0ver the coming century, the direct C02 effect will not be the only thing changing. C02 "together with several other gases that are currently increasing'' is also a greenhouse 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 are 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 either taken by itself.
Nevertheless, it is interesting to think about what might happen as both C02 and climate undergo change over the next century. 0ne model which concentrated on the USA 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. Sitch and colleagues (2008) have tried a more extensive comparison of the various climate model scenarios, and various vegetation-climate models that also incorporate a direct C02 fertilization effect on the plants. What the models tend to find is that over the next century or so, the C02 fertilization effect is all-important in preventing the world's land ecosystems from gushing out carbon into the atmosphere as C02. If the direct C02 effect is included in the model, what was previously a scenario where vegetation and soils lose C02 into the atmosphere now becomes a scenario where there is a small net uptake. This is good news in terms of avoiding a runaway global feedback, but it hangs precariously on assumptions about the balance between the effect of warming (drying out vegetation, and also promoting decay of soil carbon) and the opposite effect of direct C02 fertilization—the latter being based on models which do not find any independent validation in the results of C02 fertilization experiments (see below). And furthermore, the climate models do not all paint the same rosy picture of the direct C02 effect saving the world. The Hadley Centre model used by Peter Cox, Peter Betts and colleagues (below) suggests that the C02 effect is not enough to prevent carbon gushing out of tropical rainforest in Amazonia as the result of a hotter, drier climate.
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 C02 and thus losing less water by evaporation, this means slower less efficient recycling of rainwater (which allows more water to run straight off the land to rivers instead of getting evaporated from leaves). 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. 0n 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 C02 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! Richard Betts and his colleagues have suggested that this "stomata shutting" effect of raised C02 could make life hard for tropical forest in the Amazon Basin by the mid- 21st century. Less evaporation from the leaves of the forest in the wettest areas at the western and eastern edges of Amazonia could starve other areas farther inland of the rain that they need, by breaking down the rainwater-recycling mechanism (Chapter 5). This would be partly compensated by the reduced needs of these inland rainforests for water, due to the direct C02 effect allowing them to keep stomata partially closed—so different aspects of the direct C02 effect have both a positive and negative effect on carbon balance. Combined with the extra heat from the increasing greenhouse effect, and droughts resulting from warming the tropical oceans, the Amazon forest is forecast to begin dying back and gushing out carbon by around 2050. Some of this group's model forecasts suggest that by 2100, most of the forest of the Amazon Basin will have died. Even if global warming stops due to new policies that limit greenhouse gas emissions, the progressive dieback of the forest might continue by its own momentum, as a wave of drying climate, death of forests and collapse of the water-recycling mechanism gradually spreads across the whole Amazon Basin. It is important to emphasize, however, that this nightmare scenario is only one of a range of model forecasts. Using any one of several other different climate models, or tweaking the details of the interactive vegetation-climate scheme, predicts all or most of the Amazon forest remaining in place despite the warming climate. So the forecast for the future is all very sensitive to the detailed parameters of the model—for example, such things as the temperature sensitivity of tree leaves— which will require a lot more experimental or boots-on-the-ground observational evidence to be certain about them. Hopefully, this need will spur ecologists in the tropics to get out and make more observations that can help the models. In any case, before we dismiss the risks of an "Amazon collapse'' it is worth bearing in mind that the Hadley Centre GCM, and the vegetation model used in these forecasts, is one of the more widely respected model combinations.
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.
Was this article helpful?
Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.