Sea level rise is one of the more challenging impacts of climate change to project. Not only is the direction of change unpredictable—sea levels will rise as the world warms—but it’s very difficult to know when parts of the glaciers will slide into the ocean. There are many factors involved in temperatures, including ocean currents and the topography of the bed below the ice sheets.
As a result, the projections of sea level rise presented to agencies such as the Intergovernmental Panel on Climate Change (IPCC) have been widely released and have changed significantly over time. The 2013 IPCC report, for example, projects a higher sea level rise than the 2007 report, which it says leaves important ice processes that need more research. And the recent 2017 US National Climate Assessment also increased its predictions of sea level rise based on the current state of science.
New research from a group of researchers led by Rutgers. Bob Kopp has made for splashy headlines in recent days, some of which claim that the study shows that sea level rise will be “worse than thought” or that the study confidently predicts how many people will be drowned by rising seas in this century. Neither description is really true, because there is nothing new about the sea level rise scenarios that appear. In fact, Kopp also helped put those together string-level partition of the US National Climate Assessment, and the numbers in the new study appear to match those in the news.
That doesn’t mean the study from Kopp et al. It’s not noticeable and it’s interesting. It’s related to something the scientific community has been grappling with for the past two years—a pair of studies using an advanced ice model that simulates much faster ice loss from the vulnerable West Antarctic Ice Sheet . While this simulation is more compatible with recent research in Antarctica, it is not easy to fold the first results of the modeling phenomenon—which is not the final word—into the existing view.
In part because ice-ice models have not yet reached the point where researchers feel they accurately represent everything we know, many past efforts have produced a kind of hybrid forecast that incorporates detailed studies of experts in place. This new study predicts hybrids like that from 2014 and replace Antarctic ice-loss estimates with alarming model simulations to find out what the impacts will be. The answers are much more interesting (and absurd) than “sea level rise.” But yes, the new study definitely increases the estimate of sea level in the future.
In the high greenhouse-gas emission scenario, the 2014 forecast estimated about 0.5 to 1.25 meters of sea level rise by 2100, while this new version shows 0.9 to almost 2.5 meters. (For reference, the 2013 IPCC report projected about 0.5 to 1 meter, and the latest 2017 US National Climate Assessment puts it in the range of 1 to 2.5 meters using a different scenario.)
Once you get past 2100, the scenarios are different in a big way. By 2300, the difference in sea level between the low and high emission scenarios grows more than 10 meters. Once the ice sheets become unstable, they continue to raise sea levels for a long time, our actions this century could shape the world for many centuries to come.
Beyond that, here’s what the research really reveals: this century, the biggest change controlling the amount of sea level rise we get is the behavior of Antarctic ice. We don’t know whether to expect the high end or the low end of the range of predictions, even assuming we follow a high emission path.
And because the situation can change in Antarctica in a matter of decades, we cannot really know what will happen in the second half of the 21st century based on what we saw in the first half. Model simulations with the highest rate of sea level rise in 2100 are not necessarily those with the highest rate in the 2020s. The lesson, the researchers wrote, is that “this means that the scenarios ” “scale” of the future needs to be considered even if they measure current levels of sea level.”
Again, the researchers emphasize that these model simulations of sea-level rise provide a more realistic “worst-case scenario” than they predict the most likely outcome. But because the real possibility of a worst-case scenario is unlikely to rule out any time soon, planning should account for the uncertain future.
To add some meaning to these numbers, the researchers estimated how many people currently live in areas that could become inundated by sea level rise in 2100. Even in the lowest emissions scenario (where global warming is limited to 1 degree Celsius above current temperatures. ), that too least 75 million people worldwide. In a high emissions scenario, the “worst case” pushes that number as high as 235 million. The difference between these two futures is far from academic.
Earth’s Future2017. DOI: 10.1002/2017EF000663 (About DOIs).