The Carbon CycleThe Carbon Cycle

The Carbon Cycle

According to the Australian Academy of Sciences:

Large amounts of CO2 are continually transferred to and from the atmosphere, which exchanges carbon with the oceans and vegetation on land. Until around 200 years ago, these natural exchanges were in rough balance, shown by the nearly constant concentrations of atmospheric CO2 for most of the last two thousand years.

The importance of human-caused CO2 emissions is that they are disturbing this balance, adding carbon to the atmosphere faster than it can be removed by uptake by vegetation, the slow mixing of CO2 into the deep oceans, or the even slower weathering processes that control the carbon balance on geological timescales.

According to the NASA Earth Observatory:

Carbon is the backbone of life on Earth. We are made of carbon, we eat carbon, and our civilizations-our economies, our homes, our means of transport-are built on carbon. We need carbon, but that need is also entwined with one of the most serious problems facing us today: global climate change.

Forged in the heart of aging stars, carbon is the fourth most abundant element in the Universe. Most of Earth's carbon-about 65,500 billion metric tons-is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels.

Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth.

  NASA carbon cycle diagram

Over the long term, the carbon cycle seems to maintain a balance that prevents all of Earth's carbon from entering the atmosphere (as is the case on Venus) or from being stored entirely in rocks. This balance helps keep Earth's temperature relatively stable, like a thermostat.

This thermostat works over a few hundred thousand years, as part of the slow carbon cycle. This means that for shorter time periods-tens to a hundred thousand years-the temperature of Earth can vary. And, in fact, Earth swings between ice ages and warmer interglacial periods on these time scales. Parts of the carbon cycle may even amplify these short-term temperature changes. 

NASA change in climate over time

On very long time scales (millions to tens of millions of years), the movement of tectonic plates and changes in the rate at which carbon seeps from the Earth's interior may change the temperature on the thermostat. Earth has undergone such a change over the last 50 million years, from the extremely warm climates of the Cretaceous (roughly 145 to 65 million years ago) to the glacial climates of the Pleistocene (roughly 1.8 million to 11,500 years ago).