Box 51 Positive feedback

In environmental science it is becoming more and more apparent that a phenomenon called feedback is 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 each side of the middle the change is greater, either upwards or downwards.

Factor A (cause) response is steeper.

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. Once the flip has occurred, this 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 if 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 flip.

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.

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.

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 Arabic, 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 are 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 (Figure 5.5), rainfall records show that there was an arid phase, lasting around 20 years. Then, there was a rather abrupt increase in rainfall, and rainfall

Figure 5.4. The Sahel, at the southern border of the Sahara desert. Source: Author.

mostly stayed high up until about 1970 when it suddenly declined, and stayed low throughout the 1970s and 1980s with a partial recovery since then. The decrease in rainfall was something like 20-35% across most of the Sahel, with some even drier phases within this.

The hardship caused 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

-51900 10 20 30 40 1950 60 70 80 90 2000 2010

Averages over 20-10N, 20W-I0E; 1900-2008 climatology NOAA NCDC Global Historical Climatology Network data

Figure 5.5. Record of rainfall in the Sahel since 1900 (JJASO mean Sahel precipitation anomalies 1900-2008). A relatively moist phase lasted until about 1970, followed by a substantial reduction in rainfall through the 1970s and 1980s. Source: Mitchell (2005).

natural event. Overstocking of livestock could have set off a process of positive feedback in the vegetation-climate system, which started or greatly amplified the drought. Discussion of this idea in the case 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 upwelling 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: a cap of descending air up above 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.

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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 evaporate— transpire—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 are 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 Nino 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 state 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-perpetuating for a while. The models suggest that in fact the greener, moister 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 zone 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 moister 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.

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