Over the past 100 years, the global average sea level has risen by roughly 7 inches. This rise has been fuelled by two key factors — the added water from melting land ice, and the expansion in volume of seawater as it absorbs heat from the atmosphere.

If we examine the trend over the past 20 years alone, world's oceans have warmed at a rate of 0.12 degrees Celsius per decade — a continuation of the trend that began in the last half of the 20th century, when humans began pumping massive quantities of heat-trapping greenhouse gases into the atmosphere.

In such a scenario, there is a question climate activists have often asked — what, if anything, can be done to prevent sea levels from rising to an extent that poses an existential threat to low-lying island nations such as Fiji, the Marshall Islands and the Solomon Islands?

The answer, going by the findings of a new study published in the Proceedings of the National Academy of Sciences, is not much.

This is because oceans, once they have absorbed a certain amount of heat, take hundreds of years to cool down — a phenomenon the authors of the study called "ocean inertia."

"As the heat goes into the ocean, it goes deeper and deeper, giving you continued thermal expansion," study co-author Susan Solomon, a professor of climate science at the Massachusetts Institute of Technology, said in a statement. "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."

In order to reach their conclusions, the researchers used a climate model to simulate three scenarios —global greenhouse gas emissions ending in 2050, 2100, and 2150. Even in the most optimistic scenario, wherein anthropogenic emissions of all heat-trapping gases ceased altogether in 2050, up to 50 percent of carbon dioxide would remain in the atmosphere for over 750 years. And, even after carbon dioxide emissions cease, sea levels will continue to rise, measuring twice the level of 2050 estimates for 100 years, and four times that value for another 500 years.

Even methane — a gas that has an atmospheric lifespan of just 10 years — would continue to contribute to sea level rise for centuries after it has cleared up from the atmosphere.

Effectively, this means that even if humans were to stop emitting all greenhouse gases right now, thermal expansion and the ensuing rise of ocean levels would continue for centuries to come — quite possibly inundating several island nations and low-lying coastal areas in cities across the world.

"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?" Solomon said. "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."

There is, however, one silver lining. As part of their study, the researchers also examined what impact the 1987 Montreal Protocol, which eliminated the use of the ozone-depleting chlorofluorocarbons, had had on stemming the rise of ocean levels. They found that if the deal had never been ratified, and if countries had continued to emit CFCs, the world would have experienced up to an additional 6 inches of sea-level rise by 2050.

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