Experts anticipate that global atmospheric concentrations of carbon dioxide (C02) will have-doubled by the end of the twenty-first century. It is known that C02 can contribute to global warming by trapping solar energy that is being reradiated as heat from the Earth's surface. However, some research has suggested that elevated C02 levels could enhance the photosynthetic rates of plants, resulting in a lush world of agricultural abundance, and that this C02 fertilization effect might eventually decrease the rate of global warming. The increased vegetation in such an environment could be counted on to draw more CO, from the atmosphere.
The level of C02 would thus increase at a lower rate than many experts have predicted.
However, while a number of recent studies confirm that plant growth would be generally enhanced in an atmosphere rich in C02, they also suggest that increased
C02 would differentially increase the growth rate of different species of plants, which could eventually result in decreased agricultural yields. Certain important crops such as corn and sugarcane that currently have higher photosynthetic efficiencies than other plants may lose that edge in an atmosphere rich in C02.
Patterson and Flint have shown that these important crops may experience yield reductions because of the increased performance of certain weeds. Such differences in growth rates between plant species could also alter ecosystem stability. Studies have shown that within rangeland regions, for example, a weedy grass grows much better with plentiful C02 than do three other grasses. Because this weedy grass predisposes land to burning, its potential increase may lead to greater numbers of and more severe wildfires in future rangeland communities.
It is clear that the C02 fertilization effect does not guarantee the lush world of agricultural abundance that once seemed likely, but what about the potential for the increased uptake of C02 to decrease the rate of global warming? Some studies suggest that the changes accompanying global warming will not improve the ability of terrestrial ecosystems to absorb C02. Billings' simulation of global warming conditions in wet tundra grasslands showed that the level of C02 actually increased. Plant growth did increase under these conditions because of warmer temperatures and increased C02 levels. But as the permafrost melted, more peat
{accumulated dead plant material) began to decompose. This process in turn liberated more C02 to the atmosphere. Billings estimated that if summer temperatures rose four degrees Celsius, the tundra would liberate 50 percent more C02 than it does currently. In a warmer world, increased plant growth, which could absorb C02 from the atmosphere, would not compensate for this rapid increase in decomposition rates. This observation is particularly important because high-latitude habitats such as the tundra are expected to experience the greatest temperature increase.
According to the passage, Billings' research addresses which one of the following questions?
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