The planet naturally releases and absorbs far more carbon dioxide than humans emit by burning fossil fuels. The problem is that human activities have thrown the Earth’s carbon cycle out of balance.
The Earth’s natural carbon cycle moves a staggering amount of carbon dioxide (CO2) around our planet, says Daniel Rothman, MIT professor of geophysics. Some parts of the planet, such as the oceans and forests, absorb carbon dioxide and store it for hundreds or thousands of years. These are called natural carbon sinks. Meanwhile, natural sources of CO2 such as undersea volcanoes and hydrothermal vents release carbon. Altogether the planet absorbs and emits about 100 billion tons of carbon dioxide through this natural cycle every year, Rothman says.
That total dwarfs humanity’s contribution, amounting to ten times as much CO2 as humans produce through activities such as burning fossil fuels.
If people emit only a tenth as much CO2 as nature does, then why are scientists so concerned about our emissions driving climate change? It is because our extra chunk of carbon emissions has tipped out of equilibrium what was once a balanced cycle. “What's being taken out by natural processes is more or less equal to what's being put in—other than the extent to which we've disturbed it,” Rothman says. This is why the atmospheric level of CO2 continues to creep up as humans keep burning fossil fuels: Human activities tip the scales by adding carbon to the air faster than the planet’s sinks can absorb it.
Time is the key to understanding this problem, Rothman says, because although the natural carbon cycle balances itself, it does so over exceedingly long timescales. For example, consider one part of the natural carbon cycle: how fossil fuels are created and released. Hydrothermal vents on the seafloor provide the carbon that—via heat, pressure, and other forces below the planet’s surface—is pressed into fossil fuels such as oil and gas. Over thousands or millions of years, the creeping movement of our planet’s tectonic plates brings those fossil fuels back to the Earth’s surface and slowly emits the CO2 into the air. But mining those fossil fuels and then burning them in cars or factories shortcuts nature’s method. “That full [natural] process would eventually bring it all up—but very slowly,” Rothman says. “What we're doing with taking oil and gas out of the ground is essentially speeding up the natural process.”
Although humans have added our own emissions on top of natural carbon sources, we cannot speed up the work of most of the natural carbon sinks that absorb CO2 from the air. Rothman says it takes centuries for carbon dioxide in the atmosphere to absorb into the oceans. It takes another 10,000 years or so for natural mechanisms to remove excess carbon from the oceans and return them to equilibrium (1).
Because of the glacial pace at which natural carbon sinks absorb CO2, much of the carbon dioxide humans have emitted over the past centuries will remain in the atmosphere for many years to come. This will be true even if humans were to stop emitting all greenhouse gases tomorrow—the planet would need hundreds or thousands of years to cleanse all the excess CO2 people have pumped into the atmosphere during the industrial era.
“It doesn't have anywhere to go… and it's not going to go away for a long time,” Rothman says.
Thank you to several readers for sending in related questions, including Michael Legge of Victoria, British Columbia, and Howland Larsen of Gig Harbor, Washington. You can submit your own question to Ask MIT Climate here.
Published January 4, 2022.
1 When the oceans grow more acidic because of excess CO2, Rothman explains, the waters dissolve calcium carbonate minerals on the seafloor, mostly the shells of planktonic organisms that have settled there. Calcium also flows from rivers into the oceans. Both processes contribute to the slow formation of limestone from calcium and carbon, making the oceans less acidic over very long timescales and preventing the runaway acidification of the oceans we’re beginning to see with human-caused climate change today.