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Emerging threatsShort-lived greenhouse gases cause centuries of sea-level rise

By Jennifer Chu

Published 11 January 2017

Even if there comes a day when the world completely stops emitting greenhouse gases into the atmosphere, coastal regions and island nations will continue to experience rising sea levels for centuries afterward, according to a new study. Researchers report that warming from short-lived compounds — greenhouse gases such as methane, chlorofluorocarbons, or hydrofluorocarbons, that linger in the atmosphere for just a year to a few decades — can cause sea levels to rise for hundreds of years after the pollutants have been cleared from the atmosphere.

Even if there comes a day when the world completely stops emitting greenhouse gases into the atmosphere, coastal regions and island nations will continue to experience rising sea levels for centuries afterward, according to a new study by researchers at MIT and Simon Fraser University.

In a paper published this week in the Proceedings of the National Academy of Sciences, the researchers report that warming from short-lived compounds — greenhouse gases such as methane, chlorofluorocarbons, or hydrofluorocarbons, that linger in the atmosphere for just a year to a few decades — can cause sea levels to rise for hundreds of years after the pollutants have been cleared from the atmosphere.

“If you think of countries like Tuvalu, which are barely above sea level, the question that is looming is how much we can emit before they are doomed. Are they already slated to go under, even if we stopped emitting everything tomorrow?” says co-author Susan Solomon, the Ellen Swallow Richards Professor of Atmospheric Chemistry and Climate Science at MIT. “It’s all the more reason why it’s important to understand how long climate changes will last, and how much more sea-level rise is already locked in.”

Solomon’s co-authors are lead author Kirsten Zickfeld of Simon Fraser University and Daniel Gilford, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

Short stay, long rise
Recent studies by many groups, including Solomon’s own, have shown that even if human-caused emissions of carbon dioxide were to stop entirely, their associated atmospheric warming and sea-level rise would continue for more than 1,000 years. These effects — essentially irreversible on human timescales — are due in part to carbon dioxide’s residence time: The greenhouse gas can stay in the atmosphere for centuries after it’s been emitted from smokestacks and tailpipes.

In contrast to carbon dioxide, other greenhouse gases such as methane and chlorofluorocarbons have much shorter lifetimes. However, previous studies have not specified what their long-term effects may be on sea-level rise. To answer this question, Solomon and her colleagues explored a number of climate scenarios using an Earth Systems Model of Intermediate Complexity, or EMIC, a computationally efficient climate model that simulates ocean and atmospheric circulation to project climate changes over decades, centuries, and millenia.

With the model, the team calculated both the average global temperature and sea-level rise, in response to anthropogenic emissions of carbon dioxide, methane, chlorofluorocarbons, and hydrofluorocarbons.