Die USA sind ein großes und bevölkerungsreiches Land. Wetterextreme sind hier keine Seltenheit. Immer wieder ereignen sich schlimme Überschwemmungen, oft ausgelöst durch Starkregen. Es ist gute Tradition, dass das klimaalarmistische Lager nach einem Flutereignis öffentlich den Klimawandel als Schuldigen hinstellt. Es ist ebenso guter Brauch, dass Klimaskeptiker dies vehement abstreiten. Harte wissenschaftliche Daten fehlen bei diesen Spontanaktionen meist. Wir nehmen uns daher die Zeit, einmal die wissenschaftlich erhobenen Trends anzuschauen. Dies könnte der nächsten Runde im Blame-Game dann ein wenig mehr Substanz verleihen. Beginnen möchten wir mit einer Graphik, die von der NOAA bereitsgestellt wird. Sie zeigt die Entwicklung der Starkniederschläge für die letzten 100 Jahre in den kontinentalen USA (Abb. 1). Zu erkennen ist ein Anstieg, der etwa 1990 einsetzte.
Abb. 1: Entwicklung der Starkniederschläge für die letzten 100 Jahre in den kontinentalen USA. Quelle: NOAA.
Ob der Klimawandel hinter diesem Anstieg steckt? Was sagen andere Studien zu dieser Frage? Im Oktober 2014 veröffentlichten McCabe und Wolock in den Geophysical Research Letters eine Arbeit zu Abflussraten von US-Flüssen. Einen Langzeittrend in der maximalen Abflussrate konnten die Forscher nicht entdecken. Der Vergleich mit Standard-Klimaparametern verlief ebenfalls ergebnislos. Vorhersagen können daher keine abgegeben werden, erläutern die Wissenschaftler. Abstract:
Spatial and temporal patterns in conterminous United States streamflow characteristics
Spatial and temporal patterns in annual and seasonal minimum, mean, and maximum daily streamflow values were examined for a set of 516 reference stream gauges located throughout the conterminous United States for the period 1951–2009. Cluster analysis was used to classify the stream gauges into 14 groups based on similarity in their temporal patterns of streamflow. The results indicated that the temporal patterns in flow metrics (1) have strong spatial coherence within each region, (2) are similar among the three annual flow metrics and the four seasonal flow metrics within each region, (3) indicate some small magnitude trends over time, and (4) are only weakly associated with well-known climate indices. We conclude that most of the temporal variability in flow is unpredictable in terms of relations to climate indices and infer that, for the most part, future changes in flow characteristics cannot be predicted by these indices.
Einige Monate später, im Februar 2015, erschien in Nature Climate Change eine Studie von Iman Mallakpour und Gabriele Villarini zur Flutgeschichte der zentralen USA. Die Forscher konnte für das 20. und 21. Jahrhundert keine Veränderung der Stärke der Überschwemmungen feststellen, fanden jedoch Hinweise auf eine Steigerung der Flut-Häufigkeit. Abstract:
The changing nature of flooding across the central United States
In the twentieth and twenty-first centuries, flooding has taken a devastating societal and economic toll on the central United States, contributing to dozens of fatalities and causing billions of dollars in damage1, 2. As a warmer atmosphere can hold more moisture (the Clausius–Clapeyron relation), a pronounced increase in intense rainfall events is included in models of future climate3. Therefore, it is crucial to examine whether the magnitude and/or frequency of flood events is remaining constant or has been changing over recent decades. If either or both of these attributes have changed over time, it is imperative that we understand the underlying mechanisms that are responsible. Here, we show that while observational records (774 stream gauge stations) from the central United States present limited evidence of significant changes in the magnitude of floodpeaks, strong evidence points to an increasing frequency of flooding. These changes in flood hydrology result from changes in both seasonal rainfall and temperature across this region.
Ein weiteres Jahr später, im April 2016, eine Untersuchung von Hoerling et al. im Journal of Climate. Sie beschrieben eine Zunahme der Extremregenfälle im Norden und eine Abnahme im Süden während der vergangenen knapp 40 Jahre. Es ist also nicht so, dass die gesamten USA unter mehr Extremregen zu leiden hätten. Weiterhin fanden Hoerling und Kollegen, dass wohl die Ozeanzyklik hinter den Änderungen steckt. Eindeutige Hinweise auf eine anthropogene Beeinflussung gäbe es nicht. Abstract:
Characterizing Recent Trends in U.S. Heavy Precipitation
Time series of U.S. daily heavy precipitation (95th percentile) are analyzed to determine factors responsible for regionality and seasonality in their 1979–2013 trends. For annual conditions, contiguous U.S. trends have been characterized by increases in precipitation associated with heavy daily events across the northern United States and decreases across the southern United States. Diagnosis of climate simulations (CCSM4 and CAM4) reveals that the evolution of observed sea surface temperatures (SSTs) was a more important factor influencing these trends than boundary condition changes linked to external radiative forcing alone. Since 1979, the latter induces widespread, but mostly weak, increases in precipitation associated with heavy daily events. The former induces a meridional pattern of northern U.S. increases and southern U.S. decreases as observed, the magnitude of which closely aligns with observed changes, especially over the south and far west. Analysis of model ensemble spread reveals that appreciable 35-yr trends in heavy daily precipitation can occur in the absence of forcing, thereby limiting detection of the weak anthropogenic influence at regional scales. Analysis of the seasonality in heavy daily precipitation trends supports physical arguments that their changes during 1979–2013 have been intimately linked to internal decadal ocean variability and less so to human-induced climate change. Most of the southern U.S. decrease has occurred during the cold season that has been dynamically driven by an atmospheric circulation reminiscent of teleconnections linked to cold tropical eastern Pacific SSTs. Most of the northeastern U.S. increase has been a warm season phenomenon, the immediate cause for which remains unresolved.
Im November 2016 publizierten van der Wiel und Kollegen im Journal of Climate eine Modellierungstudie. Ganz am Ende des Abstracts das Hauptergebnis: Wie bereits Hoerling und Kollegen können auch van der Wiel et al. keine anthopogene Beeinflussung des bisherigen Extremregengeschehens in den USA finden.
The Resolution Dependence of Contiguous U.S. Precipitation Extremes in Response to CO2 Forcing
Precipitation extremes have a widespread impact on societies and ecosystems; it is therefore important to understand current and future patterns of extreme precipitation. Here, a set of new global coupled climate models with varying atmospheric resolution has been used to investigate the ability of these models to reproduce observed patterns of precipitation extremes and to investigate changes in these extremes in response to increased atmospheric CO2 concentrations. The atmospheric resolution was increased from 2° × 2° grid cells (typical resolution in the CMIP5 archive) to 0.25° × 0.25° (tropical cyclone permitting). Analysis has been confined to the contiguous United States (CONUS). It is shown that, for these models, integrating at higher atmospheric resolution improves all aspects of simulated extreme precipitation: spatial patterns, intensities, and seasonal timing. In response to 2 × CO2 concentrations, all models show a mean intensification of precipitation rates during extreme events of approximately 3%–4% K−1. However, projected regional patterns of changes in extremes are dependent on model resolution. For example, the highest-resolution models show increased precipitation rates during extreme events in the hurricane season in the U.S. Southeast; this increase is not found in the low-resolution model. These results emphasize that, for the study of extreme precipitation there is a minimum model resolution that is needed to capture the weather phenomena generating the extremes. Finally, the observed record and historical model experiments were used to investigate changes in the recent past. In part because of large intrinsic variability, no evidence was found for changes in extreme precipitation attributable to climate change in the available observed record.
Schließlich dann Slater & Villarini im Dezember 2016 in den Geophysical Research Letters. In der dazugehörigen Pressemitteilung der University of Iowa steht die Hauptnachricht gleich im Untertitel. Das Flutrisiko steigt im Norden und sinkt im Süden der USA. Das wussten wir aber schon aus der Hoerling-Studie. Hier die Pressemitteilung:
Flood threats changing across US
University of Iowa study finds flood risk growing in the North, declining in the South
A University of Iowa study has found that the risk of flooding is changing in the United States, and the changes vary regionally. The threat of moderate flooding is generally increasing in the northern U.S. (red areas) and decreasing in the southern U.S. (blue areas), while some regions remain mostly unchanged (gray areas). The findings come from comparing river heights at 2,042 locations with NASA satellite information showing the amount of water stored in the ground. The study was published in the journal “Geophysical Research Letters.” Image courtesy of the American Geophysical Union.
The risk of flooding in the United States is changing regionally, and the reasons could be shifting rainfall patterns and the amount of water in the ground. In a new study, University of Iowa engineers determined that, in general, the threat of flooding is growing in the northern half of the U.S. and declining in the southern half. The American Southwest and West, meanwhile, are experiencing decreasing flood risk. UI engineers Gabriele Villarini and Louise Slater compiled water-height information between 1985 and 2015 from 2,042 stream gauges operated by the U.S. Geological Survey. They then compared the data to satellite information gathered over more than a dozen years by NASA’s Gravity Recovery and Climate Experiment (GRACE) mission showing “basin wetness,” or the amount of water stored in the ground.
What they found was the northern sections of the country, generally, have an increased amount of water stored in the ground, and thus are at greater risk for minor and moderate flooding, two flood categories used by the National Weather Service. Meanwhile, minor to moderate flood risk was decreasing in the southern portions of the U.S., where stored water has declined. (See the above map.) Not surprisingly, the NASA data showed decreased stored water—and reduced flood risk—in the Southwest and western U.S., in large part due to the prolonged drought gripping those regions. “It’s almost like a separation where generally flood risk is increasing in the upper half of the U.S. and decreasing in the lower half,” says Villarini, associate professor in civil and environmental engineering and an author on the paper, published in the journal Geophysical Research Letters. “It’s not a uniform pattern, and we want to understand why we see this difference.”
Some of the regional variation can be attributed to changes in rainfall; a study led by Villarini published last year showed the Midwest and Plains states have experienced more frequent heavy rains in the past half-century. More rainfall leads to more groundwater, a “higher water base line,” Villarini explains. “The river basins have a memory,” adds Slater, a post-doctoral researcher and the paper’s corresponding author. “So, if a river basin is getting wetter, in the Midwest for example, your flood risk is also probably increasing because there’s more water in the system.” Why some sections of the nation are getting more, or less, rainfall is not entirely clear. The researchers say some causes could be the rains are being redistributed as regional climate changes.
The researchers hope that their findings could revise how changing flood patterns are communicated. In the past, flood risk trends have typically been discussed using stream flow, or the amount of water flowing per unit time. The UI study views flood risk through the lens of how it may affect people and property and aligns the results with National Weather Service terminology understood by the general public. “The concept is simple,” says Villarini, whose primary appointment is in IIHR–Hydroscience, a branch of the College of Engineering. “We’re measuring what people really care about.”