If direct C02 fertilization turns out to have significant effects on the natural world, will these effects 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 richness in areas that are already tropical rainforest.
However, the current picture from vegetation-climate modeling in a high-C02 world is not encouraging. Models tend to predict that there will either be no substantial increase in tropical rainforest, or a dieback, because of the effects of high temperatures and aridity overwhelming any fertilization effect of C02.
Even if rainforest does turn out to grow more lush and cover larger areas, 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 richness 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 species 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 species, 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 easily in wind storms? No-one knows the answer to the 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.
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