The vast majority of land sites in Africa show characteristic warming during the Medieval Warm Period, in line with similar warming in Europe and North America.
Global climate is currently undergoing major change. Experts agree that this change is driven by a combination of man-made and natural factors. However, full quantification of the anthropogenic and natural components is still a matter of debate.
In order to better understand the contribution of natural climate variability and distinguish this from man-made influence, researchers worldwide have gone out to the field to study Earth’s pre-industrial climate history. Of particular interest are the past thousand years, which in Europe and North America have seen the transition from a rather warm medieval period to major cooling of the Little Ice Age, followed by the temperature rebound of the Current Warm Period which was further intensified by human greenhouse gas emissions. Our understanding of medieval climate outside this well-studied North Atlantic region is unfortunately still poor.
An international team led by geoscientist Sebastian Lüning wants to change this. Lüning is a professional resources geologist who in his sparetime works on paleoclimatological studies with the Switzerland-based Institute for Hydrography, Geoecology and Climate Sciences. Together with colleagues from Poland, Nigeria, Turkey and Germany they embarked on a journey through the scientific literature to shed light on the so-called ‘Medieval Climate Anomaly’, a period comprising of the years 1000-1200 AD. The initial focus region of their study was Africa. Lüning and his team crawled through hundreds of publications and mosaiced together a fascinating picture of African medieval climate change that tracks ancient heat waves, local cooling, drought and phases of amplified rainfall. Using modern database and visualization technology, the team managed to synthesize astonishing trends from the large amount of filtered data. Lüning explains the challenge:
“This was true detective work. Obviously, we do not have any thermometer measurements or satellite data from medieval times. Scientists therefore analyze natural archives, namely sediment cores from the ocean floor, lakes and peatlands, as well as cave dripstones. Layer by layer the climate history is recorded like in a geological book, recorded by changes in geochemistry and fossil content. These property variations are then geologically translated back into climate parameters. Radiocarbon age dating provides the necessary time orientation”.
All published data were initially collected on a Google Map. The recorded climate development was then thoroughly correlated across the region and resulting temperature trends were mapped out. The vast majority of land sites in Africa showed a characteristic warming during the Medieval Climate Anomaly, in line with similar warming in Europe and North America. The situation in the coastal seas surrounding Africa, however, was different. Changes in the wind systems intensified the upward transport of deeper cooler water to the coasts, which resulted in a marked medieval cooling in the so-called ‘upwelling’ zones. Nevertheless, the areal extent of these narrow coastal sectors is rather small when compared to the total area of the African continent. The study was published in the journal Paleoceanography and is based on 44 study sites in and around Africa.
Rainfall Crucial to Understanding Medieval Warm Period
Besides temperature, Lüning and his group were also interested in medieval shifts in rainfall. An improved understanding of natural variability of rainfall is crucial, especially for the semi-arid and arid parts of Africa that heavily depend on seasonal rainfall for drinking water, agriculture and food production. Lüning describes the significance of this work:
“Our initial simplistic expectation was that we would find a uniform continent-wide medieval change in rainfall. However, this was not the case. As rainfall belts shift, some areas get drier and others wetter. It is important to understand these natural changes of rainfall patterns and their ultimate drivers. These are key calibration input data for climate models that still struggle to simulate regional rainfall correctly.”
The study identified three areas in Africa in which rainfall increased during the medieval warm period, namely Tunisia, western Sahel and the majority of southern Africa. At the same time, a reduction in precipitation occurred in the rest of Africa, comprising of Northwest and Northeast Africa, West Africa, Eastern Africa and the Winter Rainfall Zone of South Africa. The latter region includes Cape Town that currently suffers from a multi-year drought that has led to a serious shortage of drinking water. Similar droughts appear to have existed here during medieval times when the rain-bearing westerly winds were pushed so far south and away from land that they no longer intercepted the continent. The hydroclimate study has been published on 12th February in the journal Palaeo3 and is based on 99 sites in Africa.
In the absence of pre-industrial atmospheric CO2 variations, natural drivers have to be invoked to explain the observed medieval climate change. The most promising candidates are changes in solar activity as well as ocean cycles, which operate on times scales ranging from decades to millennia. The two new studies from Africa emphasize the great significance of natural climate variability on a global scale. A robust understanding of the natural ‘climatic heartbeat’ is essential. Ultimately, it will allow to better distinguishing between anthropogenic and natural contributions to modern climate change and will improve the predictive skill of climate models.
The two African climate reviews are based on a great number of published palaeoclimatic case studies which involved hundreds of scientists. Despite major progress in the field over the past 15 years, large parts of Africa are still white space when it comes to climate reconstructions of the past 1000 years. A dedicated structured research program is necessary to fill these important gaps. Climate policy depends on a robust foundation of such data. Compared to its large share of nearly one quarter of the world’s landmass, data from Africa and Arabia are significantly underrepresented in global temperature reconstructions of the past 2,000 years, an imbalance which needs to be swiftly and effectively addressed.
Lüning, S., M. Gałka, F. Vahrenholt (2017): Warming and cooling: The Medieval Climate Anomaly in Africa and Arabia. Paleoceanography 32 (11): 1219-1235, doi: 10.1002/2017PA003237.
Lüning, S., M. Gałka, I. B. Danladi, T. A. Adagunodo, F. Vahrenholt (2018): Hydroclimate in Africa during the Medieval Climate Anomaly. Palaeogeogr., Palaeoclimatol., Palaeoecol., doi: 10.1016/j.palaeo.2018.01.025.