As human activity has increased the amount of carbon dioxide in the atmosphere, carbon cycling has helped to buffer against some of the greenhouse gas's warming effects. But over time, if carbon dioxide levels continue to increase, the planet will become progressively less able to sequester CO2 in the soil or deep within the ocean.
Scott Doney, a senior scientist at the Woods Hole Oceanographic Institution in Massachusetts, compares the scenario to an open faucet flowing into a basin (of air), with drains that allow some of the CO2 to escape into the soil and ocean. As time passes and global temperatures rise, the drains eventually become smaller and smaller, so more CO2 begins to collect, he said.
Now, with the help of long-term global climate-carbon modeling, Doney and other researchers are developing a clearer understanding of when, over the next few centuries, the land and oceans are likely to behave more like those hypothetical shrinking drains, taking in less and less carbon.
Using the Community Earth System Model, the researchers mapped out how global carbon cycling would be affected if the rate of fossil fuel use continued to increase at a "business as usual" rate. The model started from the preindustrial period in 1850 and went all the way to 2300. They found that after the end of the 21st century, the oceans' ability to absorb more carbon dioxide would decrease considerably because of climate change and would just about equal the impact of terrestrial ecosystems on global carbon cycling after 2100.
The findings, published in Global Biochemical Cycles, contrast with previous models using shorter time scales that suggested ocean carbon cycles did not change much from century to century, said James Randerson, a chancellor's professor of earth system science at the University of California, Irvine, and the lead author of the study.
Lowering available space in the marine 'vault'
He explained that under normal ocean conditions, carbon dioxide that dissolves in ocean surface waters will eventually mix with deeper waters, driving the carbon downward. But as ocean temperatures warm and the layers of water become more stratified, the waters mix less.
Not only that, but "with climate change interacting with oceans, it's causing ocean currents to shut down so that carbon is no longer pushed down into the deep ocean," he said.
While scientists had known about these climate effects, no one had connected those effects to global carbon cycling hundreds of years into the future, according to Randerson.
Inez Fung, a professor of atmospheric science at the University of California, Berkeley, who was not involved with the study, said the research demonstrated that nothing on the planet is static.
"We cannot assume what's going on now will apply in the future," she wrote in an email. "It has always been assumed that the oceans, with the vast storage capacity, will dominate the uptake in the future. Not so, these authors tell us. Climate change in essence isolates the deep ocean (the vault) from the atmosphere. So in the end, the ocean storage capacity is significantly reduced."
A 20 C Arctic temperature spike
The global model also predicted increases in CO2 weakening the amount of carbon sequestered by land-based ecosystems after 2100, though the degree of weakening wasn't uniform across the planet. For instance, tropical ecosystems in South and Central America were much more affected than tropical regions in Africa and Asia. On land, the effects of climate change limited the growth and production of plants and increased the activity of microbes, he said.
Over the 550 years modeled in the study (1850 to 2300) temperatures increased 9.3 degrees Celsius (16.7 degrees Fahrenheit), and roughly half of that warming happened after 2100. The warming effect was substantially greater at higher latitudes in the Arctic, where temperature rose a whopping 20 C (36 F).
Even though the changes were well into the future, the researchers emphasized the importance of understanding future global conditions.
"The impacts of climate change don't stop at 2100; they intensify beyond that," Randerson said. "It's really important to understand the long-term stability of the planet."
For Doney, acting on climate change is a matter of stewardship.
"It's our responsibility to pass on [the planet] in a good state," he said. "With carbon dioxide, a good chunk of what we emit now will be in the atmosphere in several hundred years."