Over the coming century, the direct C02 effcct will not be the only thing changing. C02 (together with several other gases that are currently increasing) is also a grcen-
house gas, and by trapping heat these greenhouse gases will tend to warm the climate and also alter rainfall patterns (Chapter 3).
Temperature changes will also bring about changes in the water balance, whether or not the rainfall changes. All this has to be considered in relation to any forecast of global vegetation based on direct C02 effects, adding up to a very complicated mixture. While the direct C02 effect might be pulling things in one direction (towards wetter climate vegetation) by allowing more efficient water use, at the same time a decrease in rainfall or an increase in temperature which increases evaporation might be pulling things in the opposite direction (towards drier climate vegetation). It may be very difficult to predict which factor will dominate and in which direction the vegetation will change. There arc uncertainties in both the direct C02 fertilization effect and in the climate simulations for the future, and the combination of both adds up to a far wider range of uncertainty than cither taken by itself.
Nevertheless, it is interesting to think about what might happen as both C02 and climate undergo change over the next century. One model which concentrated on the IJSA during the next 50 100 years suggested that, initially, there will be an increase in overall forest extent and vegetation productivity due to the C02 fertilization effect dominating. However, the model predicts that, as the 21st century draws to a close, increasing heat and aridity from the greenhouse effect will result in a net decrease in forest extent, even though the direct fertilization effect of C02 is still increasing as its level in the atmosphere soars.
Another much more ambitious model, by Stephen Cox and colleagues at the Hadley Center in the UK. attempted to simulate the whole world s vegetation under increasing C02 and climate-warming. They used a climate model - a GCM (see Chapter 3 for an explanation of what a GCM is) including ocean circulation, plus a biome model to predict vegetation, and a C02 fertilization effect model. They also included a carbon cycle model to understand how the uptake and release of C02 by ecosystems would respond to changes in climate and C02 fertilization. Set to run for around 2050, the model suggested that by this time there will be an out-pouring of C02 into the atmosphere from the Amazon rainforest in response to greater aridity, despite the increased C02 fertilization effect. So, it looks like the net direct effect of C02 in the atmosphere may be overwhelmed by the influences of climate change. However, if it wasn't for the direct CO: effect, the amount of carbon leaving the world's vegetation would be even greater.
A further step is to consider how the direct C02 effect might set off the sorts of climate feedbacks from vegetation I mentioned in Chapters 5 and 6. If plants are opening their stomata less under increased CO? and thus losing less water by evaporation, this means slower less efficient recycling of rainwater (which allows more water to run straight oil' the land to rivers instead). Less recycling may mean an overall decrease in rainfall, which takes away some of the benefit to the water balance of the plants from having increased C02. On the other hand, the increase in vegetation leaf coverage resulting from direct C02 effects would decrease albedo—the ""lightness" of the surface. In arid areas this darkening of the surface would tend to increase rainfall by promoting convection (Chapter 5). In colder climates, the decreased albedo would also tend to warm the climate (Chapter 6). Hence, an initial boost given by the direct C02 effect can end up being magnified into a larger shift in vegetation. Some attempts at modeling such influences on the Sahara Desert margins over the next few decades suggest that, although the decreased evaporation from partially open stomata at high C'02 may tend to decrease rainfall a little, the increased efficiency of water use will promote more vegetation overall and that this will then set off an albedo feedback that actually gives more rain! Clearly, it is very hard to try to model the outcome of such complex networks of interacting factors, but what the musings of modelers do show is that there is a lot of potential for changes to be magnified, in ways that we might not initially expect.
A leaf studied in high C'02 concentration over a few minutes is not necessarily at all representative of nature. This statement might seem obvious, but modelers have not always been prepared to acknowledge it! Because there are many factors that could potentially change plants' responsiveness to C02, a good way to get a firmer idea of how wild or crop plants will behave is to do experiments on whole plants grown over weeks, months or years. For about 20 years now, plant biologists have been experimentally raising C02 levels, growing plants in small-scale systems to sec what effect future increases in C02 might have. While they are no more than isolated snapshots of the future world, these experiments at least have the advantage that they are based on actual whole plants, often growing under at least plausibly complex combinations of influences.
The earliest and simplest experiments were in closed chambers with plants growing cither in compost or natural soil, adding C02 to the air beyond the atmospheric concentration (Figure 8.2a). This would be compared with a "control" chamber where air with the C02 concentration of normal outside air (ambient air) was piped in. The trouble with these closed chambers was that they always seemed rather artificial. There was not the exchange with the outside world that might allow insects, herbivores and fungi to move in and out: the plants were effectively living in a "sterile" environment. And you couldn't grow big trees in these chambers, only small plants.
To help deal with these limitations, more sophisticated experiments were developed to look at the effects of raised C02. Open-top chambers (Figure 8.2b) of translucent plastic were used out in fields and natural vegetation: C02 was piped in to make a *'double-C02" atmosphere. Because C02 continually leaked out of the top, you had to use more C02 than in a closed chamber, making them rather more expensive to maintain (the cost of all the pure CO:, maintained over months or years, adds up to quite a lot). The trouble with these open-topped chambers has been that they tend to be warmer inside, which complicates the conclusions (in an experiment it is always best to start by holding all things constant except for the one factor you arc studying the effects of). Nevertheless, if their internal temperature is controlled with some sort of cooling system, such chambers do not necessarily have to be very different from the outside. Another problem with open-top chambers is that
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