How is carbon taken out the air?

Some of it is absorbed by sea water, where it reacts with water to form carbonic acid, decreasing the ocean pH.

In general, tree leaves and grass pull CO2 out of the air in the spring and summer months, and release it in the fall as they die. We have been able to measure and monitor this seasonal cycle since the 1950s. (See link).

In ages past vast amounts of carbon dioxide was pulled from the atmosphere by various organisms. During the Permian, 300 million years ago, trees and vegetation that died toppled into the watery muck where they did not fully decay. The carbon taken up by these plants remained buried, eventually turning into vast coal and oil deposits.

At other times small marine organisms would take up carbon in their shells, forming huge limestone deposits, eventually sequestering billions of tons of atmospheric carbon. During the cretaceous calcareous coccolithophores gradually settled out of ocean water forming deep chalk deposits. Much of this activity ended with the conclusion of the cretaceous, the name of which comes from the Greek Word "creta" for "chalk."

Fifty million years ago an arctic freshwater fern known as "Azolla" proliferated over the polar ocean surface. As the ferns died they sank to the sea floor, where they did not decay. The build up of dead fern mass pulled billions more tons of carbon out of the atmosphere, cooling the earth enough to precipitate a long series of ice ages.

Today we are pumping CO2 into the atmosphere much faster than the earth can adjust. This may not seem reasonable, as the earth is large--its diameter is nearly 8000 miles. But we have been puffing over 30 billion tons of CO2 into the air every year, and this steady increase is beginning to have not just measurable but noticeable impact. In 1900 CO2 was 290 ppm. By 1950 it had risen to 300. Today it is nearly 400, and it will exceed 500 ppm before 2050.