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Mon June 25, 2012
The Increasing Certainty of Sea Level Rise
Uncertainty is everywhere. Will it rain? Will that house flood the next time it does? What will the stock markets do? Who will win the next Red Sox game? Experts can tell us the odds, but they can’t tell us with absolute certainty what will happen. The more complex the system, the greater the uncertainty. But understanding the processes at work – whether it’s weather or economics or baseball – can help reduce the uncertainty.
Science is no different. Case in point: sea level rise.
The impact of a warming climate on sea level has a tendency to be misunderstood as simple – glaciers melt, more water goes into the ocean, sea level rises. But that’s really just the tip of the iceberg (sorry, couldn’t resist). In actuality, there are at least three climate-driven factors contributing to sea level rise. Gaps in our understanding of these processes is the source of uncertainty about future sea level rise.
The best understood source of sea level rise is ocean warming. The ocean has absorbed the vast majority- 80% or more – of the excess heat trapped by carbon dioxide emissions from human activities. As a result, the average temperature of the ocean has risen an estimated 1-2°F. And as water warms, it expands. So-called thermal expansion is thought to account for around half of the global sea level rise we’ve seen in the past century.
Glacial melting is considered a bit of a wild card. In fact, the IPCC’s 2007 assessment report didn’t even include the effects of glacial melting in sea level rise predictions because the science was considered too incomplete and unsettled. It’s still far from a closed case, but a lot of work has been done since then. Scientists now know that rising water temperatures, as well as rising air temperatures, play a role in melting coastal ice sheets. And they’ve made progress in pinning down the rate of melting. Unfortunately, virtually all of the recent research has suggested that Greenland’s glaciers and the polar ice caps are melting faster than expected.
Finally, there’s the possibility that climate change could slow ocean circulation. Variation in the temperature and saltiness of different parts of the ocean drive ocean currents that move many times more water than all the world’s rivers combined. But that could change. Most of the sea surface is warming, but the input of cold, fresh glacial meltwater is increasing in some places, and accelerating evaporation is leaving some tropical areas saltier than before. Some of the most dire climate change scenarios involve major rearrangements in ocean circulation patterns, including weakening or even reversal of the Gulf Stream.
That could be disastrous for New England, because the Gulf Stream carries water away from our coasts. A study published in 2009 suggested that changes in the Gulf Stream could impact sea levels along the coast of New England by allowing water – as much as several inches of it – to pile up along the northeast coast of the U.S. Now, a new study suggests that the process is already underway. Scientists say that’s the best explanation for the fact that sea level rise along the northeast coast of the U.S. is accelerating three to four times faster than the global average. Writing for the AP, Seth Borenstein reports that Jeff Williams and Stefan Rahmstorf – two of the world’s leading experts on sea level rise – say the study “does a good job of making the case for sea level rise acceleration.”
So the good news is, scientists are whittling away at the uncertainty surrounding sea level rise forecasts. The bad news is, everything we’re learning points to greater increases than previously thought. The emerging consensus is that we can expect at least two to three feet of sea level rise by 2100 globally, and possibly an extra foot here in New England.