Regen ist lebensnotwendig für Menschen, Tiere und Pflanzen. Umso wichtiger ist die korrekte Prognose der Niederschlagsentwicklung. Gerne werden hierzu theroretische Klimamodelle herangezogen. Bartlein et al. zeigten im September 2017 am Beispiel der Niederschläge Eurasiens vor einigen Jahrtausneden, dass selbst grundlegende Prozesse offenbar noch vollkommen unverstanden sind:
Underlying causes of Eurasian midcontinental aridity in simulations of mid-Holocene climate
Climate model simulations uniformly show drier and warmer summers in the Eurasian midcontinent during the mid-Holocene, which is not consistent with paleoenvironmental observations. The simulated climate results from a reduction in the zonal temperature gradient, which weakens westerly flow and reduces moisture flux and precipitation in the midcontinent. As a result, sensible heating is favored over evaporation and latent heating, resulting in substantial surface-driven atmospheric warming. Thus, the discrepancy with the paleoenvironmental evidence arises initially from a problem in the simulated circulation and is exacerbated by feedback from the land surface. This region is also drier and warmer than indicated by observations in the preindustrial control simulations, and this bias arises in the same way: zonal flow and hence moisture flux into the midcontinent are too weak, and feedback from the land surface results in surface-driven warming. These analyses suggest the need to improve those aspects of climate models that affect the strength of westerly circulation.
Eine NASA-Studie untersuchte die Prognoseleistung von 23 Modellen für den Zeitraum 1995-2005 und fand, dass mehr als zwei Drittel aller Modelle die real gemessenen Regenmengen unterschätzt hatten. Kein richtig gutes Ergebnis, das Vertrauen in die Vorhersagekraft der Klimamodelle stärken würde. Siehe Bericht auf The Daily Caller aus dem Juni 2017:
Study: Climate models have been estimating rainfall incorrectly this whole time
Most global climate models are underestimating increased rainfall caused by global warming, according to a study released Monday.
NASA and four universities compared climate data from 1995 to 2005 to 23 climate model simulations for the same period. More than 70 percent of the climate models underestimated the amount of rain compared to the real world observations. “Precipitation is vital to life on Earth and regional precipitation changes accompanying anticipated global warming could exert profound impacts on ecosystems and human society,” reads the study’s abstract, adding that “we infer that most CMIP5 models underestimate the hydrological sensitivity under global warming.” The climate models that came closest to matching real world observations indicated that global warming will greatly increase rainfall in the future. Climate models in the study were underestimating rainfall because of the way they examined high altitude in the atmosphere. Global warming will mean fewer of these clouds, causing more rainfall.
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