About Carbon Dioxide: We Got One Thing Right

A while ago we wrote several posts about the ongoing rise in atmospheric concentration of carbon dioxide. We said nothing about the impact this human-caused change in the chemical composition of the atmosphere might have on climate, as most people seem already to have a more definite opinion on that question than most of the rather small number of people somewhat familiar with the facts. But we did say, what is much less widely discussed or even recognized as a public policy issue, that rising carbon dioxide concentration would have a large impact on the biosphere, for the following reason:

... plants extract carbon dioxide from the atmosphere by diffusion. Assimilating atmospheric carbon dioxide by diffusion entails a challenge because, if there is a path for the carbon dioxide to diffuse from the atmosphere to the plant cell, there must, unavoidably, be a path for the loss of water by vapor diffusion from the plant cell to the atmosphere. This means that plants exchange water, which is usually in limiting supply, for carbon. Moreover, the rate of exchange depends directly on the concentration gradients of the two gases between plant cell and atmosphere. Therefore, if the atmospheric carbon dioxide concentration rises, the amount of carbon fixed by plants in exchange for the water available also rises.

This effect is greatest where plant growth is most severely restricted by drought. Much of Southern Africa, and particularly the Sahel, a broad belt of mixed shrubs and grasses to the South of the Sahara desert, is a severely water-limited habitat. By promoting carbon assimilation and hence plant growth in such water-limited habitats, rising atmospheric carbon dioxide concentration can have a big impact on ecosystem structure. That is because:

... some plants respond more vigorously to rising carbon dioxide concentration than others. Particularly responsive are woody species of arid habitats such as the Australian outback, the Sahel to the immediate south of the Sahara Desert, and the South American Savanna. This means that rising carbon dioxide concentration is changing the species composition of plant communities and thereby changing the composition of the animal communities that depend on the plants for food and shelter. The extent and significance of these changes has thus far, received barely any attention, but they will become increasingly obvious as the atmospheric carbon dioxide concentration continues its accelerating rise.

And has it ever become increasingly obvious. Or so it has been reported by scientists from the UK's Herriot Watt University. Put shortly, what they claim to have found is that almost 8,000 sub-tropical African plant species from an estimated total of about 23,000 species could become extinct within the next few decades.

Wow. Maybe we should be taking the CO2 thing more seriously.