The Green Sahara Period
The Sahara is the largest hot desert in the world.
The first noteworthy point here is that deserts need not be hot. The Sahara, while being the largest hot desert, is only the third largest desert in the world. The first two are in Antarctica and the Arctic. The defining characteristic of deserts is their aridity, or perhaps more appropriately, their hyperaridity. Deserts receive very little precipitation, whether it be in the form of rainfall or snowfall.
The second noteworthy point is the size of the Sahara. At 9.2 million km2, it is almost three times the size of India (3.3 million km2). So before reading further, it is important to keep in mind that we are going to talk about climate change in a huge region and of a huge scale.
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| Source: https://mapfight.xyz/compare/in-vs-sahara/ |
Around 11,000 years ago, something strange happened. The Sahara Desert started receiving more rainfall than before. Among other things, precipitation dictates what vegetation can be supported in a region. Thus, the Sahara also saw a rather dramatic change in terms of biome, going from desert to grassland. There were large lakes, rivers and wetlands in the middle of a region which gets less than 100 mm of rain in a year now. This went on for a few thousand years, and is called the Green Sahara Period or the African Humid Period.
Needless to say, if the largest hot desert of the (modern) world stops being a desert for a few thousand years, it's an intriguing puzzle for climate scientists. What made the Sahara turn into a grassland? What made it turn back into a desert? Can this happen again? But before we delve into these questions, the reader may wonder - how do we know the Sahara took a break from being a desert? There are many independent lines of evidence for this, but let's focus on pollen records for now.
Today, northern Africa has distinct bioclimatic zones which support distinct vegetational groups: Saharan vegetation, Sahelian vegetation, Sudanian vegetation and Guineo-Congolian vegetation. These zones follow broad precipitation patterns. Thus, Saharan vegetation comprises of those plant species that can survive in very little precipitation (<150 mm/year). The Sahel region receives more precipitation, plants of this region are used to 150-500 mm of rain each year. Similarly, the Sudanian and the Guineo-Congolian groups are adapted to 500-1500 mm/year and >1500 mm/year of rain, respectively.
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| Source: https://eros.usgs.gov/westafrica/node/147 |
Each zone is represented by different plant groups and thus, different types of pollen. Plant pollen is made up of very resistant material which can be preserved in sediments for millions of years. The disciplines of micropaleontology (study of microscopic fossils) or paleopalynology (study of ancient pollen) have several techniques to reconstruct the past climate, by identifying ancient pollen in ancient sediments. Researchers study sediments dated to the Green Sahara Period, estimate what pollen was in situ and what was brought to their sampling site by the wind, and then make inferences about what vegetation was present at their sampling site during the Green Sahara Period.
A multitude of such studies have conclusively shown that the savanna (or the grassland) biome grew larger during the Green Sahara Period, encroaching upon what is the desert today. Such plants could survive in that region for a long period of time (several millennia) only if the region consistently received precipitation like the Sahel does today. A simplistic way of imagining this is that the biomes shifted northwards (the actual picture is a bit more nuanced -- but let's not go into that right now).
Scientists who conduct such studies have a long list of arduous and difficult tasks. They must first obtain samples from an appropriate site, figure out the scope of the paleoclimatic signature (i.e. whether it represents a local record or a regional record), get samples dated to make sure they represent the right time period, learn to identify fossil pollen, correctly identify hundreds of pollen from each sample, and then conduct statistical analyses on pollen data to tease out trends through time. But such work is very rewarding - it has informed us that climatic variability can result in astonishing and drastic changes in the Sahara and the Sahel. This is a body of pollen data that has grown through decades, with newer and subsequent records benefiting from newer and improved techniques of microscopic analysis, dating techniques and statistical methods. The inferences are also supported by studies using other paleoclimatic archives -- dust, speleothems, diatoms, macrofossils and many more.
What does this mean for climate change in the region today? And revisiting our previous questions -- What made the Sahara turn into a grassland? What made it turn back into a desert? Can this happen again? We know these answers to some extent, but I'll leave these explorations for a future blog post.
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If you want to explore this further:
(1) Check out the images for the different bioclimatic regions on this page. Imagine what a difference this made in the lives of animals and humans in the region!
(2) I found this video very enjoyable and accurate. Strongly recommended!
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If you want to read papers, I suggest:
Hoelzmann et al. (1998) [Global Biogeochemical Cycles]
Watrin et al. (2009) [Comptes Rendus - Geoscience]
Bartlein et al. (2011) [Climate Dynamics]
Hély et al. (2014) [Climate of the Past]
All of these are compilations of a large number of pollen studies, with references for individual studies within.
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