Unbalancing the Cycle
We humans are unbalancing a fundamental Earth process: the carbon cycle. Carbon moves continuously through the atmosphere, plants and animals, soils, the ocean and the rocks of the solid Earth, in time-spans ranging from hours to tens of millions of years. Throughout Earth's history, the balance of carbon in those various reservoirs has kept the atmosphere's carbon dioxide (CO2) level, and Earth's temperature, within relatively narrow ranges.
Now, we're withdrawing carbon--in the form of coal and oil--from the long-term rock reservoir in which it sat for tens of millions of years, and pumping vast amounts of it into the other reservoirs, particularly the atmosphere, where it resides as carbon dioxide.