Speaking of Science: UW scientists offer sea level rise projections, investigate icy uncertainty

Come the year 2100, Washington State’s coastline could be in for some noticeable changes. Coastal cities from Bellingham to Olympia are already planning for the possibility of rising waters over the next 50-100 years.

Earlier this year, in response to frequent requests for sea level rise projections along Washington’s coasts, UW’s Climate Impacts Group (CIG) offered a range of estimates to municipalities and the public.

“The report provides very low, medium and very high estimates of sea level rise over the next 100 years,” said Philip Mote, a UW research scientist and lead author of the report. “It’s not intended as a scientific document, rather for planning purposes.”

Although the report presented specific numbers for three broad coastal regions and gave most credence to the middle estimate, its authors were unable to quantify the probability associated with them. This came in acknowledgment of unknowns in the models the estimates were built on.

A team of researchers, led in part by UW glaciologist Ian Joughin, has offered a refining perspective that may help limit the bounds on future projections of sea level rise. Joughin’s research focuses on fieldwork and modeling, using observations to constrain and understand processes contained in numerical models of the world’s largest ice sheets, Greenland and Antarctica.

“Rapid changes in these ice sheets were the big asterisk in the latest sea level rise projections from the Intergovernmental Panel on Climate Change (IPCC),” Joughin said. “Speed up in the movement of their glaciers wasn’t built into their projections, which used current rates of ice discharge to estimate their contributions to sea level rise.”

Although precise observations of ice melt and discharge from the Greenland ice sheet have been measured, the period of record is short. Mote, who was also a lead author of the most recent IPCC assessment, said in preparing their projections that he and other authors were cautious not to use limited data to estimate changes far into the future.

“The question became, ‘How comfortable are you in extrapolating to the next 100 years based on a few years of observations?’” Mote said. “In our case, not very.”

While the observational record of ice discharge lengthens, basic research by scientists like Joughin contributes more immediately to our understanding of ice sheet dynamics. Two forthcoming papers in the journal Science, co-authored by Joughin and colleagues, have offered a lucid picture of the relative importance of surface melting and discharge off glaciers that comprise the Greenland ice sheet.

Using a variety of techniques to monitor the speed of glacial advance, the scientists have demonstrated that the seasonal speed up of iceflows occurs throughout the sheet’s periphery, but that outlet glaciers around Greenland have increased in speed only modestly.

“What we’ve done is suggest that a process that’s been characterized as catastrophic isn’t as important,” Joughin said. “But not necessarily in a radical kind of way. Greenland still isn’t safe. There are other processes at work that we are studying.”

Although ice sheets could contribute largely to sea level rise, Susan Solomon, co-chair of the IPCC, said their contribution is small, roughly 15 percent of the total. Most of the observed rise has resulted from the melting of smaller glaciers elsewhere, as well as from thermal expansion associated with warmer waters.

“Rapid discharge from ice sheets could be a big term, so it’s natural to ask the question ‘How much bigger can it get in the future?’” Solomon said. “This work is helpful in beginning to offer an answer.”

[Reach columnist Brian Smoliak at news@thedaily.washington.edu.]