A few thousand years ago, the Earth’s climate was quite different. Some regions were warmer and moister – and the Sahara was considerably greener. A lush canopy of plants drew water from the soil and “sweated” it back out, which produced monsoons on a regular basis. But, over the course of millennia, the Earth’s orbit changed, and with it the sun’s influence on the climate. This also impacted the Sahara: the monsoons ended and the vegetation gradually disappeared.
Today the Earth is growing warmer again. The cause: the greenhouse effect, which has been intensified by our use of fossil fuels, releasing more and more CO2 into the atmosphere. The increased CO2 levels are warming the Earth – and far more than thousands of years ago. The question is: with a warmer climate, will vegetation return to the Sahara?
My colleagues at the Cluster of Excellence CliSAP and I have explored this question with the help of three different climate models. The projections indicate that, through the end of the 21st century, the central and western Sahel, as well as the southern edge of the Sahara, actually will become greener. Nevertheless, the equation “warm climate = green desert” doesn’t really work, because the increased vegetation is not only a result of the higher temperatures; two of our three models indicate that it is primarily due to the higher CO2 concentrations in the atmosphere.
Plants need carbon dioxide: they “breathe” it in and, with the help of sunlight and water, convert it into building materials for their cells. In theory, then, more CO2 should promote more vegetation in the Sahel zone. But our calculations show that major sections of this region will also become more arid in the wake of climate change. This will harm the plants, which need water in order to process the CO2. So we see two opposing effects: more CO2 “fertilizes” plants, but higher temperatures dry them out.
In order to better grasp this interaction, we examined the two factors again separately – unfortunately with mixed results. In some models CO2 is the decisive factor for increased vegetation growth at the edges of the desert regions; in others, only those regions in which the precipitation and temperature conditions were conducive from the outset will become greener.
In this regard it’s important to know that plants take in carbon dioxide through tiny pores in their leaves. When plants have plenty of water, these pores are wide open, allowing them to absorb a great deal of CO2. At the same time, some water is released through the pores and evaporates, which cools their immediate vicinity. In contrast, plants surrounded by CO2-rich air only need to open their pores slightly; higher CO2 levels mean less evaporation. The effect: the air close to the ground grows warmer. Further, vegetation is darker and absorbs more sunlight than bright desert sand. This intensifies the warming – bad news for heat-sensitive plants.
Even if the different models portray this complex interplay in different ways, we can say the following: it’s quite possible that the “CO2 fertilization” will lead to the formation of a green belt in the Sahara. But if so, it most likely won’t last for long; at some point, the heat and lack of water will gain the upper hand and the vegetation will dwindle once again.
Author: Prof. Dr. Martin Claußen
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