There’s a pretty story we tell ourselves about environmental problems: Once you fix them, they immediately start to improve.
Smog works like this. In cities where air quality is a problem, smog tends to worsen on weekdays, because millions of people are commuting and factories are fully productive. On weekends, when fewer people drive, the air tends to clear.
Likewise, when the country chose to address its smog problem, it got better. In 1970, Congress passed the Clean Air Act and told the EPA to start regulating air pollution. Smog across the country began to dissipate, and certain lung conditions became less common. Air pollution is not the problem today that it was in the 1960s and early 1970s because the United States addressed it.
It is a pleasant story. It’s true for some issues. For global warming, it is a fable.
Greenhouse gases released into the atmosphere today will cause the seas to rise for centuries to come, even if those gases leave the atmosphere relatively rapidly, finds a new study published Monday in the Proceedings of the National Academy of Sciences.
The study specifically examined short-lived greenhouse gases like methane. Methane is the main ingredient in natural gas, and it contributes about a third of modern-day global warming. It’s very powerful, trapping heat 25 times more efficiently than carbon dioxide, but it’s also ephemeral. On average, a molecule of methane is absorbed in the soil or destroyed in the atmosphere 12 years after it is emitted. A molecule of CO₂ can float around for centuries.
Yet the paper shows that the consequences of that methane molecule will last for more than a millennium, causing the the seas to rise higher and higher all the time. That’s because sea-level rise is not only caused by extra water, but by hotter water. As the oceans absorb heat, they expand—and it takes a very long time for this heat to leave.
“The ocean remembers, and that’s really the key message,” says Susan Solomon, an author of the paper and a professor of atmospheric science at the Massachusetts Institute of Technology. “The sea takes a very, very long time to cool down once you’ve heated it up.”
Solomon is a luminary in the field of atmospheric sciences: She led the first Antarctic expedition to study the hole in the ozone layer, and she was among the first to identify how that hole actually formed.
Her and her colleagues’ current finding comes at an unfortunate time: Evidence increasingly suggests that the planet’s methane problem is only getting worse. Last month, an international group of researchers presented a new analysis showing that methane emissions increased after 2007 and began surging in 2014 and 2015. Scientists aren’t yet sure whether melting permafrost, the worldwide growth in natural-gas fracking, or some other source are responsible for the sudden upswing.
The study informs an ongoing discussion among people who care about global warming. Some people advocate for reducing emissions of CO₂, the most abundant greenhouse gas. Carbon dioxide lasts for centuries in the atmosphere, and will be responsible for most of climate change’s ill effects.
But in the short term, carbon dioxide will be aided by “short-lived greenhouse gases,” like methane, nitrous oxide, and the synthetic chemicals called halocarbons. These don’t last longer than a century in the atmosphere, but they can trap much more heat than CO₂. Some think we should tackle them first.
“Our study shows we need to mitigate both as soon as possible. There are no trade-offs,” says Kirsten Zickfeld, an author of the new paper and a professor at Simon Fraser University.
She cited recent research that showed that the heat trapped by gases like methane lasted far longer than 10 years. “There’s this misconception that as we stop emitting these gases, the climate effects will actually go away,” she told me. But that’s not true. Short-lived greenhouse gases, she says, “must be mitigated as soon as possible if future warming and sea-level rise are to be mitigated.”
In their study, Solomon, Zickfeld, and their co-author Dan Gilford projected what would happen if methane emissions accelerated to 2050, and then dropped off entirely. They found that though much of the methane had left the atmosphere relatively quickly, its trapped heat was still causing inches of sea-level rise in 2900.
If emissions of carbon dioxide, nitrous oxide, and methane are all allowed to accelerate to 2050, they could cause three feet of sea-level rise by 2900 through thermal expansion alone. Though they ended their model that year, it showed that the seas would still be rising.
They also ran a similar experiment examining what would happen if the Montreal Protocol did not exist. The Montreal Protocol is a 1980s-era international environmental treaty that restricted the use of some halocarbons, the chemicals causing the hole in the ozone layer. Halocarbons are short-lived, but they trapped a lot of heat in the atmosphere. The authors show that the world avoided much higher sea levels by passing the Montreal Protocol.
For Zickfeld, the study also suggests that it’s risky to plan on removing carbon dioxide in the future. Most UN climate projections already anticipate that the world will develop and use “negative-emissions technologies” at some point in the future—that is, some technology that can scrub carbon from the air.
“What our study shows is that even if we were able to successfully develop these technologies, they won’t allow us to undo sea-level rise,” she told me. Sucking CO₂ or methane from the air may reduce global warming, but some sea-level rise will already be baked in.
“I think this paper will be an eye-opener for policymakers in showing that this ‘short-lived’ perspective does not apply to sea-level,” says Peter Clark, a professor of geophysics at Oregon State University. Clark was a lead author of the sea-level section of the UN’s 2013 assessment report on climate science. He was not involved in this paper.
“As with CO2, the sooner we reduce [short-lived greenhouse-gas] emissions, the less the amount of sea-level rise there will be. This is thus another example of how our actions now as well as in the future are committing the Earth to changes that will have long-lasting impacts,” Clark added in an email.
Kevin Trenberth, a climate analyst at the U.S. National Center for Atmospheric Research, and a lead author of two UN assessment reports, said that “short-lived” has always been a misnomer for these greenhouse gases.
“The short life of some [greenhouse gases] is irrelevant because it is the integrated accumulated effects” that matter, he said in an email. “But the community that this paper is directed toward does not deal with [...] energy budgets. The paper is likely to be new and useful to that community, and perhaps to the policy community.”
It depends which policy community you’re talking about though, as the EPA may soon surrender the ability to legally regulate any methane emissions at all. Since November 8, Congressional Republicans have indicated they want to repeal the Obama administration’s set of environmental protections meant to reduce methane emissions. If they do so under a law called the Congressional Review Act, they will essentially strip the EPA of its ability to regulate methane in any way.
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