Sahelregen durch Ozeanzyklen gesteuert

Tolle Nachrichten aus Südtunesien: Die bewässerte Oasenfläche hat sich in den letzten Jahrzehnten stark ausgedehnt. In der Mai-Ausgabe 2017 des Journal of African Earth Sciences erscheint hierzu der folgende Artikel eines Teams um Emilio Rodríguez-Caballero:

Irrigated land expansion since 1985 in Southern Tunisia
The causes of agricultural land expansion and its impacts on dryland ecosystems such as the oasis regions of Southern Tunisia, are fundamental problems challenging the sustainability of irrigated agriculture on water limited ecosystems. Consequently, a thorough understanding of this phenomenon is necessary to avoid future problems. With the objective of identifying irrigated land expansion dynamics and the primary drivers, two representative oasis regions in Southern Tunisia, Mareth and Fatnassa, were selected. Changes in irrigated lands in both regions between 1985 and 2011 were analyzed, and the land uses from which expansion occurred were identified using Landsat images from different years (1985, 1996 and 2011). The results indicate that the surface occupied by irrigation agriculture has doubled in Mareth, while in Fatnassa, it has increased fourfold. During that period, there was a simultaneous increase in total population, as consequence of human migration that came along with an increase in income. Thus, we could identify human migration and economic development as potential drivers of irrigated agriculture expansion, though further research is warranted to ascertain a quantification that would assist in obtaining the sustainability of these regions.

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Im Juni 2016 erschien im IPCC-nahen Fachblatt Nature Climate Change ein kurioser Artikel von Jong-yeon Park, Jürgen Bader und Daniela Matei vom Hamburger Max Planck Institute für Meteorologie zur Niederschlagsentwicklung in der Sahelzone. Zunächst wird behauptet, die Sahel-Dürren der 1960er und 70er Jahre wären eine Folge der Klimaerwärmung, dann wird eingeräumt, dass es in den letzten Jahrzehnten dort wieder mehr geregnet hat – und das obwohl die Klimaerwärmung doch weiter fortschritt. Die Hamburger zeigen sich kreativ und zaubern einen weiteren Hasen aus dem Zylinder: “Schuld” an dem vermehrten Sahel-Regen hat wiederum der Mensch, nämlich durch die anthropogene Erwärmung des Mittelmeeres. Anstatt die natürliche Variabilität und Ozeanzyklik als wichtigen Steuerungsfaktor anzuerkennen, begeben sich die alarmhungrigen Forscher auf seltsame Interpretationswege. Abschließend wagen Park und Kollegen sogar eine Prognose: Es könnte in Zukunft sogar noch mehr im Sahel regnen, verursacht durch den ungezügelten Treibhausgas-Ausstoß der Menschen. Schmunzeln erlaubt. Hier der Abstract:

Anthropogenic Mediterranean warming essential driver for present and future Sahel rainfall
The long-lasting Sahel drought in the 1970s and 1980s caused enormous human and socio-economic losses1, driving extensive research on its causes2, 3, 4, 5, 6, 7, 8. Although changes in global and regional sea surface temperatures (SSTs) are thought to be dominant drivers of the severe Sahel drying trend9, 10, 11, 12, the mechanisms for the recent recovery trend are not fully clear yet, but are often assumed to be akin to the previous SST–Sahel drought linkage13, 14, 15. Here we show, by analysing observational and multi-model data and conducting SST-sensitivity experiments with two state-of-the-art atmospheric models, that the SST key area causing the recent Sahel rainfall recovery is the Mediterranean Sea. Anthropogenic warming of this region has driven the shift from the tropical Atlantic and Indo-Pacific oceans, which historically were the main driver of Sahel drought. The wetting impact of Mediterranean Sea warming can become more dominant in a future warming climate and is key to understanding the uncertainty in future Sahel rainfall projections.

Drei Monate später zeigten Catherine Pomposi und drei weitere New Yorker Kollegen in den Geophysical Research Letters die wahren Zusammenhänge auf. Sie machten pazifische Ozeanyzklen als wichtigen Steuerungsfaktor des Sahelregens aus:

Understanding Pacific Ocean influence on interannual precipitation variability in the Sahel
Moisture budget decomposition is performed for the Sahel (10°–20°N and 20°W–40°E) in order to understand the processes that govern regional hydroclimate variability on interannual time scales and frame them in the context of their primary ocean driver. Results show that warm conditions in the Eastern Tropical Pacific remotely force anomalously dry conditions primarily through affecting the low-troposphere mass divergence field. This behavior is related to increased subsidence over the tropical Atlantic and into the Sahel and an anomalous westward flow of moisture from the continent, both resulting in a coherent drying pattern. Understanding the physical processes relating remote sea surface temperature anomalies to atmospheric circulation changes and the resulting complex local convergence patterns is important for advancing seasonal prediction of precipitation over West Africa.

Das Hamburger Max-Planck-Institut hat in den vergangenen Jahren wichtige neue Erkenntnisse zu Aerosolen und anderen Themen beigesteuert. Die Sahel-Arbeit gehört leider nicht zu den besten Arbeiten, genausowenig wie die MPI-Studien zum arktische Meereis.