What Accounts for the Varying Rates of Sea Level Rise in Different Locations?

Ask a Scientist - June 2013

Jess C. of Salem, MA, asks "I've read that sea level rise is occurring faster than expected with increases of three to six feet expected this century. But I've also heard that the sea level is rising more quickly in some areas than in others. What accounts for that?" and is answered by Melanie Fitzpatrick, Ph.D, climate scientist in the Union of Concerned Scientists' Climate and Energy Program.

Sea level rise is already posing a threat in many coastal areas and you are right that the rate of increase varies considerably in different locations.

Let's start by addressing what’s driving sea level rise in the first place. The story begins over a century ago with the Industrial Revolution in the mid-1800s. That's when we started to transformed our energy usage by burning large amounts of coal, then oil, and a little later natural gas, significantly increasing the amount of carbon dioxide we were emitting into the atmosphere. As carbon dioxide emissions have risen dramatically since then, the effect has been for the heat-trapping gas to essentially wrap the Earth in a blanket that is warming the planet.

As the Earth has warmed over these years, some 85 percent of this excess heat has gone into Earth’s oceans. So, one driver of sea level rise is what scientists call “thermal expansion.” The added heat actually causes the sea to expand. A good analogy is to think about a pot of water on the stove. As the temperature of the water increases, the water expands slightly. The same phenomenon occurs with the world’s oceans. And, since oceans cover roughly 70 percent of the Earth’s surface, even relatively small amounts of expansion can have a significant effect in coastal areas. Overall, we’ve seen an average of eight inches of sea level rise globally over the past century and roughly half of that has been caused by thermal expansion.

The other major driver is the current melting of some of the planet’s huge ice sheets in Greenland and Antartica as well as the planet’s remaining glaciers. As the Earth has warmed, shrinking land ice—glaciers, ice caps, and ice sheets—has contributed an increasing share of total global sea level rise since the early 1990s as the pace of ice loss has accelerated. This melting is now occurring even faster than most scientists anticipated. Recent studies suggest that land ice loss has accounted for some three quarters of the total sea level rise that has occurred over the past five years.

So, that’s the big picture. Now, as for the local variations. It is true that some places—such as the East Coast of the United States and the eastern Gulf Coast off Florida, for instance—are experiencing higher than average rates of sea level rise. Two major factors account for these sea-level hotspots: local topography and changes in ocean currents. Depending on local geological factors, land in some coastal areas is actually slightly sinking, or “subsiding” as scientists describe it. This localized phenomenon can occur for a variety of reasons, but this natural settling of land can increase the rate of sea level rise in certain coastal regions. This is happening, for example in the Gulf region, where subsiding land is allowing the ocean to penetrate further inland.

Equally important is the role of ocean currents. As the oceans warm globally, currents in many places can shift, resulting in changes that either tend to pull water away from the shore or pull it in. Along the East Coast, changes in the path and strength of ocean currents are contributing to faster-than-average sea level rise.

Scientists have been closely studying so-called "king tides"—the highest measured tides over the course of the year in a given local area. As we have already seen from storm surges during Superstorm Sandy, for instance, sea level rise accompanied by storms is already having a destructive and devastating effect on some coastal regions. King tides present communities with challenges but they also help force these communities to prepare because king tides offer a preview of sea level rise to come. Given the current pace of sea level rise, the water level reached now during a king tide will eventually become the water level reached at high tide on an average day.

Sea level rise presents coastal communities around the world with many difficult choices ahead. While a fair amount of sea level rise is already locked in due to past emissions, the extent to which we are able to reduce our carbon emissions now will have great bearing on sea level rise in the next century and beyond. That’s why, as a scientist specializing in sea level rise, I try to help people understand the issue and what they can do to ultimately slow down the trend. My ultimate goal, in other words, is to work myself out of a job—making concern about sea level rise a thing of the past. Unfortunately, for the moment at least, I’ve got my work cut out for me.

Melanie Fitzpatrick is a climate scientist with the UCS Climate and Energy Program. Her work encompasses basic climate research, the local and global consequences of climate change, and communicating scientific findings to policy makers, the media and citizens.Dr Fitzpatrick completed her Ph.D. in geophysics at the University of Washington in Seattle, where she researched the interaction between sea ice, clouds and climate. As a polar research specialist, she has worked as a field-based climate scientist with the Australian Antarctic Program, the U.S. Antarctic Program, and with the Department of Earth and Space Sciences at the University of Washington in Seattle.

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