Der ergrünende Sahel: Neues aus der afrikanischen Dürrekunde

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.”

The scientists used the chemical composition of leaf waxes preserved in sediments from Lake Bosumtwi, Ghana, to create a reconstruction of precipitation in humid tropical West Africa for the past 20,000 years. “We have a brand new record from West Africa and from it we found that the end of the humid period was more nuanced than previously believed,” said co-author Konrad Hughen, a geochemist with Woods Hole Oceanographic Institution. “Rather than collapsing abruptly across the entire region, we determined that the monsoon decreased more gradually and migrated southward over a period of several thousand years.” This southward shift could appear as abrupt changes locally – for example, a lush jungle reverts to grassland – but does not mean the entire region underwent abrupt changes at the same time.

The mechanisms that contribute to the changing monsoon are complicated and include orbital shifts of the Earth around the sun, which impact the intensity of solar radiation. For example, if solar energy is reduced, the monsoon circulation is weaker and the whole system gets dryer. However, a greener landscape will boost the monsoon more than a sandier landscape, and even though the sun’s energy in the region is getting weaker, widespread forest cover can help maintain a strong monsoon for a time. But eventually a threshold is reached and the system shifts. When and how that happens can be challenging to parse. By synthesizing a large array of records across northern Africa for this study, the scientists could observe large spatial and temporal patterns of variability. When viewed together, rather than one coherent shift in the West African monsoon, the team saw changes occurring at different times in different places.

“These are important components to understand and they must be included in climate models of the future,” said Hughen. “We need to understand these behaviors to be able to model them.” In addition to Shanahan and Hughen, the study involved scientists from Northern Arizona University, the University of Arizona, the National Center for Atmospheric Research, the University of Rhode Island, Syracuse University, and the University of Akron. “Africa’s tropical rainbelt supplies a significant (>60–90%) portion of northern and equatorial Africa’s annual moisture,” the authors wrote, “and as a result, changes in the timing or intensity of the seasonal rainfall influence food and water security for more than 150 million people.”

Über die Anfänge der Grünen Sahara berichtete am 28. Januar 2015 die Neue Zürcher Zeitung:

Abrupter Klimawandel in der Sahara: Auf einmal kam die Wüste
Der einst grüne Osten der Sahara scheint früher und schneller ausgetrocknet zu sein als bisher angenommen. Das könnte in der Region die Sesshaftwerdung der Menschen begünstigt haben.

Weiterlesen in der Neue Zürcher Zeitung.


Der guten Ordnung halber hier noch weitere interessante Mosaiksteinchen zur Dürrekunde aus dem Rest Afrikas:


Shanahan et al. wiesen bereits 2009 auf die starke natürliche Niederschlagsvariabilität in West Afrika hin:

„Thus the severe drought of recent decades is not anomalous in the context of the past three millennia, indicating that the monsoon is capable of longer and more severe future droughts.”

Ibrahim und Kollegen versuchten sich 2014 an Niederschlagsmodellierungen in Burkina Faso und stellten bestürzt fest, dass jedes der von ihnen verwendeten fünf Klimamodelle ein komplett anderes Ergebnis herausbrachte. Upps.



Ironie oder ernst gemeint? Am 12. Januar 2015 brachte Die Welt einen Beitrag zum Klimawandel in Südafrika:

Zeitraffer-Fotos belegen dramatischen Klimawandel
Vorher-Nachher-Fotos zeigen: So drastisch wirkt sich der Klimawandel in Südafrika aus. Forscher entwickeln aus den Bildern Vorhersagen für andere Regionen. Doch was ist überhaupt noch zu machen?

Im Artikel selber lernt man dann, dass sich die Waldgebiete ausgebreitet haben und auch die Waldbrände weniger geworden sind. Das sind wirklich eklatante Klimawandelfolgen der übelsten Sorte.



Oldie but Goldie: Im Jahr 2000 publizierte Sharon Nicholson in Global and Planetary Change diese interessante Arbeit zur natürlichen Variabilität der Niederschläge in Afrika:

The nature of rainfall variability over Africa on time scales of decades to millenia

This paper begins with an overview of the African rainfall regime, noting in particular the contrast among various regions of the continent, followed by a description of the nature of climatic (i.e., rainfall) variability over Africa on time scales of decades and centuries. The decadal scale is examined using modern data covering the twentieth century. The century scale is examined using historical reconstructions of climate, based on a combination of geologic, geographic and historical information (e.g., lake chronologies, landscape descriptions, archives and diaries).

The presentation includes some results of an analysis of a new historical semi-quantitative data set for Africa covering the last two centuries. It was produced using a combination of historical information, nineteenth century rainfall records, and statistical relationships among various sectors of Africa. Presented here are reconstructions of lake level fluctuations for numerous lakes of eastern and southern Africa.

This overview of climatic fluctuations is utilized to uncover inherent spatial and temporal characteristics of the rainfall variability. The dominance over time of various spatial modes is emphasized and the questions of synchroneity of the hemispheres and the abruptness of change are considered. The contrast between the two hemispheres is also surveyed, notably the different time scales of variability and potential causal factors in the variability. One of the most important contrasts is the multi-decadal persistence of anomalies over most of northern Africa. This has implications for the causes of long-term fluctuations, even those historical and paleo-time scales.