Short-term greenhouse gases cause long-term sea level rise
Methane and other gases with a 10-year lifetime can cause enduring sea-level changes, thanks to the ocean's capacity to store heat.
Even if we stop pumping greenhouse gases into the atmosphere, sea levels will continue to rise for hundreds of years afterwards – and it's not just carbon dioxide, which lingers in the atmosphere for centuries, that's to blame.
Emissions such as methane and chlorofluorocarbons, which dissipate within a year to a decade, also play a big part in this extended effect, new modelling shows.
Kirsten Zickfeld from Canada's Simon Fraser University and Susan Solomon and Daniel Gilford, both from Massachusetts Institute of Technology in the US, found oceans retain heat long after these short-term greenhouse gases have disappeared.
The work was published in the Proceedings of the National Academy of Sciences.
Recent studies have shown that even if we ended all carbon dioxide emissions today, the atmosphere would continue to warm and sea levels rise for another millennium – partly as ice sheets melted and partly from thermal expansion.
This is partly because carbon dioxide hangs around in the atmosphere. If we stopped emitting carbon dioxide starting in 2050, up to half the gas would remain in the atmosphere more than 750 years afterwards.
But no one had modelled the effects of shorter-lived gases, such as methane and chlorofluorocarbons, which only hang around for a few years.
So Zickfeld, Solomon and Gilford modelled sea level response if we cut methane emissions entirely in 2050, 2100 and 2150.
The gas quickly cleared from the atmosphere, along with its associated atmospheric warming. But even though it had gone, methane continued to contribute to sea-level rise for centuries afterward. And the longer the world waits to reduce methane emissions, the longer seas will stay elevated.
How? The driver is "ocean inertia". As the world warms due to greenhouse gases, including carbon dioxide and methane, waters oceans heat and expand and sea levels rise.
Removing the extra ocean heat caused by even short-lived gases such as methane, and consequently lowering sea levels, is extremely slow.
"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."
It's not all bad news, though. The researchers also found the 1989 Montreal Protocol, which was designed to protect the ozone layer by phasing out pollutants such as chlorofluorocarbons, also helped slow 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 15 centimetres 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."