To prevent rapid sea-level rise, reduce emissions now

The timing of emissions reductions, even more so than the rate of reduction, will be key to avoiding catastrophic thresholds for ice-melt and sea-level rise, according to a new Cornell study.

The study, published Oct. 10 in Nature Climate Change, models the impacts of different emissions trajectories, finding that emissions and uncertainties around ice sheet dynamics will have the most impact on sea-level rise through 2200.

"Roughly speaking, we found that somewhere between 2065 and 2075, emissions really start to become the dominant factor, as well as uncertainties related to emissions like Antarctic Ice Sheet tipping points," said Vivek Srikrishnan, assistant professor of biological and environmental engineering in the College of Agriculture and Life Sciences. "As we get to 2060, 2065, the mitigation we do today will start to really materially impact the range of sea-level rise outcomes."

It's well understood that future carbon dioxide emissions will impact global temperatures, the melting of ice sheets and sea-level rise, but there's a great deal of uncertainty around what that will look like: We don't know how or when emissions and increased temperatures will precipitate a rapid melting of ice sheets and surge in sea levels, or the best approach to avoid those tipping points.

Previous studies and models focused on or made central emissions' impact on warming temperatures, without considering how those changes might interact with ice sheet and ocean dynamics over time in a detailed way. Srikrishnan and his team, including first author Chloe Darnell, M.S. '23, wanted to understand how emissions might have a nonlinear relationship to sea-level rise - as temperatures surpass thresholds and initiate a more rapid melting of ice sheets.

For more granular detail, the researchers used their own model of emissions and integrated it with a number of existing climate models, including ones that model ice sheet dynamics, to determine the factors impacting sea-level rise through 2200. They found that delaying emissions reductions by even a decade substantially reduces the chances of avoiding thresholds for sea-level rise, with the year emissions peak greatly influencing the probability of surpassing ice sheet tipping points.

They estimate that a failure to reduce emissions by 2050 will result in a greater than 50% probability of reaching the threshold that would raise sea-levels by 0.4 meters, which could surpass 0.5 meters depending on how heat is absorbed into the oceans and other complex geophysical dynamics. A sea-level rise of 0.5 meters could increase the entire distribution of flood risk at most tidal gauges by 10-fold, with a 100-fold increase in risk at more than half of those gauges.

"It's not worth waiting for a silver bullet," Srikrishnan said. "Obviously the faster we can reduce emissions the better, but any decrease is better than nothing. That's not a new insight, but this reinforces it."

The study also found that while uncertainties around the Antarctic Ice Sheet explain the most variability of 21st century sea-level rise, Greenland could play a larger role in the 22nd century.

"The overall volume of sea-level rise that Greenland could contribute could be quite large, and that matters a lot in a number of different places on the planet," Srikrishnan said.

More broadly, the modeling approach provides a new tool to assess shifts in emissions and policy, and to help communities allocate resources.

"One of the goals is building a machinery that allows us to look more granularly at how changes in emissions - that we may see in response to changes in policy - might impact climate risk," Srikrishnan said. "We need to have ongoing conversations around what emissions trajectories are more plausible and how that might impact our assessment of adaptation needs."

Srikrishnan said the approach attempts to identify signposts in a future that no one can precisely predict.

"Trying to refine our understanding of these uncertainties, the ones we can refine, is important," Srikrishnan said. "Then we can ask: What can we observe that would give us enough lead notice for decision-makers to make appropriate accommodations to their planning? What clear signposts can we try to identify that point to an impending instability? These are central questions."

Additional co-authors include researchers from Stanford University and the Rochester Institute of Technology.

Funding for the study came from the U.S. Department of Energy and the National Science Foundation.