understand to what extent climate change might make it more or less likely that those events occur in the future,“ Cook said. The abnormal high-pressure system is one lesson from the past that informs scientists‘ understanding of the current severe drought in California and the western United States.

„What you saw during this last winter and during 1934, because of this high pressure in the atmosphere, is that all the wintertime storms that would normally come into places like California instead got steered much, much farther north,” Cook said. “It’s these wintertime storms that provide most of the moisture in California. So without getting that rainfall it led to a pretty severe drought.“ This type of high-pressure system is part of normal variation in the atmosphere, and whether or not it will appear in a given year is difficult to predict in computer models of the climate. Models are more attuned to droughts caused by La Niña’s colder sea surface temperatures in the Pacific Ocean, which likely triggered the multi-year Dust Bowl drought throughout the 1930s. In a normal La Niña year, the Pacific Northwest receives more rain than usual and the southwestern states typically dry out.

But a comparison of weather data to models looking at La Niña effects showed that the rain-blocking high-pressure system in the winter of 1933-34 overrode the effects of La Niña for the western states. This dried out areas from northern California to the Rockies that otherwise might have been wetter. As winter ended, the high-pressure system shifted eastward, interfering with spring and summer rains that typically fall on the central plains. The dry conditions were exacerbated and spread even farther east by dust storms. „We found that a lot of the drying that occurred in the spring time occurred downwind from where the dust storms originated,“ Cook said, „suggesting that it’s actually the dust in the atmosphere that’s driving at least some of the drying in the spring and really allowing this drought event to spread upwards into the central plains.“

Dust clouds reflect sunlight and block solar energy from reaching the surface. That prevents evaporation that would otherwise help form rain clouds, meaning that the presence of the dust clouds themselves leads to less rain, Cook said. „Previous work and this work offers some evidence that you need this dust feedback to explain the real anomalous nature of the Dust Bowl drought in 1934,“ Cook said. Dust storms like the ones in the 1930s aren’t a problem in North America today. The agricultural practices that gave rise to the Dust Bowl were replaced by those that minimize erosion. Still, agricultural producers need to pay attention to the changing climate and adapt accordingly, not forgetting the lessons of the past, said Seager. „The risk of severe mid-continental droughts is expected to go up over time, not down,“ he said.

Reference: Cook, B.I., R. Seager, and J.E. Smerdon, 2014: The worst North American drought year of the last millennium: 1934. Geophys. Res. Lett., 41, no. 20, 7298-7305, early on-line, doi:10.1002/2014GL061661.