But Deserts Make Themselves
In addition to all of the more traditional climatology, there is another factor whose importance is only now becoming understood. Deserts partly owe their existence to the fact that they themselves exist. The desert makes the desert, internally modifying its own climate so that less rain falls!
So the link from climate to vegetation, in Chapter 1, has been turned on its head. A fundamental fact of the earth system, that climate scientists arc only now becoming fully aware of, is that vegetation can make the climate too. The mechanisms by which deserts reinforce their own dry climate was apparently first explored in the mid-1970s by Ottcrman. who represented his ideas in a landmark paper which inspired a whole new way of thinking. He took apart the basic physics of the local atmosphere and surface the "mcsoscalc climate" that is built up from the microclimatic factors explained in Chapter 4 and he thought about what might happen if you changed the vegetation covcr. One thing that he knew was important was the brightness of the surface, the albedo (roughly meaning "whiteness" in Latin).
Seen from above, green leaves look a lot darker than a bare soil surface. For example, if you look out of the window of a plane living high above dry country, areas of dense tree and shrub cover look almost black, in contrast to the blinding brightness of patches of bare soil. The brightness of that bare soil is solar energy— sunlight reflected straight back up into space. This is energy that might have gone into heating the surface if it had been absorbed, but instead it has been wasted. Thus, the lighter the surface, the less energy is absorbed and more is thrown right out into space. Table 5.1 shows a range of typical albedo values found for different surface types in the world.
Above bare soil with high albedo, the atmosphere is deprived of some of the warm air that would otherwise be rising up from the ground, so it is cooler than it would be if it was darker. There is less convection to carry heat aloft, and the atmosphere is relatively calm and stable. The relatively shallow convection above high-albedo land tends to let air, coming in from above, descend and form a "cap" on the top, which supresses rain cloud formation. Normally, rain clouds tend to form where warm air keeps rising up from the surface carrying some water vapor and then begins cooling, causing water droplets to condense out to give clouds and then rain. Over a bare, bright surface, air tends to do the opposite thing—descending and heating as it does so. These arc the sort of conditions which prevent any rain from falling (Figure 5.1).
To anyone who has traveled a lot around the world, it is intuitively hard to believe that lack of vegetation covcr would make the surface cooler. Arid areas of the world tend to get very hot, whereas the rainforest zones of the tropics have fairly mild temperatures most of the lime. But, in fact, areas with plenty of dark vegetation are absorbing a lot more energy overall than deserts. The air temperature at ground level
Table 5.1. Range of values for various land surface types. The important thing in the context of understanding arid lands is that hare ground typically has a much higher albedo than forest vegetation. Although ranges of values overlap considerably between different surface types, actual means (not shown) are quite distinct. Source: Bonan.
Fresh snow 0.80 0.95
Old snow 0.45 0.70
Grassland 0.16 0.26
Deciduous forest 0.15-0.20
Coniferous forest 0.05-0.15
Road 0.05 0.20
Wall 0.10 0.40 Paint
Red, brown, green 0.20 0.35
in these densely vegetated zones would be even higher than in a bare arid desert, except that the leaves of the forest are usually evaporating water which sucks away heat as "latent heat of evaporation". If you need to be convinced of the difference that albedo makes to the amount of sunlight converted into heat at the surface, try walking across from a light concrete surface to dark recently-laid asphalt on a hot sunny day. The air hovering above the dark asphalt will be much hotter than that over the light concrete—often almost unbearably hot. If they were not continually evaporating water from their leaves, forests would also be even hotter than bare land.
In a desert, then, because the land surface is bare of vegetation, this tends to give descending air which docs not give rain. And, of course, without rain there can be no vegetation. So the chain of causes goes in two different directions depending on the starting position;
Bare land => sinking air no rain bare land Vegetated land rising air rain vegetated land
In effect, once there is a lack of vegetation cover in an already fairly arid environment, it stabilizes its own aridity in a vicious cycle. Indeed, it may well exaggerate its own
Rising air condenses out clouds and
Rising air condenses out clouds and
aridity, making things more arid than they would have been to start off with. This process of reinforcement is known as positive feedback (see Box 5.1).
So, there is a positive feedback process involving surface reflectivity in deserts, and this can intensify the arid desert climate. However, the mechanism is also liable to break down sometimes and produce a sudden flip between moist and dry climates, if a slight triggering change in the environment occurs. This triggering change could be something natural, or something caused by humans.
Box 5.1 Positive feedback
In environmental science it is becoming more and more apparent that feedback processes are key to understanding global processes. What do we mean by feedback? It is a process that (in some cases) "feeds off" itself, gathering momentum like a snowball rolling downhill, or conversely (in other cases) damping itself down and moderating its own effects like the central heating system of a house, controlled by a thermostat.
One example of feedback often occurs if you go to watch some awful band at the local pub. They will tend to set up the stage with the singer's microphone too close to the speakers. The mike picks up the sound of guitar and amplifies it. The more it amplifies the sound, the louder it comes out of the speakers, and the more goes back into the mike, and so on eventually it arrives at an ear-piercing screech. This self-reinforcing process is positive feedback. It is a process that "magnifies" change, once the initial small triggering event (the first little noise that went into the microphone) occurs. It can be just the same with the natural environment: a slight initial change is picked up and amplified into a big shift in climate.
Or consider another type of feedback: a central heating system in a house. When the thermostat senses that the house is too hot. above the set point, it turns down the heating. If it turns the heat down too much, the thermostat senses the decrease in temperature that results, and turns it back up slightly. The temperature oscillates slightly around a fixed point. Also, if the weather outside changes, becoming warmer or hotter and tending to alter the temperature of the house, the thermostat senses the change inside the house and adjusts the heating. The operation of a central heating system is a different type of feedback: negative feedback. This is a process that "damps down" change. In the natural environment, negative feedback loops help keep the climate stable, resisting knocks and the destabilizing influences of positive feedbacks.
Where do we see positive feedback in nature? It is everywhere. One important example is in snow cover and ice sheet extent. An area of ice or snow cools the surface (by reflecting back most of the sun's heat and preventing it warming the surface), ensuring that more ice can form. This runaway effect helps explain why the earth went through ice ages in the past, when huge ice sheets made of solidified snow spread to cover the high latitudes of the world. Patches of snow that failed to melt could reflect sunlight and cool the air, making it easier for more snow to survive the next year, and so on. Other important positive feedbacks that involve living vegetation are explained in this chapter and the next.
So, positive feedback factors are amplifiers that intensify differences in the climate from one place to another, and also increase variability in earth's climate and environment over time. They must surely have been important in bringing about the many sudden changes in climate that have happened in the geological history of the earth, particularly during the last couple of million years (Chapter 3).
It takes a triggering event to set the positive feedback loop rolling. Particular factors that could have acted as triggers for positive feedback in the past climate include Milankovitch rhythms, which are changes in sunlight distribution due to the Earth's wobbling orbit. The change in the seasonal sunlight intensity sets a wide range of positive feedback loops rolling, involving ice, involving vegetation (Chapters 5 and 6) and involving plankton in the oceans (Chapter 7). If it moves in one direction, the Milankovitch change can trigger global cooling through all these loops. A slight shift in the other direction and the earth can suddenly warm through positive feedbacks operating in reverse.
Although positive feedbacks in climate can greatly intensify change, there will be constraints on how far things can move. A positive feedback cycle is generally "damped" beyond a certain point the amplification loop stops responding and the system doesn't move beyond that extreme. A screeching guitar amplifier does not keep getting louder and louder until the whole building is demolished by the sound waves. It reaches its limit in terms of volume because the wattage of the speakers becomes limiting to the power of the feedback. Where a positive feedback cycle causes warming, the temperature doesn't go running away until the earth burns up completely, just because the feedback is operating to heat it up. Nor does it go down to absolute zero when a feedback that causes the cooling is in operation. All the positive feedback does is amplify the amount of change that occurs, up to a certain point.
If you think of it all in terms of a graph, what a positive feedback mechanism does is increase the slope of the responsiveness of the system to external changes for example, to a Milankovitch rhythm, to random variation in regional weather patterns or to changes in the composition of the atmosphere. If you vary the amount of a certain factor (e.g., increasing the amount of greenhouse gas in the atmosphere) along the bottom axis A, there is a certain amount of response seen in terms of the vertical axis B (e.g., the temperature, responding to the increased greenhouse gases) giving a sloping line (Figure 5.2). Adding in a positive feedback changes the slope of the response: it has become much steeper. In the middle of the graph that shows the amount of response there will be a "hinge point" that remains the same, but either side of the middle the change is greater, either upwards or downwards.
Figure 5.2. How positive feedback affects the slope of a response. Factor B affects A, and with a positive feedback working the slope of the response is steeper.
With positive feedback
Figure 5.2. How positive feedback affects the slope of a response. Factor B affects A, and with a positive feedback working the slope of the response is steeper.
Factor A (cause)
Without positive feedback
With positive feedback
Factor A (cause)
State 1 t t
Figure 5.3. A metastable system has multiple states. It will stay stable in one state until it is "kicked" into the other one by some temporary triggering event.
Positive feedbacks on climate can be set off by a slow, long-lasting change in background conditions. This might, for example, be a change in seasonal sunlight distribution due to the Milankovitch rhythms, moving the earth from a glacial to an interglacial state. This type of shift in the system tends to be fairly stable over thousands of years because the change in sunlight is so slow.
However, the trigger for a positive feedback could also just be a temporary set of conditions, some random event such as a much wetter than average summer or a much colder than average winter. Or it might even be too much goat-grazing in a particular year on the edge of a desert. The feedback then amplifies this initial switch until it reaches its limits, and what would have been a small temporary change has become amplified and settled into a new steady state. The new steady state is not fixed forever, though. It can always be thrown back in the other direction by a random temporary change, taken hold of by positive feedback loops running in reverse. Thus, the state of such a system is never stable, but metastable, liable to flip suddenly given a small push (Figure 5.3). In a system that is metastable, both alternative states are really just as stable, and what things are like at any one time just depends what particular "peg" things have come to rest on. Such metastable states are very important in understanding the history of climates in arid regions, and many other changes over time, including some of the large global climate changes in the fossil record. As we shall see, vegetation is probably often involved in making the system metastable and liable to Hip.
5.2.1 The Sahel and vegetation feedbacks
At the southern edge of the Sahara desert is a zone of open scrub, known as the Sahel. In local languages, the word Sahel means "shoreline", and this is a good metaphor
for this long, thin strip—perhaps 200 km in depth—that borders the great desert (Figure 5.4). For thousands of years, the Sahel has been inhabited by herders and farmers, but their survival has always been made precarious by fluctuations in rainfall. In most other arid regions of the world, dry and wet years arc randomly interspersed, so if one year is especially dry it is no predictor that the next year will be dry too. But, in the Sahel the climate tends to go through distinct wet or dry "phases" lasting several decades. If one year in the Sahel is drier than the long-term average, it is a good bet that the next year will be relatively dry too. and also the year after that. In the late 1800s, the region went through a moist phase, with rather good agricultural yields. Around 1900, rainfall records show that there was an arid phase, lasting around 20 years. Then, there was a rather abrupt increase in rainfall, and rainfall stayed high up until about 1970 when it suddenly declined, and stayed low throughout the 1970s and 1980s. The decrease in rainfall was something like 20-35% across most of the Sahel, with some even drier phases within this.
The hardship causcd by this arid phase in the Sahel was well documented by the world's media. Human populations in the Sahel had expanded in response to an abundance of food, under the high-rainfall conditions of the mid-20th century. Suddenly, there was less to eat, and nowhere for the farmers and herders to go. The whole situation was exacerbated by civil war in some parts of the Sahel. and the combination of events led to famine and many thousands of deaths.
The environmental movement, still finding its feet in the 1970s, was given a jolt when some respected climate scientists suggested that this drought was not an entirely natural event. Overstocking of livestock could have set off a process of positive feedback in the vcgctation-climatc system, which started or greatly amplified the drought. Discussion of this idea in the ease of the Sahel seems to have started with that study by Otterman. He suggested that overgrazing by livestock had removed the dark vegetation cover of the Sahel, decreasing rainfall through the albedo effect mentioned above. So. with the vegetation removed there would be less upward movement of the air within the atmosphere. The lack of upvvclling warm surface air would also mean that the atmosphere high above tended to stay cooler. This cool air would tend to sink gently downwards, compressing and holding its water vapor more tightly as it descended. The result would be the desert climate: descending air and no rain. So, essentially, the high reflectivity of the surface, caused by lack of vegetation, would produce a dry climate which would not support vegetation, ensuring high reflectivity of the bare surface, and so on ...
The idea that there is a positive feedback loop behind the rainfall cycles is a compelling one, and in fact it is not necessarily anything caused by humans. It could be that the albedo feedback operates almost independently of whatever humans do that they and their livestock cannot affect albedo enough to bring about a lasting drought. In this case the trigger for a drought might be some sort of event imposed from outside the region. For example, there could be natural changes in wind flow patterns that produce an initial drought that is then reinforced by changes in vegetation cover. So, the albedo feedback loop involving vegetation might still be important in producing dry or wet phases in the Sahel, but humans might not be a significant part of the trigger.
After Otterman started talking about albedo, climate scientists thought about other ways in which deserts and arid zones might make their own climate. They brought in some extra factors that might alter the influence of vegetation on climate, including "roughness" which is the bumpiness of the vegetation surface compared with bare ground. The greater this roughness is, the more it tends to produce turbulence as the wind blows over it. This alters wind speed, and the vertical movement of the atmosphere. Vertical movement in the air is all-important to producing rainfall, and in transferring heat up from the surface; so, this might be another important way that vegetation modifies its own climate.
The climate modelers also thought about how evaporation might differ between vegetated areas and bare ground. Evaporation gives the water vapor that can condense out as rain again up in the sky, either over the same area or hundreds of kilometers away. It can also affect the strength of the rising convection of surface air up into the atmosphere, acting as "fuel" for the upwards motion by supplying latent heat that keeps the air warmer and thus rising. Vegetated surfaces cvaporate-transpirc—more water, because they catch more as rain in the root mat and in the loose soils underneath, and then let it out from the leaves. And, whereas a bare soil tends to evaporate all of its (relatively small) store of water very quickly, a vegetated area loses it gradually over a much longer period. Including such extra factors in climate models has suggested that in these respects too, vegetation tends to "make" its own climate.
So, here arc two additional feedbacks on rainfall, beyond the albedo effect:
THE EVAPORATION FEEDBACK Vegetated land -> more evaporation more rain -> vegetated land
THE ROUGHNESS FEEDBACK Vegetated land => rougher surface more turbulence more rain vegetated land
The more sophisticated modeling studies which included these extra feedbacks agreed with the initial conclusion that in an arid area the climate you end up with can depend very much on what you start from in terms of vegetation cover. For example, in the Sahel, if you start from a dense blanket of vegetation, this landscape generates more rain than it would if you started from sparse vegetation. In fact, in the models a more densely vegetated Sahel such as existed in the moister phases of the last century generates enough extra rain to sustain its denser vegetation cover. So, this is a self-stabilizing system: once set up, it can perpetuate itself (through a positive feedback loop). It is the same the other way round too—starting with a bare Sahel, the climate that the land surface makes for itself is drier, and the vegetation remains spare. So, there are two potential steady states following on from these two different sets of starting conditions.
However, these states are not truly stable; they are "metastable" (see Box 5.1), liable to flip if given a knock from external influences in climate that alter the broader atmospheric circulation across the region to give a few wetter years or drier years than normal. The sort of factors that can supply this knock are changes in ocean circulation and temperature, such as might come about in an El Niño event or during other broad climate oscillations in the climate system such as the North Atlantic Oscillation (a shift in the relative strength of air pressure systems between the north and central Atlantic). Once the surface vegetation cover has been forced to change by such strong external factors, the vegetation-climate feedback system can suddenly tumble in a different direction, to end up in the "other" state.
We see signs of the existence of these two steady states in the decadal-timescale oscillations in rainfall that are recorded by climate station records, and the famines of the Sahel. Oscillations on this longer time scale cannot be explained by external influences such as temperature shifts originating in the Atlantic (which tend to be very short-lived); they must be internally generated. So, once the Sahel gets dry it tends to stay that way for several years at least. The dry slate is self-perpetuating because of the sparser vegetation influencing climate. Only when it is overwhelmed by an external change in climate (due to sea surface temperature changes forcing rainfall to come its way) does everything finally flip into a new state of greenery and higher rainfall, which is also self-perpctuating for a while. The models suggest that in fact the greener, moistcr state is more stable than the died-back dry one, because the bushes in the Sahel can put up with a lot of drought before they gradually give up and die. Conversely, it does not take much rain to bring about a regrowth of vegetation, dragging the system quickly out of a long drought. Observations agree with this expectation: dry phases start reluctantly and slowly as the vegetation dies back, whereas dry phases end suddenly as the vegetation responds to rain.
The variability in rainfall is also thought to have an effect on the abruptness of the boundary between desert and non-desert at the southern edge of the Sahara. A vegetation-climate model by Ning Zeng of the University of Maryland suggests that the reason a transition /one of scrub like the Sahel can exist at all is the shifting lottery of rainfall, which comes farther north in some years than others. If the rain came north to the same point every year, there would form a denser wooded cover of small trees able to exploit the moistcr conditions south of that point, which would suddenly give way to very sparse open scrub where the rain could no longer sustain it. The very sharpness of the transition would be amplified by vegetation climate feedbacks from the presence of the trees themselves. In the real world, the year-to-year variability in rainfall, which is itself amplified by vegetation climate feedbacks, produces a broad zone where moisture supply is too unstable to favor the growth of trees. The more conservative, drought-tolerating shrubs of the Sahel are able to hold on in this variable environment, and it is they that themselves favor both the variable environment and more broader gradient in vegetation! The chain of causes then goes something like this ...
Variable rainfall caused by variability in sea surface temperatures => variability is amplified by vegetation feedbacks from the Sahelian vegetation => favors Sahelian shrubs that tolerate variability variability is amplified by Sahelian vegetation feedbacks => etc.
5.2.2 Have humans really caused the Sahelian droughts?
To return to the original question which set Otterman and others wondering: Are humans a large part of the blame for dry periods in the Sahel? Could they sometimes be the "kick" that sets the climate system tumbling towards a dry stale after overgrazing takes place? The conclusion of all the models so far is that humans and their animals do not in fact have a big influence on the Sahel vegetation-climate system. While overgrazing may occur, it tends to alter details of structure and composition of the vegetation, towards thornier and less edible species, more than it aflccts the overall vegetation coverage. There is always at least some effect on overall vegetation coverage from grazing, and in this sense humans may have some small part to play in reinforcing droughts in the Sahel. However, it is an effect that is dwarfed by, and very difficult to disentangle from, the other vegetation-climate processes that cause wet and dry cycles in that region.
Interestingly, there is at least a possibility that events in the Sahel could be influenced by humans changing the vegetation outside the region, hundreds of miles to the south in West Africa. One model suggests that if the forests in West Africa arc cleared, the loss of re-evaporation of water cuts oil' the supply of moisture for rainfall over the Sahel. There has already been a considerable amount of forest loss in West Africa in the last few decades and it is not certain how this might have a fleeted the recent climate history of the Sahel. It seems that most of those working on this area presently feel that the forest removal has not had much effect, and that the variability in the Sahel is mainly due to variable sea surface temperatures in the Atlantic plus internally generated vegetation-climate feedbacks in the Sahel.
Continue reading here: Could The Sahara Be Made Green
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