Klimamodelle überschätzen Zunahme der globalen Niederschläge um fast die Hälfte

So langsam setzt sich das Verständnis durch, dass in der Klimamodellierungswelt nicht alles so rosig aussieht wie lange Jahre behauptet. Ein bisschen ist das wie bei der Tour de France, wo man lange seinen Helden zujubelte, bis dann die Dopingwirklichkeit schließlich herauskam. Stehen wir bei den Klimamodellen kurz vor diesem Umschwung und neuem Realismus? Ein lesenswertes Editorial in Nature vom 3. Mai 2017 lässt aufhorchen. Dort wird der Erwämungshiatus klar eingeräumt, eingeordnet und Kommunikationsfehler auf beiden Seiten der Klimadiskussion ausgemacht:

Increased scrutiny of climate-change models should be welcomed
The apparent slowdown in global warming has provided a spur for better understanding of the underlying processes. […] Some background: the El Niño weather event in 1997 and 1998 belched a great bolus of heat from the ocean into the atmosphere, a release that was entirely consistent with expectations — as was the heady spike in global mean surface temperature that followed. From the top of the Himalayas, the rest of Earth is downhill. And, in a similar way, the 1998 peak in temperature offered an easily visualized time that climate sceptics could cherry-pick as a starting point for a ‘hiatus’, ‘pause’ or ‘slowdown’ in climate change. It’s true (of course) that the next few years saw a reduced rate of warming, or maybe even a slight cooling. And it’s also true that, soon after, some analyses showed that these observations were beginning to diverge from the suite of projections made by climate models. A few responses emerged. First: yawn — “This is nothing more than the sort of normal variability one should expect in the climate system, and models should not be expected to predict any specific dip or peak.” Second: hysteria — “Climate scientists have no idea what controls the climate system.” Third: interesting  — “Let’s figure this out.” Happily, most of the climate-science community adopted the third option. The result was a flood of publications on the topic, and the only half-joking suggestion that Nature’s publisher should launch a new journal called Nature Hiatus.

Weiterlesen in Nature

Bereits im März 2016 hatte Nature das Thema im Programm:

Where climate models fall short
Climate models tend to overestimate the extent to which climate change contributes to weather events such as extreme heat and rain. Omar Bellprat and Francisco Doblas-Reyes at the Catalan Institute of Climate Sciences in Barcelona, Spain, used an idealized statistical model to compare the frequency of weather extremes in simulations with and without climate warming. Extreme events seemed to be more closely linked to climate change when the model was forced to run at low levels of reliability than when the model error was kept to a minimum. To account for models‘ biased representation of climate variability, studies should rely on calibrated model ensembles, which are commonly used by weather forecasters, the authors suggest.

Sympathischer Realismus auch in einer Pressemitteilung des Lawrence Livermore National Laboratory vom 10. Dezember 2015. Klimamodelle überschätzen die Zunahme des Niederschlags systematisch um 40%:

Climate models overestimate rainfall increases

Lawrence Livermore researchers and collaborators have found that most climate models overestimate the increase in global precipitation due to climate change. Specifically, the team looked at 25 models and found they underestimate the increase in absorption of sunlight by water vapor as the atmosphere becomes moister, and therefore overestimate increases in global precipitation. The team found global precipitation increase per degree of global warming at the end of the 21st century may be about 40 percent smaller than what the models, on average, currently predict. The research appears in the Dec. 10 edition of the journal Nature.

Evaluation of model-predicted global precipitation change with actual precipitation observations is difficult due to uncertainties arising from many sources, including insufficient spatial and historical data coverage. As an alternative approach, the team, made up of LLNL scientist Mark Zelinka and colleagues from the University of California, Los Angeles, including lead author Anthony DeAngelis, evaluated model-simulated global precipitation change through consideration of the physical processes that govern it.

The team found that the increase in global precipitation simulated by models is strongly controlled by how much additional sunlight is absorbed by water vapor as the planet warms: Models in which more sunlight is absorbed by water vapor tend to have smaller increases in precipitation. They demonstrated that model-to-model differences in increased absorption of sunlight were not controlled by how much their humidity increased, but by how much additional sunlight was trapped in the atmosphere for a given increase in humidity. Conveniently, this quantity can be measured from space, allowing the team to assess how well the models capture the physics controlling changes in global precipitation.

„This comparison with observations allowed us to see quite clearly that most models underestimate the increased absorption of sunlight as water vapor increases,“ Zelinka said. „Because this acts as such a strong lever on global precipitation changes, the models are likely overestimating the increase in global precipitation with global warming.“ The intensification of the hydrologic cycle is an important dimension of climate change that can have significant impacts on human and natural systems, perhaps more so than rising temperatures alone, according to Zelinka.

Commonly measured by the increase in globally averaged precipitation per degree of surface warming, hydrologic cycle intensification predictions vary substantially across global climate models. „We sought to understand the sources of this uncertainty and use the best available observations to narrow in on the most likely response,“ Zelinka said. „We cannot expect to make useful predictions of local water cycle changes that are most relevant for societal impacts if we do not understand and accurately simulate the change in globally averaged precipitation.“ The absorption of sunlight by water vapor is vital to understand future global precipitation changes.

Condensational heating by precipitation, absorption of sunlight by water vapor and fluxes from the Earth’s surface all combine to heat the atmosphere, keeping it in energy balance with cooling due to thermal emission up to space and down to the Earth’s surface. As the planet warms and the atmosphere emits more thermal radiation, the heating components also must increase to maintain atmospheric energy balance, and the two that matter most are absorption of sunlight and precipitation. The more heating provided by absorption of sunlight as the planet warms, the less heating is required by precipitation increases. The study notes that more reliable predictions of future precipitation change can be made by improving the representation of how radiation is transmitted through the atmosphere in global climate models. The models that have more sophisticated representations better agree with observations.

Paper: Anthony M. DeAngelis, Xin Qu, Mark D. Zelinka, Alex Hall. An observational radiative constraint on hydrologic cycle intensification. Nature, 2015; 528 (7581): 249 DOI: 10.1038/nature15770

Modellversagen auch bei der Simulation von Lebensräumen. Die Tiere der letzten Eiszeit setzten sich stur über die Vorgaben des Computersimulators hinweg und zeigten in der Realität eine Verbreitung, die gänzlich unerwartet war. Pressemitteilung der University of Oregon aus dem November 2014:

Fossils cast doubt on climate-change projections on habitats

Mammals didn’t play by the rules of modeling on where they migrated to survive last ice age, says UO researcher

Leave it to long-dead short-tailed shrew and flying squirrels to outfox climate-modelers trying to predict future habitats. Evidence from the fossil record shows that gluttonous insect-eating shrew didn’t live where a species distribution technique drawn by biologists put it 20,000 years ago to survive the reach of glaciers, says University of Oregon geologist Edward B. Davis. The shrew is not alone. According to a new study by Davis and colleagues, fossil records of five ancient mammalian species that survived North America’s last glacial period point to weaknesses in the use of ecological niche models and hindcasting to predict future animal and plant habitats. As a result, Davis says, the modeling needs to be fine-tuned for complexities that might be harvested from fossils.

Ecological niches use modern habitat distributions and climate; hindcasting adds predictive power by adding major past climate shifts into the models. That modeling combination — as seen in a 2007 study led by Eric Waltari, then of the American Museum of Natural History in New York — had the short-tailed shrew surviving the last ice age in mostly Texas and the Deep South. Conclusions drawn in other studies, Davis noted in the new study, also are biased toward southern locations for ice-age surviving mammals of the Pleistocene Epoch. Short-tailed shrew, according to fossil records, did not live in the predicted ranges. Instead they lived across north central and northeast United States, closer to the glaciers and where they are widely found today.

„It’s almost as though it is living in all of the places that the model says it shouldn’t be living in and not in any of the places that the model says it should be living in,“ said Davis, who also is manager of the paleontological collection at the UO Museum of Natural and Cultural History. „This suggests to me that whatever the model is keying on is not actually important to the shrew.“ Nor to the American marten (Martes americana), two species of flying squirrels and the Gapper’s red-backed vole (Myodes gapperi), all of which lived mostly outside of predicted ranges, according to the fossil record. Northern (Glaucomys sabrinus) and southern (Glaucomys volans) flying squirrels, the Davis study found, shared a compressed geographic region. It may be, Davis said, that some species tolerate competition under harsh conditions but separate when abundant resources are available.

Davis noted that an important but under-cited 2010 paper on rodents by Robert Guralnick of the University of Colorado and Peter B. Pearman of the Swiss Federal Research Institute also showed problems with hindcast projections. Those for lowland rodents in the last ice age did not hold up, but those for a higher elevation species did. „Our findings say that we need to pay more attention to the potential problems we have with some of our modern methods, and the way that we can improve our understanding of how species interact with the environment,“ said Davis, who added that his study was inspired by Waltari’s. „The way to improve our forecasting is to include data from the fossil record. It can give us more information about the environments that species have lived in and could live in.“ The findings appear in the November issue of the journal Ecography. In a special section of the journal, the Davis paper is packaged with four papers on research initially presented in a symposium on conservation paleobiogeography in 2013 at a biennial meeting of the International Biography Society. The Davis paper is co-authored by Jenny L. McGuire, now at Georgia Tech University, and former UO doctoral student John D. Orcutt, who is now at Cornell College in Iowa.

Davis and McGuire co-hosted the symposium, edited the special issue and penned an editorial that accompanies the five papers. Conservation paleobiogeography, Davis said, „is the idea that we can help people understand questions that arise from conservation needs using data from the fossil record.“ Doing so, he said, may explain how species shift their ecological roles, or evolve, to survive amid abrupt changes in their habitats. „Our paper raises questions about some of the work on projecting future ranges of mammals, and we suggest some directions forward,“ Davis said. „We have concerns about the precision of the modeling techniques now being used. We don’t have any concerns about climate change happening and that it going to cause geographic range shifts for mammals and plants. The thing I want to do, as a scientist, is to have the best models possible so as we’re making informed decisions as a society.“

Paper: Edward Byrd Davis, Jenny L. McGuire, John D. Orcutt. Ecological niche models of mammalian glacial refugia show consistent bias. Ecography, 2014; DOI: 10.1111/ecog.01294