With greenhouse gases increasing in the atmosphere, it is likely that the world's climate will change significantly over the next century or two. Vegetation feedbacks will surely have a role to play in this change too—perhaps amplifying change, perhaps damping it.
There has not been much discussion in the scientific literature so far on how vegetation feedbacks in arid zones will alter the course of changing climate, but it is a reasonable guess that they will have some significant effects. Part of the problem in forecasting these future feedbacks is that different GCM simulations tend to predict quite different changes in the background climate in the drier parts of the world. So, in a particular semi-arid or arid region, there might either be less rainfall or more as the climate warms. Plus, even if there is an increase in rainfall, the warming caused by the greenhouse effect might increase evaporation so much that the climate actually ends up drier overall.
In a general way, some people have speculated that the greenhouse world of the coming centuries might closely resemble the warmer world of the early Holocene "optimum", between about 9,000 and 6,000 years ago, or of the early Eemian interglacial about 125,000 years ago. At both times, the Saharan, Arabian and Asian deserts were much moister than they are now. However, it is important to realize that there is a major difference from the future greenhouse world: during these past "moist" phases there was a lot more summer sunlight over the northern hemisphere and this is what helped to bring the monsoon rains in, and enabled vegetation-climate feedbacks to get going in making the climate even moister. There is nothing in the coupled vegetation-climate models that predicts a similar drastic change in Saharan climate as a result of the increased greenhouse effect alone. One of the coupled models does produce a slight increase in monsoon rainfall at the southern edge of the Sahara during the next century, and forecasts that feedbacks from the vegetation (such as decreased albedo, increased roughness and recycling of rainfall by evapotranspiration) will then amplify the amount of rainfall and result in some greening up of the southern edge of the Sahara. But this spread of vegetation and moistening of climate is not even as much as the "green" Sahara forecast by some models for the present-day state, let alone the "Green" Sahara of the past.
Something that needs to be explored further by the models is how other arid regions of the world might undergo feedbacks between climate and vegetation cover, that will either amplify or damp change. Another factor to consider is the effect of very high CO2 levels (Chapter 8) on the feedbacks from vegetation. If CO2 concentrations are more than double their previous background level, as they will be in another 60 years or so, this is likely to affect the way vegetation behaves. For one thing, the plants will not need to keep the stomatal pores in their leaves open for as long to get enough carbon to photosynthesize, and this means that they will tend to lose less water to the atmosphere. This means less latent heat uptake, and less recycling of rain to the atmosphere, which could surely feed back on climate. Presumably, it would tend to heat up the air near the ground (with less latent heat uptake), and also decrease the amount of rainfall (with less recycling of moisture to the atmosphere). But this scenario assumes that the types of plants growing there stay just the same. In fact, if less water is used up by each photosynthesizing leaf it might mean that small trees bearing more leaves can push their way into the vegetation, migrating in from other regions. The whole structure of the vegetation might change, and this can itself change the albedo and the roughness of the surface. Trees will also unavoidably transpire more water than smaller shrubs, counteracting the effect of increased CO2. This could now change the climate, most likely towards increased rainfall.
Although our understanding is still evolving, the climate system of arid lands turns out to be subtly and inextricably linked to vegetation. In the next chapter we will explore the links which are also present at the other end of the moisture spectrum, involving forests.
Was this article helpful?