Short-lived greenhouse gases cause centuries of sea-level rise

The researchers’ estimates for carbon dioxide agreed with others’ predictions and showed that, even if the world were to stop emitting carbon dioxide starting in 2050, up to 50 percent of the gas would remain in the atmosphere more than 750 years afterward. Even after carbon dioxide emissions cease, sea-level rise should continue to increase, measuring twice the level of 2050 estimates for 100 years, and four times that value for another 500 years.

The reason, Solomon says, is due to “ocean inertia”: As the world warms due to greenhouse gases — carbon dioxide included — waters heat up and expand, causing sea levels to rise. Removing the extra ocean heat caused by even short-lived gases, and consequently lowering sea levels, is an extremely slow process.

“As the heat goes into the ocean, it goes deeper and deeper, giving you continued thermal expansion,” Solomon explains. “Then it has to get transferred back to the atmosphere and emitted back into space to cool off, and that’s a very slow process of hundreds of years.”

Stemming tides
In one particular climate modeling scenario, the team evaluated sea level’s response to various methane emissions scenarios, in which the world would continue to emit the gas at current rates, until emissions end entirely in three different years: 2050, 2100, and 2150.

In all three scenarios, methane gas quickly cleared from the atmosphere, and its associated atmospheric warming decreased at a similar rate. However, methane continued to contribute to sea-level rise for centuries afterward. What’s more, they found that the longer the world waits to reduce methane emissions, the longer seas will stay elevated. 

“Amazingly, a gas with a 10-year lifetime can actually cause enduring sea-level changes,” Solomon says. “So you don’t just get to stop emitting and have everything go back to a preindustrial state. You are going to live with this for a very long time.”

The researchers found one silver lining in their analyses: Curious as to whether past regulations on pollutants have had a significant effect on sea-level rise, the team focused on perhaps the most successful global remediation effort to date — the Montreal Protocol, an international treaty ratified by 197 countries in 1989, that effectively curbed emissions of ozone-depleting compounds worldwide.   

Encouragingly, the researchers found that the Montreal Protocol, while designed to protect the ozone layer by phasing out pollutants such as chlorofluorocarbons — has also helped stem rising seas. If the Montreal Protocol had not been ratified, and countries had continued to emit chlorofluorocarbons to the atmosphere, the researchers found that by 2050, the world would have experienced up to an additional 6 inches of sea-level rise.

“Half a foot is pretty significant,” Solomon says. “It’s yet another tremendous reason why the Montreal Protocol has been a pretty good thing for the planet.”

In their paper’s conclusion, the researchers point out that efforts to curb global warming should not be expected to reverse high seas quickly, and that longer-term impacts from sea-level rise should be seriously considered: “The primary policy conclusion of this study is that the long-lasting nature of sea-level rise heightens the importance of earlier mitigation actions.”

Reprinted with permission of MIT News