Keine langfristige Zunahme von Hitzewellen in Nordamerika

Es ist ein eingespieltes Muster: Nach dem Auftreten einer Dürre, Hitzewelle oder Überschwemmung überschlagen sich Journalisten und Klimaalarmisten mit schrillen Warnungen, dass dies ja nur der Anfang sei und man jetzt Gewissheit habe, dass der böse Klimawandel hinter dem Ereignis stecke. Wir haben es diesen Sommer (2018) mit der mitteleuropäischen Hitzewelle wieder miterlebt. Belastbare Statistiken mit der Häufigkeit von Hitzewellen während der letzten 100 und 1000 Jahre bleiben die Akteure in der Regel schuldig.

Wir helfen hier gerne aus und stellen Ihnen neueste Ergebnisse zur Entwicklung der Hitzewellen in Nordamerika vor. Wir beginnen mit einer Frühlings-Hitzewelle in den USA von 2012, die Dole & Hoerling (2014) in einen langjährigen Kontext einordneten. Die Autoren sehen einen rein natürlichen Auslöser der ungewöhnlichen Hitze:

The Making of an Extreme Event: Putting the Pieces Together
We examine how physical factors spanning climate and weather contributed to record warmth over the central and eastern United States in March 2012, when daily temperature anomalies at many locations exceeded 20°C. Over this region, approximately 1°C warming in March temperatures has occurred since 1901. This long-term regional warming is an order of magnitude smaller than temperature anomalies observed during the event, indicating that most of the extreme warmth must be explained by other factors. Several lines of evidence strongly implicate natural variations as the primary cause for the extreme event. The 2012 temperature anomalies had a close analog in an exceptionally warm U.S. March occurring over 100 years earlier, providing observational evidence that an extreme event similar to March 2012 could be produced through natural variability alone. Coupled model forecasts and simulations forced by observed sea surface temperatures (SSTs) show that forcing from anomalous SSTs increased the probability of extreme warm temperatures in March 2012 above that anticipated from the long-term warming trend. In addition, forcing associated with a strong Madden–Julian oscillation further increased the probability for extreme U.S. warmth and provided important additional predictive information on the timing and spatial pattern of temperature anomalies. The results indicate that the superposition of a strong natural variation similar to March 1910 on longterm warming of the magnitude observed would be sufficient to account for the record warm March 2012 U.S. temperatures. We conclude that the extreme warmth over the central and eastern United States in March 2012 resulted primarily from natural climate and weather variability— a substantial fraction of which was predictable.

Auch bei der Sommerhitze lohnt sich ein Rückblick auf die vergangenen 100 Jahre. Die 1930er Jahre waren in den USA besonders reich an Hitzewellen, wie die offiziellen USHCN-Daten zeigen (z.B. Beiträge von Judith Curry, Steven Goddard, John Christy, Anthony Watts). Die heutige Zeit ist daher nicht extremer als die Vergangenheit, wie auch Kunkel et al. 2014 beschrieben:

Is the monthly temperature climate of the United States becoming more extreme?
A new data set of monthly temperatures, adjusted for detected inhomogeneities, was used to examine whether the monthly temperature climate of the U.S. has become more extreme. During the past two to three decades, there has been a shift toward more frequent very warm months, but less frequent very cold months. Thus, overall the monthly temperature climate has not become more extreme. Midtwentieth century including the 1930s was an earlier period of frequent very warm months, a result of very warm daytime temperatures, while nighttime temperatures were not unusual. Regionally, there is a lack of century‐scale warming in the southeast U.S. annually and in parts of the central U.S. in the summer, characterized by lack of daytime warming while there has been nighttime warming. Compared to the earlier midcentury warm period, recent decades have been more (less) extreme in the summer (winter) in the west while Midwest summers have been less extreme.

Auch sogenannte ‘Flash Droughts’ sind in den USA offenbar auf dem Rückzug, wie Mo & Lettenmaier 2015 zeigen konnten:

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.

Im Mittleren Westen der USA sind die sommerlichen Extremtemperaturen gesunken, was Mueller et al. 2016 der Landwirtschaft zuschreiben:

Cooling of US Midwest summer temperature extremes from cropland intensification
High temperature extremes during the growing season can reduce agricultural production. At the same time, agricultural practices can modify temperatures by altering the surface energy budget. Here we identify centennial trends towards more favourable growing conditions in the US Midwest, including cooler summer temperature extremes and increased precipitation, and investigate the origins of these shifts. Statistically significant correspondence is found between the cooling pattern and trends in cropland intensification, as well as with trends towards greater irrigated land over a small subset of the domain. Land conversion to cropland, often considered an important influence on historical temperatures, is not significantly associated with cooling. We suggest that agricultural intensification increases the potential for evapotranspiration, leading to cooler temperatures and contributing to increased precipitation. The tendency for greater evapotranspiration on hotter days is consistent with our finding that cooling trends are greatest for the highest temperature percentiles. Temperatures over rainfed croplands show no cooling trend during drought conditions, consistent with evapotranspiration requiring adequate soil moisture, and implying that modern drought events feature greater warming as baseline cooler temperatures revert to historically high extremes.

Leary et al. 2015 wiesen an einem Beispiel aus Florida darauf hin, dass es nicht mal gegenwärtig genug Daten gibt, um eine Hitzewelle genau genug zu beschreiben. Ist das nicht seltsam: Modelle sagen genau voraus, um wieviel Hitzewellen stärker werden, aber die Grundlage heute ist nicht genug bekannt. Zitat aus der Studie:

“  This study highlights challenges in creating a general methodology to identify periods of extreme heat for Florida….For future studies, it is recommended to use a spatio-temporal model to impute missing values, leading to more precise estimates of percentiles and more accurate identification of heat waves.”

Scannell et al. 2016 analysierten Hitzewellen im Nordatlantik und Nordpazifik, sogenannte “marine heatwaves”. Einen allgemeinen Trend konnten sie für die letzten Jahrzehnte nicht feststellen:

Frequency of marine heatwaves in the North Atlantic and North Pacific since 1950
Extreme and large‐scale warming events in the ocean have been dubbed marine heatwaves, and these have been documented in both the Northern and Southern Hemispheres. This paper examines the intensity, duration, and frequency of positive sea surface temperature anomalies in the North Atlantic and North Pacific Oceans over the period 1950–2014 using an objective definition for marine heatwaves based on their probability of occurrence. Small‐area anomalies occur more frequently than large‐area anomalies, and this relationship can be characterized by a power law distribution. The relative frequency of large‐ versus small‐area anomalies, represented by the power law slope parameter, is modulated by basin‐scale modes of natural climate variability and anthropogenic warming. Findings suggest that the probability of marine heatwaves is a trade‐off between size, intensity, and duration and that region specific variability modulates the frequency of these events.

Und hier die dazugehörige Pressemitteilung der University of Washington.