Immer wieder tappen Teilzeitaktivisten in die Sahelfalle. Sie behaupten gerne, die Sahelzone wäre akut vom Klimawandel bedroht, die Sahara würde sich immer weiter ausbreiten. Intuitiv verständlich, faktisch jedoch absolut falsch. Die Wahrheit: Die Regenmengen haben sich in der Sahelzone nach den schweren Dürren der 1970er und 80er Jahre wieder deutlich gesteigert, die Sahelzone ist grüner geworden. Nachzulesen z.B. im Oktober 2015 in den Geophysical Research Letters in einer Arbeit von Maidment et al.:
Recent observed and simulated changes in precipitation over Africa
Multiple observational data sets and atmosphere-only simulations from the Coupled Model Intercomparison Project Phase 5 are analyzed to characterize recent rainfall variability and trends over Africa focusing on 1983–2010. Data sets exhibiting spurious variability, linked in part to a reduction in rain gauge density, were identified. The remaining observations display coherent increases in annual Sahel rainfall (29 to 43 mm yr−1 per decade), decreases in March–May East African rainfall (−14 to −65 mm yr−1 per decade), and increases in annual Southern Africa rainfall (32 to 41 mm yr−1 per decade). However, Central Africa annual rainfall trends vary in sign (−10 to +39 mm yr−1 per decade). For Southern Africa, observed and sea surface temperature (SST)-forced model simulated rainfall variability are significantly correlated (r~0.5) and linked to SST patterns associated with recent strengthening of the Pacific Walker circulation.
In die gleiche Richtung geht eine Arbeit von Kaptué und Kollegen, die einen Monat zuvor in PNAS erschien. Fazit: Die Sahelzone ist in den letzten 30 Jahren ergrünt:
On regreening and degradation in Sahelian watersheds
Over many decades our understanding of the impacts of intermittent drought in water-limited environments like the West African Sahel has been influenced by a narrative of overgrazing and human-induced desertification. The desertification narrative has persisted in both scientific and popular conception, such that recent regional-scale recovery (“regreening”) and local success stories (community-led conservation efforts) in the Sahel, following the severe droughts of the 1970s–1980s, are sometimes ignored. Here we report a study of watershed-scale vegetation dynamics in 260 watersheds, sampled in four regions of Senegal, Mali, and Niger from 1983–2012, using satellite-derived vegetation indices as a proxy for net primary production. In response to earlier controversy, we first examine the shape of the rainfall–net primary production relationship and how it impacts conclusions regarding greening or degradation. We conclude that the choice of functional relationship has little quantitative impact on our ability to infer greening or degradation trends. We then present an approach to analyze changes in long-term (decade-scale) average rain-use efficiency (an indicator of slowly responding vegetation structural changes) relative to changes in interannual-scale rainfall sensitivity (an indicator of landscape ability to respond rapidly to rainfall variability) to infer trends in greening/degradation of the watersheds in our sample regions. The predominance of increasing rain-use efficiency in our data supports earlier reports of a “greening” trend across the Sahel. However, there are strong regional differences in the extent and direction of change, and in the apparent role of changing woody and herbaceous components in driving those temporal trends.
Auch Hoscilo et al. hatten im Oktober 2015 im International Journal of Climatology eine Zunahme der Sahelniederschläge in den letzten Jahrzehnten berichtet (siehe Pressemitteilung hier).
Kurios ist der Erklärungsversuch der ergrünenden Sahelzone. Im Juni 2015 schlugen Dong & Sutton in Nature Climate Change doch allen Ernstes vor, die Verbesserung wäre auf den Anstieg des CO2 in der Atmosphäre zurückzuführen. Das CO2 nimmt und das CO2 gibt, Amen.
Interessant sicher auch der weitere Blick zurück. Millan & Rodrigo werteten alte Schriften aus Mauretanien, Mali und Niger aus und fanden bereits vor 300 Jahren starke natürliche Schwankungen:
Results show wet conditions in the 17th century, as well as dry conditions in the 18th century (interrupted by a short wet period in the 1730s decade).
Ganz frisch aus dem Januar 2016 ist eine Arbeit von Ruan et al. in Climate of the Past. Die Autoren dokumentieren darin eine langanhaltende Dürreperiode vor 4200 Jahren in Algerien.
Tausend Jahre zuvor war die Sahara noch grün und Elefanten und Giraffen stolzierten durch die Gegend. Im Februar 2015 erinnerte die University of Texas at Austin an diese Phase in einer Pressemitteilung:
Study Supplies Insight into Behavior of African Monsoon
Think of the Sahara and you will conjure images of a vast desert landscape, with nothing but sand as far as the eye can see. But for a period of about 10,000 years, the Sahara was characterized by lush, green vegetation and a network of lakes, rivers and deltas. This “green Sahara” occurred between 14,800 and 5,500 years ago during what is known as the “African Humid Period.” Why and how it ended is the subject of scientific study that holds important information for predicting the region’s response to future climate change.
In a study published Jan. 26 in Nature Geoscience, a team of researchers provides new insight into the behavior of the African monsoon at the end of the African Humid Period and the factors that caused it to collapse. “Our work suggests that the African monsoon’s response to climate forcing is more complicated than previously understood,” said lead author Tim Shanahan, assistant professor at The University of Texas at Austin Jackson School of Geosciences. “Really big forcings like a collapse in the circulation of the Atlantic can cause synchronous drought across North Africa – and the current generation of climate models do a very good job at simulating events like this.
“However, the monsoon response to more gradual forcing is more complex. For example our data show that gradual changes in radiative forcing over the past 10,000 years produced dramatic changes to climate that are locally abrupt but asynchronous. But climate models don’t do a great job of simulating the complex mechanisms behind these changes, such as seasonality. Understanding how future changes in the monsoon will occur in response to gradual increases in greenhouse gases will require a better understanding of these processes.”