Short-term greenhouse gases, long-term impact: What does that mean for Earth's oceans?

The notion that greenhouse gases are warming the Earth is nothing new, as is the notion that methane, while having 23 times the heat retention capabilities of carbon dioxide, is rarer and breaks down relatively quickly. Many efforts to curb climate change, therefore, tend to focus on CO2, which can linger in the atmosphere for centuries, rather than methane, which is generally expected to be absorbed into soil or broken down in the atmosphere after about 12 years.

But a new study indicates that the negative effects of short-lived gases such as methane might last longer than previously thought, at least as far as the ocean is concerned.

According to researchers, methane and other rarer greenhouse gases contribute a lot of extra heat to water, which is much harder to cool down once it has been warmed up. If the new study is accurate, eliminating greenhouse gases within a relatively short span of time will not be able to cool down the oceans sufficiently to avoid further ice melt at the poles. As a result, ocean levels could continue to rise for centuries, even after greenhouse gas emissions are finally reduced to zero.

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

Air in the atmosphere actually has a fairly low heat capacity, which is the amount of energy that must be put into something in order for it to change its temperature. Air, therefore, does not hold much heat compared to the ocean, which has a much higher heat capacity.

"You probably have a feel for this if you've ever tried to boil a pot of water," Josh Willis, an oceanographer at NASA's Jet Propulsion Laboratory, explained in a 2006 article for NASA's Earth Observatory. "You have to burn a lot of gas or wood to heat up the water. But if you had a similar quantity of air, it would take a lot less energy to heat it up to the same temperature."

As a result, the ocean heats up a lot less quickly than the air around it, and its huge mass helps absorb a lot of the heat added by greenhouse gases. But unlike the air, the ocean will hold on to the heat it does absorb for a lot longer.

In the recent study published online on Monday in the Proceedings of the National Academy of Sciences, researchers explored scenarios that used an Earth Systems Model of Intermediate Complexity (EMIC), a type of computer model that simulates ocean and atmospheric circulation to project climate changes, in order predict the effect of eliminating greenhouse gases by 2050, 2100, and 2150. They found that even in their best-case scenario of fully eliminating emissions by 2050, sea levels would continue to rise through thermal expansion for more than 500 years afterward.

"[Methane, like CO2] contributes to temperature rise," Keah Schuenemann, a meteorology professor at Metropolitan State University of Denver, tells The Christian Science Monitor in an email. "The somewhat interesting piece is that even though methane concentrations could come down more quickly than carbon dioxide concentrations due to its shorter lifetime, because methane contributes to global warming, it contributes to the long term commitment of sea level rise."  

While the study may carry an element of depressing inevitability, it should be noted that strong climate change policy can and does have a positive effect on curbing human-caused climate change. An international treaty known as the Montreal Protocol went into force in 1989 to eliminate chemicals like chlorofluorocarbons (CFCs) that deplete Earth's ozone layer. In addition to significant progress toward mending the hole in the ozone layer over Antarctica, the researchers on the methane sea level project created models that found that if the Montreal Protocol had not gone into effect, oceans would have been 1.4 inches higher by 2050 if all other conditions remained the same.

"It's really quite an achievement that the world has to celebrate that we did agree on the Montreal Protocol," Solomon told The Washington Post. "And the challenge now is for us to think about other gases."

Currently, international efforts like the Paris Agreement are underway in order to minimize the effects of climate change by reducing the amount of greenhouse gases being produced worldwide, including CO2 and methane. This study comes amid concerns about President-elect Donald Trump's stated view that human-caused climate change is a "hoax," despite a near-consensus in the scientific community that this view is incorrect.

"The longer we wait to curb our emissions of all greenhouse gases, the larger our long term commitment to sea level rise," says Dr. Schuenemann. "This is why the Paris agreement is so vital and the United States needs to keep our commitment to it. Every year that we contribute to the problem, we are committing to large impacts down the road."