Neue Modellierungsergebnisse: Wirbelstürme werden in Zukunft wohl seltener und weniger intensiv

Immer wenn ein Hurrikan wütet, gibt es stets einen aufmerksamen Klimaaktivisten, der den Auslöser ganz genau kennt: Der Klimawandel! Und er ist sich dann ganz sicher: In Zukunft wird es alles noch viel ärger. Haltet inne, übet Verzicht, kehret um! Ihr seit auf dem Weg des Teufels, elendiger CO2-Sünder. Zahle und tue Buße!

Nun ja, aber es gibt zum Glück auch noch eine wissenschaftliche Seite, die leider viel zu selten in den Medien zu Wort kommt. Denn die Wahrheit ist sehr viel unspektakulärer als uns eingeflüstert wird. Am 4. Oktober 2017 erschien eine Studie von Yoshida und Kollegen. Darin enthalten sind die Ergebnisse von Simulationen der tropischen Wirbelstürme und ihre zukünftige Entwicklung. Laut Modellen wird sich die Anzahl der tropischen Wirbelstürme im globalen Maßstab um 33% verringern. Zudem wird es wohl deutlich weniger Stürme der besonders starken Kategorien 4 und 5 geben. Letzteres ist bemerkenswert, denn lange glaubte man, dass zwar die Anzahl der Wirbelstürme abnehmen könnte, dafür aber die Intensität zunimmt. Die neuen Ergebnisse lehnen diese Sichtweise nun deutlich ab. Abstract der Studie:

Future Changes in Tropical Cyclone Activity in High-Resolution Large-Ensemble Simulations
Projected future changes in global tropical cyclone (TC) activity are assessed using 5,000 year scale ensemble simulations for both current and 4 K surface warming climates with a 60 km global atmospheric model. The global number of TCs decreases by 33% in the future projection. Although geographical TC occurrences decrease generally, they increase in the central and eastern parts of the extra tropical North Pacific. Meanwhile, very intense (category 4 and 5) TC occurrences increase over a broader area including the south of Japan and south of Madagascar. The global number of category 4 and 5 TCs significantly decreases, contrary to the increase seen in several previous studies. Lifetime maximum surface wind speeds and precipitation rate are amplified globally. Regional TC activity changes have large uncertainty corresponding to sea surface temperature warming patterns. TC-resolving large-ensemble simulations provide useful information, especially for policy making related to future climate change.

Eine Verringerung der Wirbelstürme in der Zukunft hatten bereits Murakami et al. 2014 prognostiziert:

Influence of Model Biases on Projected Future Changes in Tropical Cyclone Frequency of Occurrence
The influence of model biases on projected future changes in the frequency of occurrence of tropical cyclones (FOCs) was investigated using a new empirical statistical method. Assessments were made of present-day (1979–2003) simulations and future (2075–99) projections, using atmospheric general circulation models under the Intergovernmental Panel on Climate Change (IPCC) A1B scenario and phase 5 of the Coupled Model Intercomparison Project (CMIP5) models under the representative concentration pathway (RCP) 4.5 and 8.5 scenarios. The models project significant decreases in global-total FOCs by approximately 6%–40%; however, model biases introduce an uncertainty of approximately 10% in the total future changes. The influence of biases depends on the model physics rather than model resolutions and emission scenarios. In general, the biases result in overestimates of projected future changes in basin-total FOCs in the north Indian Ocean (by +18%) and South Atlantic Ocean (+143%) and underestimates in the western North Pacific Ocean (−27%), eastern North Pacific Ocean (−29%), and North Atlantic Ocean (−53%). The calibration of model performance using the smaller bias influence appears crucial to deriving meaningful signals in future FOC projections. To obtain more reliable projections, ensemble averages were calculated using the models less influence by model biases. Results indicate marked decreases in projected FOCs in the basins of the Southern Hemisphere, Bay of Bengal, western North Pacific Ocean, eastern North Pacific, and Caribbean Sea and increases in the Arabian Sea and the subtropical central Pacific Ocean.

Und auch Kim et al. 2014 sehen weniger tropische Wirbelstürme für die Zukunft vorher:

Tropical Cyclone Simulation and Response to CO2 Doubling in the GFDL CM2.5 High-Resolution Coupled Climate Model
Global tropical cyclone (TC) activity is simulated by the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model, version 2.5 (CM2.5), which is a fully coupled global climate model with a horizontal resolution of about 50 km for the atmosphere and 25 km for the ocean. The present climate simulation shows a fairly realistic global TC frequency, seasonal cycle, and geographical distribution. The model has some notable biases in regional TC activity, including simulating too few TCs in the North Atlantic. The regional biases in TC activity are associated with simulation biases in the large-scale environment such as sea surface temperature, vertical wind shear, and vertical velocity. Despite these biases, the model simulates the large-scale variations of TC activity induced by El Niño–Southern Oscillation fairly realistically. The response of TC activity in the model to global warming is investigated by comparing the present climate with a CO2 doubling experiment. Globally, TC frequency decreases (−19%) while the intensity increases (+2.7%) in response to CO2 doubling, consistent with previous studies. The average TC lifetime decreases by −4.6%, while the TC size and rainfall increase by about 3% and 12%, respectively. These changes are generally reproduced across the different basins in terms of the sign of the change, although the percent changes vary from basin to basin and within individual basins. For the Atlantic basin, although there is an overall reduction in frequency from CO2 doubling, the warmed climate exhibits increased interannual hurricane frequency variability so that the simulated Atlantic TC activity is enhanced more during unusually warm years in the CO2-warmed climate relative to that in unusually warm years in the control climate.

Es gibt wohl keinen Hurrikan-, Taifun- oder anderen Wirbelsturmforscher, der hier eine abweichende Meinung hat. So gehen auch Walsh et al. 2015 von einer zukünftig sinkenden Anzahl von tropischen Wirbelstürmen aus:

Hurricanes and Climate: The U.S. CLIVAR Working Group on Hurricanes
While a quantitative climate theory of tropical cyclone formation remains elusive, considerable progress has been made recently in our ability to simulate tropical cyclone climatologies and to understand the relationship between climate and tropical cyclone formation. Climate models are now able to simulate a realistic rate of global tropical cyclone formation, although simulation of the Atlantic tropical cyclone climatology remains challenging unless horizontal resolutions finer than 50 km are employed. This article summarizes published research from the idealized experiments of the Hurricane Working Group of U.S. Climate and Ocean: Variability, Predictability and Change (CLIVAR). This work, combined with results from other model simulations, has strengthened relationships between tropical cyclone formation rates and climate variables such as midtropospheric vertical velocity, with decreased climatological vertical velocities leading to decreased tropical cyclone formation. Systematic differences are shown between experiments in which only sea surface temperature is increased compared with experiments where only atmospheric carbon dioxide is increased. Experiments where only carbon dioxide is increased are more likely to demonstrate a decrease in tropical cyclone numbers, similar to the decreases simulated by many climate models for a future, warmer climate. Experiments where the two effects are combined also show decreases in numbers, but these tend to be less for models that demonstrate a strong tropical cyclone response to increased sea surface temperatures. Further experiments are proposed that may improve our understanding of the relationship between climate and tropical cyclone formation, including experiments with two-way interaction between the ocean and the atmosphere and variations in atmospheric aerosols.

Weshalb werden tropische Wirbelstürme in Zukunft wohl seltener werden? Eine Studie von Lim et al. 2015 fand, dass eine Abkühlung der mittleren und höheren Troposphäre zu mehr Wirbestürmen führt, nicht etwa eine Erwärmung:

Sensitivity of Tropical Cyclones to Parameterized Convection in the NASA GEOS-5 Model
The sensitivity of tropical cyclones (TCs) to changes in parameterized convection is investigated to improve the simulation of TCs in the North Atlantic. Specifically, the impact of reducing the influence of the Relaxed Arakawa–Schubert (RAS) scheme-based parameterized convection is explored using the Goddard Earth Observing System version 5 (GEOS-5) model at 0.25° horizontal grid spacing. The years 2005 and 2006, characterized by very active and inactive hurricane seasons, respectively, are selected for simulation. A reduction in parameterized deep convection results in an increase in TC activity (e.g., TC number and longer life cycle) to more realistic levels compared to the baseline control configuration. The vertical and horizontal structure of the strongest simulated hurricane shows the maximum wind speed greater than 60 m s−1 and the minimum sea level pressure reaching ~940 mb, which are never achieved by the control configuration. The radius of the maximum wind of ~50 km, the location of the warm core exceeding 10°C, and the horizontal compactness of the hurricane center are all quite realistic without any negatively affecting the atmospheric mean state. This study reveals that an increase in the threshold of minimum entrainment suppresses parameterized deep convection by entraining more dry air into the typical plume. This leads to cooling and drying at the mid to upper troposphere, along with the positive latent heat flux and moistening in the lower troposphere. The resulting increase in conditional instability provides an environment that is more conducive to TC vortex development and upward moisture flux convergence by dynamically resolved moist convection, thereby increasing TC activity.

Auch Sugi et al. 2015 konnten zeigen, dass Wirbelstürme in kälterem Klima häufiger werden, nicht etwa in wärmerem:

More tropical cyclones in a cooler climate?
Recent review papers reported that many high-resolution global climate models consistently projected a reduction of global tropical cyclone (TC) frequency in a future warmer climate, although the mechanism of the reduction is not yet fully understood. Here we present a result of 4K-cooler climate experiment. The global TC frequency significantly increases in the 4K-cooler climate compared to the present climate. This is consistent with a significant decrease in TC frequency in the 4K-warmer climate. For the mechanism of TC frequency reduction in a warmer climate, upward mass flux hypothesis and saturation deficit hypothesis have been proposed. The result of the 4K-cooler climate experiment is consistent with these two hypotheses. One very interesting point is that the experiment has clearly shown that TC genesis is possible at sea surface temperature (SST) well below 26°C which has been considered as the lowest SST limit for TC genesis.

Fazit: Wirbelsturm-Alarmisten sind eine aussterbende Spezies. Nicht einmal die Ersatzhypothese zieht mehr, denn auch die Intensität der Stürme wird wohl in Zukunft abnehmen. Für einige Zeitgenossen unbequeme Ergebnisse, die jedoch auf jeden Fall anzuerkennen sind.