As carbon dioxide levels rise natural sinks are likely to keep the atmosphere in balance by negative feedback mechanisms.
True up to a point but in the long run the effects of drought and temperature rise are likely to overwhelm any short term increase in plant growth caused by increasing carbon dioxide concentrations.
Amazonia accounts for more than half the world’s rainforest and has been highly important in the past in removing an estimated 2 billion tonnes of carbon dioxide from the atmosphere each year. However recent droughts, particularly the harsh one of 2005, have killed many trees and drastically reduced the forest’s ability to take up carbon dioxide. Furthermore, drought has increased the rate at which carbon is lost from Amazonian soil to the atmosphere. These effects have combined to turn the Amazon forest from being a net remover (‘sink’) of carbon dioxide to a net producer, adding an estimated 5 billion tonnes of carbon dioxide to the atmosphere.
In addition, higher atmospheric carbon dioxide levels are making the oceans more acidic. Although acidification is seriously damaging corals, surprisingly, it does not appear to alter the ability of some planktonic plants to remove carbon dioxide from the atmosphere by photosynthesis and there appears to be no net change in how much carbon dioxide they remove.
Tropical rainforests and other mechanisms could provide positive or amplifying feedback mechanisms leading to progressively accelerating (runaway) climate change. Other possible positive feedbacks include the reduced ability of warmer sea water to dissolve and hence store carbon dioxide and the increased warming of the Arctic Ocean resulting from the decreased reflection of heat into space, as the ice melts, known as the albedo effect. Most seriously, melting of the permafrost and warming of the oceans could lead to accelerated release of huge quantities of methane, an even more powerful greenhouse gas than carbon dioxide, from sediments.