Scientists estimate that natural “sinks” like plant growth and ocean absorption remove about half of the CO2 humans add to the atmosphere by burning fossil fuels. The rest stays in the atmosphere, amplifying solar-driven warming. That overshoot has led to the current levels of greenhouse gases in our atmosphere – more than 400 parts per million – unprecedented in the past 800,000 years.
When plants grow, they absorb CO2 from the air to make food. When they die, their remains decay, releasing carbon dioxide into the soil or water. Those cycles balance each other over time. But when people disrupt those natural processes by deforesting land or burning fossil fuels, carbon emissions jump into the atmosphere and cause global warming.
The natural CO2 cycle is shown in this graph of measured atmospheric carbon dioxide at Mauna Loa, Hawaii. The wiggles in the trend line show annual changes caused by seasonal cycles in photosynthesis. Each spring, plants in the Northern Hemisphere begin growing and drawing in CO2 from the air for their food. That slows down and then reverses in autumn. The boom-and-bust cycle in the graph is why CO2 levels vary from year to year and why the long-term trend line shows a steady climb upward.
Most of the human-caused carbon dioxide emissions come from the transportation sector, which includes cars, trucks, ships, and planes. They use fossil fuels like coal and oil to burn, releasing carbon dioxide in the process. Also, petroleum-based products like plastics, solvents and lubricants are created from fossil fuels, and when they evaporate, dissolve or wear out, release more carbon dioxide.