Even if the world suddenly stopped emitting greenhouse gases today, the seas would still rise tomorrow — and for centuries to come.
Though scientists have understood that message for many years, new research puts a bigger, more dire number on the amount of “committed” ice loss and subsequent sea-level rise from the Greenland ice sheet — the amount that’s baked in by the greenhouse gases we’ve already put in the atmosphere. Even if humans could cut emissions down to zero immediately, the world is more or less locked in to nearly a foot of sea-level rise, thanks to a loss of about 3.3 percent of Greenland’s ice. And that doesn’t even count sea-level rise from the melting of the much larger Antarctic ice sheet.
“It’s not surprising … but it really does shine a light on that problem,” said Yarrow Axford, an associate professor of earth and planetary sciences at Northwestern University, who was not involved with the new study. “Indicating that we’ve already committed to losing three percent of the Greenland ice sheet is devastating.”
The idea of committed impacts is widespread in climate change. From ice sheets to rainforests, even the most optimistic of warming timelines will still involve serious harm, baked-in effects that lag behind the greenhouse gases emitted and the actual degrees of warming so far observed. Experts agree, though, that the quicker those emissions come down to zero and the quicker the thermometer stops rising, the less committed calamity there is.
“Even though some degree of change is committed, the total amount of change will still be lower if we reduce our emissions,” said Chris Jones, a scientist with the U.K.’s Met Office and a co-author of several of the United Nations’ major climate reports. “There’s always a benefit from reducing our emissions.”
The new research on Greenland, led by Geological Survey of Denmark and Greenland scientist Jason Box and published Monday, took a different approach than other efforts to estimate the massive ice sheet’s rate of melting. Essentially, the researchers examined how much ice the sheet will lose in order to regain an equilibrium with the warming climate.
This produced some bigger numbers than in other studies that used complicated, forward-looking computer modeling: The 3.3 percent loss would total more than 20,000 cubic miles of ice, an enormous chunk that would weigh more than 100 trillion tons, even if there were a big red OFF button for greenhouse gas emissions at the ready. The study did not examine the specific timeline for that amount of loss, but the authors estimated that most of it could occur within this century.
“If you take an ice cube out of the freezer, it doesn’t melt instantly,” Jones explained. “It’s pretty much committed to melting. You know it’s going to happen, it just takes a while.”
That volume of ice melting will raise global sea levels by a minimum of almost 11 inches, enough on its own to threaten many low-lying cities and countries around the world. And that’s the rosiest scenario.
The researchers also described what would happen if the conditions seen in 2012, a “high-melt year,” were applied to the Greenland ice sheet in perpetuity. In that scenario, the committed sea-level rise jumps to a devastating 31 inches.
Though even the lower estimate is dire, given the vast uncertainties still remaining for the speed of the enormous Antarctic ice sheet’s melt, Axford said some people are focusing on the wrong number.
“To me, the more important number is the rest,” she said. “So, 97 percent of the ice sheet, almost seven meters of additional sea level rise [that] we’re not committed to causing. ... If we’re committed to losing three percent of the ice sheet, we’ve got to worry about that other 97 percent now.”
Ice sheets are among the most important of committed impacts because of their potential effect on global sea levels, and because their sheer immensity builds in a relatively long time lag between warming temperatures and significant ice loss. But there are other baked-in climate change impacts as well.
“There’s many, many kinds of systems that are thrown way out of equilibrium by the warming that we’ve already seen,” said John Erich Christian, a postdoctoral fellow at the University of Texas and Georgia Tech. “Understanding these commitments is an important part of understanding where we’re headed.”
Christian led a study of another brand of ice, mountain glaciers, in 2018. By modeling what were essentially an “idealized” set of glaciers, he and his colleagues found that even absent any further warming, many mountain glaciers around the world are locked into “kilometer-scale” retreat.
Committed impacts are not restricted to the frozen parts of the planet. Jones, of the Met Office, led research published in 2009 showing that the Amazon rainforest also has a time lag in its response to warming. Stabilization of the climate by 2050 — an important part of global goals — would still warm and dry the region enough to commit the Amazon to a “significant dieback” over the subsequent one to two centuries. With the Amazon already under severe threat from deforestation and climate change-induced changes to dry seasons and drought, the research suggests an even more urgent need to protect the world’s largest rainforest.
Another study looked at tree cover in the tropics and found it would drop by as much as 15 percent even in an “equilibrium future” where the climate has stabilized. Others have examined warming itself; in one study from June of this year, researchers found there is a 42 percent chance that the 1.5 degrees Celsius of warming that the Paris Agreement set as an ambitious target is already committed today.
“There’s a risk with these committed changes that you’ve entered into a level of dangerous climate change before you can see it happen,” Jones said.
The technofix options
While experts all stressed that the concept of committed impacts only highlights the urgency of emissions reductions further, there are also two possible approaches that could — in theory — stave off these apparently inevitable impacts. One is expensive, difficult, and largely popular, while the other is not so expensive, likely fairly easy, and almost impossibly controversial: carbon dioxide removal, and solar geoengineering.
Carbon dioxide removal, if it could work at scale, would be an unassailable good. Just as it sounds, the idea involves technology that can pull the most important greenhouse gas — CO2 — out of the atmosphere, where it would otherwise stay up there for centuries. This would thin the gaseous blanket encircling the earth and bring temperatures down.
“The climate doesn’t care if we emit some CO2 and remove it, or if we don’t emit it,” Jones said. The big U.N. climate reports assume some degree of carbon dioxide removal, in fact, in order to reach target temperatures. The problem is that, so far at least, it has proven far too expensive to scale up toward the massive amount actually needed. And even if it reaches true widespread viability, zeroing out emissions remains priority number one.
“Any carbon removal has to be additional to emissions reductions; it can’t be instead of,” Jones said.
The other option is solar geoengineering, where aerosol particles are intentionally injected into the stratosphere, more or less mimicking a volcanic eruption. This would likely cool the planet quickly — within the year the project begins — and thus reduce some of the impacts both already occurring and committed for the future. But the idea is so rife with disagreement and potential conflict, and carries so many unknowns and theoretical hazards, that the international community may never agree on its use — and notably, the technology to make it work, involving planes to drag the particles up past where they usually fly, doesn’t yet exist.
Even with the technofixes looming, climate change’s baked-in hazards are another reason to zero out emissions as quickly as possible. Axford said that all of these committed impacts, from ice loss and sea-level rise to changes to rainfall and drought and flooding, are directly connected to the amount of warming humans allow to occur.
“We can look at our power over those problems as existentially devastating, as discouraging,” she said. “Or we can say, ‘Hey, we actually are in charge of these problems.’”
Thanks to Dave Tepps for copy editing this article.