Rising Atmospheric Carbon Dioxide Concentration, Part II: Ecosystem Disruption

Some global warming "skeptics" dismiss the climatic effect of carbon dioxide by claiming that the gas exists in the atmosphere in such a negligible quantity that even several times that negligible quantity is still negligible. In terms of climate, this may even be true, but the atmospheric concentration of carbon dioxide has a massive effect on the biosphere. Each year, worldwide plant dry biomass production totals around 170 billion tons, of which about half, or 85 billion tons, comprises carbon. All of that carbon is derived from atmospheric carbon dioxide at a rate that is closely dependent on the concentration of carbon dioxide in the atmosphere.

The reason for the dependence of photosynthetic production on the atmospheric carbon dioxide concentration is that 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 also be a path for water vapor to diffuse 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 to them also rises. (When available water has been exhausted, plants close down the path for gaseous diffusion between photosynthetic tissue and atmosphere and both photosynthetic production and water loss ceases.)

Australia's outback greening up: a response to rising carbon 
dioxide concentration.

Thus, an increase in atmospheric carbon dioxide concentration makes possible something like a proportional increase in plant biomass production. This effect has already been observed in the case of some crop species, and in both tropical and boreal forests. We know, for certain, therefore, that rising atmospheric carbon dioxide concentration is raising the carrying capacity of the planet for mankind and other animals by increasing yields of crops and the productivity of natural ecosystems.

Rising atmospheric carbon dioxide concentration is also changing the structure of ecosystems, because it has been shown that 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.

Related: 

CanSpeccy: Rising Atmospheric Carbon Dioxide Concentration, Part IV: Reversing the Trend

CanSpeccy: Rising Atmospheric Carbon Dioxide Concentration, Part III: Induced Stupidity and the Decline of the West

CanSpeccy: Rising Atmospheric Carbon Dioxide Concentration, Part I: Carbon Dioxide Is Not a Greenhouse Gas