When will we see a sea level rise of three feet?
Recently, the Columbia Climate Center had the chance to participate in an event aiming to improve public awareness on climate change. On the weekend of the 6th and 7th of February, the CCC had a table in the Polar Fair in the International Polar Weekend at the American Museum of Natural History. It was a wonderful opportunity to talk with a lot of kids and not-so-kids.
Our activity explored two scenarios for greenhouse gas (GHG) emissions and the likely sea level rise resulting from the melting of the polar ice sheets in Greenland and West Antarctica. The kids could choose between two options: trying very hard to reduce emissions and continuing with business as usual. Sometimes I got a laugh in response when I asked the 6 to 10 year-olds if they drove a lot, but they usually felt the need to explain they didn’t actually have a driver’s license.
Depending on which course of action they chose, we used IPCC results that project the atmospheric GHG emissions to end of century to 450ppm and 850ppm. Then they chose from one of two jars of tokens that reflected the expected probability function for the temperature distribution for each scenario, which we converted into global sea level rise values. When they had chosen a few tokens, they could go to two great online sources to watch the impact of sea level rise at specific locations. One source uses google maps and is fabulous to look at city streets, the other source from the University of Arizona uses red to indicate sea level change and is spectacular for larger geographic areas.
The question that we got most frequently was: when will this happen? And the answer is: we don’t know.
We can tell you that current models that project climate into the future anticipate sea level rises of between 30 and 60 centimeters in the next century. We can tell you that Susan Solomon and coworkers found that even if we reduce emissions rather drastically, sea level is likely to rise at least a meter (3 feet) due to thermal expansion alone Although that’s on the lower end of what might happen, it is still devastating for coastal and deltaic regions.
And we can say that amplifying processes, known among scientists and engineers as positive feedbacks, and among lay people as vicious circles, conspire such that warming leads to melting which in turn leads to more warming and more melting.
We can also tell you that GHGs emitted by our grandparents will still be in the atmosphere in several hundred years, almost half of them will still be there in a 1000 years. Clearly, the decisions we make today will have consequences for climate for such long time periods that it almost would qualify as forever.
We also know that the expected assumption that sea level will rise uniformly throughout the globe is mistaken. In fact, in a recent study, Jerry Mitrovica and colleagues noted that the effect of the gravitational pull of the West Antarctic Ice Sheet is such that if it were to collapse, the sea level would rise more in the north Pacific, north Atlantic and southern Indian Ocean. In the case of Washington DC, the sea level rise goes from 5m (15 feet) in the case of uniform sea level rise to 6.3m, almost 19 feet, after applying the the fingerprint of ice change.
We can also say that the models fail to take into account many processes that are responsible for much faster rates of melting of polar ice sheets. Some of these processes are the presence of meltwater atop the ice sheets, the movement of this water through crevasses and the formation of moulins, and the role of subglacial water accelerating the coastward flow of the ice sheets. In a meeting this week of the American Association for the Advancement of Science (AAAS), Richard Alley pointed out that ice sheets are currently modeled as “white mountains“, noting that the rate of melting is unknown. As discussed in the USGS Report on Abrupt Climate Change, when models start incorporating these processes, the projected rates of sea level rise are likely to be much higher than in current models.
But we still don’t know when the sea level might rise to one meter or more. And that lack of knowledge of when underlines the need for continuing research in basic science, including fieldwork, satellite observations, and modeling.
Do we know enough to strongly recommend reducing GHG emissions? Most definitely. Do we know enough to start planning for higher sea levels? Most definitely. See above for the things we do know. But there’s still a long way to go before we can tell you when.