Greenhouse gases to leave a long-lasting legacy

As the world waits to see whether US President-elect Donald Trump will scrap the Obama administration's Climate Action Plan, a new study suggests that like a bad smell that continues to linger in a room, even when short-lived greenhouse gases disappear from the atmosphere, they'll continue to have effects in the form of rising sea levels for centuries to come.

The study, which was conducted by MIT and Simon Fraser University researchers, focuses on the impact of short-lived greenhouse gasses, such as methane and chlorofluorocarbons, on sea levels. Though most of the attention on global warming has focused on carbon dioxide (CO₂), methane is proving itself to be just as formidable a foe.

For starters, it's a heat absorbing powerhouse that, according to the Intergovernmental Panel on Climate Change, can warm the planet up to 86 times more than CO₂ during its decade-long spell on earth. Its long-term effects are equally worrisome. According to the study, while methane-induced atmospheric warming might decrease as the gas dissipates, the same cannot be said about its warming effect on oceans, which manifests itself in the form of rising sea levels, even centuries after the gas molecules have disappeared.

This is due to ocean inertia, says study author Susan Solomon, a professor of atmospheric science at MIT. There's a reason, despite all the warnings from climate scientists, that we are not experiencing any noticeable change in ocean temperatures or sea levels yet and that's because it takes a very long time to warm up the waters given their vast size. However once they do heat up, the bad news is that it will take an equally long time for them to cool down again. And you know the drill: once the waters expand, sea levels will follow suit and rise as well.

"As the heat goes into the ocean, it goes deeper and deeper, giving you continued thermal expansion," explains Solomon. "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 their study, the researchers used a climate model called the Earth Systems Model of Intermediate Complexity to project various climate change scenarios. In the case of methane, even if countries found a way to stop emissions by 2050, rising sea levels would still be a problem as far down the road as 2900 due to the residual heat trapped in the oceans.

The projections get grimmer when carbon dioxide and nitrous oxide are added to the equation. If emissions continue to accelerate till 2050, sea-levels would rise by three feet (0.9 m) via thermal expansion alone by 2900, and continue increasing after that. For island nations that are barely above sea level, like Tuvalu, this is worrying news, indeed.

The study also corroborated previous predictions about the impact of carbon dioxide and showed that even if the world managed to curb emissions by 2050, up to half the gas would still be in the atmosphere more than 750 years later. Sea levels would also rise, measuring twice the level of 2050 estimates for 100 years, and four times that value for another 500 years.

In a nutshell: climate effects are here to stay. Slashing greenhouse gas emissions doesn't mean we'll see a corresponding turnaround in climate change problems. In fact, things might have to get worse before they get better.

"Amazingly, a gas with a 10-year lifetime can actually cause enduring sea-level changes," says Solomon. "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."

This study comes at a time when an increasing body of research is showing that atmospheric methane and carbon dioxide levels are rising, a problem that might be compounded by thawing Arctic permafrost, which scientists such as Peter Wadhams, head of the Polar ocean physics group at Cambridge University, have warned could cause severe global economic repercussions, as The Guardian reports.

This threat reinforces why it is vital for governments to commit to regulating greenhouse gases. While cynics might question whether countries can put their self-interests aside, the success of the Montreal Protocol, an international treaty drawn up to protect the Earth's ozone layer by phasing out the production of ozone-depleting pollutants such as chlorofluorocarbons, shows that such regulations can and do work. Not only is the ozone layer recovering, the researchers found that it has also had an unintentional, albeit positive, impact on sea levels, which would have risen an extra six inches (15 cm) by 2050 if the treaty ban had not come into effect.

"Half a foot is pretty significant," says Solomon, who led the first Antarctic expedition to study the hole in the ozone layer and whose work helped pave the way for the treaty. "It's yet another tremendous reason why the Montreal Protocol has been a pretty good thing for the planet."

The Kyoto Protocol, the world's first international agreement to combat climate change and greenhouse gas emissions, has been far from successful in addressing the issue. While 21 countries, including Latvia and Romania, met their emission targets, they are hardly the world's biggest emitters. In hindsight, the fact that the three countries with the world's highest CO₂ emission rates – China, India and the US – were not part of the treaty shows how flawed it was right from the start. With the lessons learnt from this first agreement, which ought to be seen as an experiment instead, can the Paris Agreement do for climate change what the Montreal Protocol has done for the ozone layer?

The issue of global warming is one that invokes polarizing opinions across the board and one of the problems, where skeptics are concerned, is that predictions are just that – predictions. However if the authors of this study are correct, then it would probably be advisable to not wait for these forecasts to become a reality before coming up with a contingency plan. Curbing greenhouse emissions now will not magically lower sea levels immediately but the idea is to focus on playing the long game.

"The primary policy conclusion of this study," say the study's authors, "is that the long-lasting nature of sea-level rise heightens the importance of earlier mitigation actions."

The study was published in PNAS.

Source: MIT