The process is called “thermal expansion,” and the science behind it is relatively simple: When greenhouse gases go into the atmosphere, they cause air temperatures to rise. Some of the heat ends up being absorbed into the oceans, causing the water to expand in volume.
“If you’ve ever made a cup of tea on the stove you know that hot water expands,” said Susan Solomon, a professor of environmental studies at the Massachusetts Institute of Technology. “It’s simply basic physics, and it’s something that the planet can’t get away from.”
Thermal expansion is a well-documented phenomenon that climate scientists generally take into account when making modeled projections about future sea level rise. But an issue that may have received less attention is just how long this process lasts. Even if humans stopped emitting greenhouse gases into the atmosphere tomorrow, the expansion effect would continue in the oceans for centuries more, making it effectively irreversible in our lifetimes.
A new study in the journal Proceedings of the National Academy of Sciences, written by Solomon and colleagues Kirsten Zickfeld of Simon Fraser University and Daniel Gilford of MIT, underscores the fact that even greenhouse gases that don’t last long in the atmosphere — methane, for instance — can have centuries-long impacts on the expanding oceans. So although the atmospheric warming they cause may taper off comparatively quickly after their emissions are halted, their effects in the oceans are much longer-lived.
“The ocean never forgets — that’s the essential message of this paper,” Solomon said.
The researchers used a climate model to examine the effect of various greenhouse gases on thermal expansion in the oceans. They started with a “business-as-usual” scenario, which assumes high emissions into the future. They applied this scenario to emissions of carbon dioxide, methane, nitrous oxide and various halocarbons, a group of chemicals including the chlorofluorocarbons and hydrofluorocarbons.
In their simulation, they allowed the emissions to continue until the year 2050 and then abruptly cut them all off so their atmospheric levels would begin to decline. Compared with carbon dioxide, the other gases have relatively short atmospheric life spans — methane, for instance, stays in the atmosphere for only about a decade, compared to carbon dioxide’s potential 200 years or more.
Even so, their effects persist in the ocean for hundreds of years. One hundred years after the emissions stop, the model suggests that 75 percent of the peak amount of thermal expansion caused by methane still persists — and 40 percent remains even after 500 years. Allowing the greenhouse gas emissions to continue unabated for longer periods of time produced even more severe effects.
The reason these effects last so long has to do with a fundamental aspect of the way the ocean moves. Major ocean currents all over the world rely on a process known as “overturning circulation.” Warm water flows from the equator to the poles, where it cools down, sinks to the bottom of the ocean, and joins a stream of cooler water flowing back toward the equator. This process carries heat all over the world.
When heat goes into the ocean from the atmosphere, it tends to get caught up in this overturning circulation, which carries it all over the planet, Solomon explained. It can take a time scale of hundreds of years before the heat is released from the water back into the air.
The study underlines the importance of curbing greenhouse gas emissions as quickly as possible to avoid an even longer sentence, the authors note. In fact, they included a separate modeling experiment in their study that demonstrates the significance of early climate action.
In the late 1980s, nations around the world agreed on an international treaty called the Montreal Protocol, which aimed to protect the earth’s ozone layer by phasing out certain damaging chemicals — notably, chlorofluorocarbons and hydrochlorofluorocarbons. (The treaty was recently updated to include the phasing out of hydrofluorocarbons as well.) The researchers decided to investigate what would have happened to the oceans if this treaty had never been adopted. So they ran two simulations in which ozone-depleting emissions were allowed to continue at high levels until the year 2015 and the year 2050, respectively — then they cut the emissions off.
The model indicated that, if not for the treaty, we would have seen about 3.7 centimeters, or 1.4 inches, of extra sea-level rise by the year 2100 — and nearly 14 centimeters, or more than 5 inches, in the 2050 scenario. This experiment shows what the world avoided, Solomon said.
Recent discussions about climate mitigation strategies have begun to include the possibility of carbon dioxide removal — a form of geoengineering that would use technology to pull carbon back out of the atmosphere. This technique can be viewed as a kind of Hail Mary strategy to prevent more global warming from occurring after the greenhouse gases have been emitted — but the authors of the new study caution that it won’t stop all the effects of climate change.
“A scenario that reduces atmospheric temperature cannot be assumed to simultaneously eliminate future sea-level rise, due to the time scales associated with release of stored energy in the ocean,” they note in the paper.
However, the technology isn’t developed enough for wide-scale use. For now, the researchers suggest that the best strategy is to follow through with commitments to halt greenhouse gas emissions as soon as possible, keeping in mind that their influence on the planet will far outlast our lifetimes.
“It’s really quite an achievement that the world has to celebrate that we did agree on the Montreal Protocol,” Solomon said. “And the challenge now is for us to think about other gases.”