Boulder scientists have teamed on a study which shows that uneven sea level rise over the past 25 years, including higher than average increases in places such as the United States' Eastern Seaboard and the Gulf of Mexico, are partly the result of climate change and that it can be expected to intensify with future warming of the planet.
"One of the implications from our study, a key take-home, is that the patterns we have seen over the last 25 years are likely to persist into the future, much more than we would expect from just random variation," said John Fasullo, a project scientist at Boulder's National Center for Atmospheric Research.
He said many people understand that sea levels are rising, but imagine the situation comparable to a "bathtub, gradually filling up." That's not an accurate picture of what's really happening, he has found.
"There is spatial structure to sea level rise such that some regions experience different rates of rise than the global average," Fasullo said. "We're at the early stages of understanding the reasons for that structure. If you are in one of those places that rises quicker, this will likely increase the related impacts under climate change."
The study, published Monday in the Proceedings of the National Academy of Sciences, and authored by Fasullo and Steve Nerem at the University of Colorado, comes fast on the heels of the National Climate Assessment, released Nov. 23, which concluded that if steps are not taken to alleviate global warming, its effects could slash up to 10 percent of the nation's gross national product by 2100.
"Our work provides greater clarity regarding the drivers of elevated sea level rise cited in the NCA, such as in the Gulf of Mexico and along the Eastern Seaboard, and suggests that these enhanced rates are unlikely to reverse substantially over the coming decades," Fasullo said.
He added, "One of the areas of the world that has smaller than average sea level rise is a place that nobody lives, and that's around Antarctica. But other places with elevated rate of rise are heavily populated and particularly sensitive to tropical cyclones, such as the Philippines.
For the study, Fasullo and Nerem, both of whom are members of the NASA Sea Level Change Team, analyzed the satellite altimetry sea level record, which includes measurements of sea surface heights dating back to 1993. They mapped global average sea level rise, and also how specific regions deviated from the average, according to a news release.
"We now have a new tool — long-term satellite altimeter measurements — that we can use to help stakeholders who need information for specific locations," Nerem, a fellow of CU's Cooperative Institute for Research in Environmental Sciences and a professor of aerospace engineering, said in a statement.
The scientists turned to two sets of climate model runs, known as "large ensembles," to probe the role of climate change, one created by using the NCAR-based Community Earth System Model and one created using the Earth System Model at the National Oceanic and Atmospheric Administration.
These large ensembles, which are many model simulations by the same model, describing the same time period, enable researchers to disentangle natural variability from the impacts of climate change. With sufficient runs, these impacts can be isolated — even when they are relatively small compared to the impacts from natural variability.
The climate models suggest that in regions that have seen more or less sea level rise than average, as much as half of that variation may be attributed to climate change. Researchers also discovered that the impacts from climate change on regional sea level rise sometimes mimic the impacts from natural cycles, the release stated.
Fasullo said it was found, for example, that the sea level rise response to climate change in the Pacific resembles what happens during a particular phase of the Pacific Decadal Oscillation, explaining why until now it has been difficult to determine how much of the pattern was natural, or not.
Future work, Fasullo said, will be focused in improving the level of confidence he has in the findings to date.
"Right now, we have a model estimations of the rate of sea level rise to expect, but we know there are some biases in the model as well. We want to develop a method that extracts the pattern of sea level rise directly from the observations," Fasullo said.
"That would give us a greater confidence in statements I have made about exactly which regions should expect elevated rates of rise, going into the future. We know that some of the patterns from the climate models are likely incorrect."