The Future Direct C02 Effect A Good Or A Bad Thing For The Natural World
If dircct COi fertilization turns out to have significant cffccts on the natural world, will these cffccts be good or bad? The effects are likely to be complex and multi-faceted, and whether they are. on balance, likely to be good or bad is a subjective issue that depends on one's priorities.
Some scientists, and groups supported by the fossil fuel lobby, have argued that C02 fertilization might turn out to be a very good thing for nature in general. By allowing plants to thrive on less water, it might enable tropical rainforests to spread into drier climates. This might help to counterbalance the damaging effects of humans in logging and clearing tropical forests, preserving many of the species that live within them against extinction. There is a general relationship between rainfall and species richness even within tropical forest regions, and rising C02 might tend to act rather like an increase in rainfall, maximizing species richncss in areas that arc already tropical rainforest.
However, it is not certain that increasing C02 will make it easier for large numbers of species to coexist. It is fairly widely established in ecology that if vegetation grows too vigorously, stronger species can triumph and push the weaker ones out. For example, throwing a mineral fertilizer on a grassland will often cause a crash in the species richncss of the plant community, as a few fast-growing species that respond particularly well to fertilizer push all the others out. At lower nutrient levels, all spccics grow relatively slowly but are fairly evenly matched against one another in competition; none can push the others out. The fear is that increasing C02 will act as a fertilizer in just the same way, causing plant communities all around the world to undergo a burst of growth that will eliminate many "weaker" species. The result might be massive-scale extinctions of plant species and the insect and fungal life forms which depend upon them. 'The tropical rainforests could end up far poorer in spccics, despite conditions that favor their growth.
It is also very difficult to know what "knock-on" effects there will be through the food chain as the primary productivity of terrestrial ecosystems increases. Will food quality for animals increase (with a resulting increase in population densities), or decrease? Will this support more species of animals, or fewer? Also, will forests that have grown under high C02 levels burn more easily, or less easily, or be about the same? Will they fall over more easilv in wind storms? No-one knows the answer to the r ¥
many questions that concern the high-C02 world. The many nagging uncertainties give us a broad range of possible future scenarios, which frustrate our desire to know exactly what will happen in the future.
8.18 CONCLUSION: THE LIMITS TO WHAT WE CAN KNOW
In terms of painting a picture of the complexity of the living earth, the direct C02 effect serves as a final brush stroke, on top of everything else. If there is an overall message from this book, it is that the earth system is immensely subtle and complex, and that life has a key role in just about every aspect of it. And yet, as we marvel at the elegance of the mechanisms that we can see at work, we should not expect to know exactly how they will behave as we tinker with our planet.
The more effort—and the more money—that goes into research on the earth system, the more light will be shed on these processes and how they interact. This can allow us to consider a range of possibilities with more confidcnce. to spot new possible scenarios that had never been noticed before, and give either more or less credence to each of the various scenarios we already have. But, given the sheer complexity of what we arc dealing with here, scicncc can never provide the detailed certainty that politicians crave.
And so we enter a vast experiment, raising greenhouse gas levels beyond anything that has occurred in tens of millions of years. All science can do is offer a range of plausible scenarios, and then try to lay out these possibilities in an accessible way. It is up to people around the world to decide whether to take the gamble on any particular scenario coming true.
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