The term “carbon cycle” refers to the natural two-way flows of carbon that are driven by physical, chemical, and biological processes on Earth. Each year plant photosynthesis and ocean dissolution remove 25% of the carbon dioxide from the atmosphere, an amount equivalent to 200 billion tons of carbon. But an almost equal amount—195 billion tons of carbon—is released back to the atmosphere by respiration and ocean outgassing. That near balance is akin to an individual person maintaining a relatively constant body weight despite annually consuming, according to the US Department of Agriculture, nearly a ton of food. The difference between the natural removals and releases reflects the carbon cycle’s response to human activities, including fossil-fuel combustion, cement production, and land-use changes, such as the conversion of forests to agricultural lands.
The excess CO2 that’s accumulating in the atmosphere comes primarily from fossil-fuel combustion. Because carbon-14 has a half-life...
