Carbon dioxide, or CO₂, is a greenhouse gas.

The global carbon cycle

Greenhouse gases are constantly cycling between the land, ocean, and atmosphere.

There are different "stocks" of carbon, such as in the atmosphere or permafrost. They are measured as "gigatons of carbon", or GtC. 1 Gt is 1 x 10⁹ metric tons. "Fluxes" are the movement of carbon from one stock to another. These are generally measured using GtC/year. Currently we see a net positive flux of 5.1GtC/year.

Carbon sinks

A "sink" draws carbon out of the atmosphere. The main carbon sinks are vegetation (plants using CO₂ for photosynthesis) and the ocean, which dissolves massive amounts of CO₂ from the atmosphere.

Carbon sources

A "source" is a stock that emits carbon to the atmosphere. The largest source is the burning of fossil fuel.

While sinks have grown somewhat over time to absorb the additional carbon in the atmosphere, both sinks and store may become less reliable over time. Wildfires are one example of this. As wildfires become more frequent and intense, more trees that had been absorbing and storing carbon are burned, releasing that carbon straight back into the atmosphere.

It is now likely that the Amazon is a net source of greenhouse gases, with more than half of Amazonia impacted by humans. In California, emissions from the massive wildfires of 2020 were more than enough to cancel out the state's years-long efforts to reduce GHGs.

Another issue is ocean acidification. As the ocean absorbs carbon, it becomes more acidic. This is already affecting coral reefs and broader food chains, and it may lead to new feedbacks and complications that are hard to predict. Permafrost melting is another concern, as it releases vast amounts of methane.

Impact of atmospheric carbon

High atmospheric carbon dioxide can impact food quality as it lessens the amount of nutrients.