Sauberere US-Luft und AMO+ steigern Regenfälle in der Sahelzone. Wetlands International warnt vor Wassermissmanagement

Lange hatte man gedacht, der menschengemachte Klimawandel würde hinter den Saheldürren der 1970er und 80er Jahre stecken. Dann jedoch erkannte man, dass sich die Situation in Wirklichkeit wieder besserte und heute wieder mehr Regen fällt. Sollte hierfür auch der Mensch verantwortlich sein? Natürlich nicht. Nun weiß man, dass die Saheldürren stark von den Ozeanzyklen abhängen. Eine positive Atlantische Multidekadenoszillation (AMO+) bringt Regen in den Sahel. Zudem gibt es weitere Zusammenhänge, die langsam immer besser verstanden werden. Das erlaubt nun auch, erfolgreichere Regenprognosen für die kommenden Jahr abzugeben. Pressemitteilung der University of Exeter vom 25. Mai 2017:

Summer rainfall in vulnerable African region can be predicted

Summer rainfall in one of the world’s most drought-prone regions can now be predicted months or years in advance, climate scientists at the Met Office and the University of Exeter say.

The Sahel region of Africa – a strip across the southern edge of the Sahara from the Atlantic Ocean to the Red Sea – is a semi-arid landscape between the desert to the north and the savannah to the south. Much of the food produced in the Sahel depends on summer rainfall, and the region experienced major droughts during the 1970s and 1980s. The new research used the Met Office Hadley Centre’s Decadal Prediction System and found that the model was good at predicting summer Sahel rainfall over the forthcoming five years. Forecasting years ahead relies on sea surface temperature in the North Atlantic, whereas the El Niño Southern Oscillation is important for a shorter-term forecast before each summer.

“Our study suggests that skilful predictions of summer rainfall in the Sahel are now possible months or even years ahead,” said Dr Katy Sheen, formerly of the Met Office but now of the University of Exeter’s Penryn Campus in Cornwall.  “With a population reliant on agriculture, the Sahel is particularly vulnerable to major droughts, such as those of the 1970s and 1980s. “Improved understanding and predictions of summer rainfall in the Sahel has the potential to help decision makers better anticipate future cycles of summer droughts and floods, helping local communities become increasingly resilient to the region’s notoriously variable and changing climate.” By analysing data from the past, the researchers checked whether the model would have predicted the lack of rainfall associated with the prolonged droughts of the 1970s and 1980s – and they found it was very effective. “Our study improves our understanding of the driving mechanisms of summer rainfall variability and shows they are predictable,” Dr Sheen added. The study, published in the journal Nature Communications, is entitled: “Skilful prediction of Sahel summer rainfall on inter-annual and multi-year timescales.”

Paper: K. L. Sheen, D. M. Smith, N. J. Dunstone, R. Eade, D. P. Rowell, M. Vellinga. Skilful prediction of Sahel summer rainfall on inter-annual and multi-year timescales. Nature Communications, 2017; 8: 14966 DOI: 10.1038/ncomms14966

Mit Ozeanzyklen läufts einfach besser. Der Sahel-Regen wird aber wohl auch durch einen weiteren Faktor bestimmt, nämlich die Luftverschmutzung in den USA. Diese nimmt derzeit ab, was zu mehr Sahel-Regen führen wird, fanden Forscher der Lamont-Doherty Earth Observatory, Columbia University. Pressemitteilung vom 22. Mai 2017:

Reduced U.S. Air Pollution Will Boost Rainfall in Africa’s Sahel, Says Study

New Research Highlights Wider Benefits of Clean Air Policies

Falling sulfur dioxide emissions in the United States are expected to substantially increase rainfall in Africa’s semi-arid Sahel, while bringing slightly more rain to much of the U.S., according to a new study in the Journal of Geophysical Research: Atmospheres.

Pollution filters placed on coal-fired power plants in the United States starting in the 1970s have dramatically cut emissions of sulfur dioxide, a toxic gas that contributes to acid rain and premature deaths from respiratory and cardiovascular diseases. If U.S. sulfur dioxide emissions are cut to zero by 2100, as some researchers have projected, rainfall over the Sahel could increase up to 10 percent from 2000 levels, computer simulations published in the study suggest. “Reducing emissions in one region can influence rainfall far away because our global atmosphere is interconnected,” said the study’s lead author, Dan Westervelt, an atmospheric scientist at Columbia University’s Lamont-Doherty Earth Observatory. “We show that the health and environmental benefits of U.S. clean air policies extend to global climate as well.”

Sulfur dioxide simultaneously cools and dries earth’s climate by reflecting sunlight back to space and suppressing heat-driven evaporation near the ground. Though prior research has linked high sulfur emissions in Europe and Asia to the Sahel’s severe droughts of the 1970s and 1980s, this study is the first to look at how U.S. emissions influence precipitation in various regions globally. The researchers ran three independent global climate models to compare the relative impact of the United States cutting its human-caused sulfur emissions to zero and keeping its emissions at 2000-2005 levels. In the zero-emissions scenario, all three models showed a slight increase in average global rainfall, with higher levels in the United States and other northern-hemisphere regions. In the Sahel, two models found that wet-season rainfall increased by 5 to 10 percent, with one producing a rainy season two-and-a-half days longer.

“We were surprised to find that removing sulfur emissions in just one country would significantly influence rainfall on another continent, thousands of miles away,” said study coauthor Arlene Fiore, an atmospheric scientist at Lamont-Doherty. The added rainfall came as the tropical rain belt returned to its normal, northernmost position above the equator during northern hemisphere summer, the models showed, consistent with earlier research. The rain belt ordinarily shifts north when the northern hemisphere heats up during summer, but when sulfur emissions are high, cooler temperatures in the north stop the rain belt from migrating as far.

Cutting U.S. emissions to zero was enough to move the rain belt roughly 35 kilometers north, placing more of the Sahel in its path, the researchers found.  “We did not expect to see such a clear, significant influence on the Sahel,” said Westervelt. “This northern shift of the tropical rain belt could mean that cropland at the Sahel’s northern edge could become more productive in the future.” Though two of the three models were generally consistent, they disagree on exactly how much rain different regions can expect as U.S. sulfur emissions go to zero, says study coauthor Drew Shindell, an atmospheric scientist at Duke University. “We have just one real-world example — historic data — to rely on, making it very challenging to quantitatively link emissions to response,” he said.

The influence of rising carbon emissions is another complicating factor. The technology to trap carbon dioxide, unlike sulfur dioxide, is still far from being cost-effective. So while carbon dioxide levels continue to climb, falling sulfur emissions impose a climate “penalty” — less human-caused cooling to offset human-caused warming from carbon dioxide. “It’s still a good idea to cut SO2 with pollution control equipment on coal-fired power plants for the sake of public health, but even better would be to move away from coal-fired power plants entirely to reap the benefits of public health and climate change mitigation,” said Shindell. The study’s other authors are Andrew Conley and Jean-François Lamarque of the National Center for Atmospheric Research; Michael Previdi and Gus Correa of Lamont-Doherty; Greg Faluvegi of NASA Goddard Institute for Space Studies and Columbia’s Center for Climate Systems Research; and Larry Horowitz of Princeton’s Geophysical Fluid Dynamics Laboratory.

Study: Multimodel precipitation response to removal of U.S. sulfur dioxide emissions

Wetlands International recherchierte kürzlich die wahren Gründe für Migration in der Sahelzone. In einem 66-seitigen Sonderbericht (pdf hier) fanden die Autoren vor allem einen Grund: Wassermissmanagement. Zusammenfassung:

Water Shocks: Wetlands and Human Migration in the Sahel
This report calls attention to the worsening condition of wetlands in the Sahel and explains how this decline is undermining human well-being and compelling people to migrate, including to Europe. The rivers, lakes, floodplains and deltas of the Sahel are highly productive and biologically diverse ecosystems, fed by seasonal floods. These dynamic wetlands have long shaped human culture and been the basis for local and regional economies. Tens of millions of people still depend on their vitality. But these natural assets are degrading, often due to ill-advised economic development projects which divert water resources. Consequently, some wetlands have ceased to be a refuge in hard times and have instead become sources of out-migration, as people look elsewhere for alternative livelihoods.