Atlantic Coast Sea Level Rise: Past, Present, and Future

Three things you should know:

1) Along the U.S. East Coast, the sea level has been rising since the late 1800s at a faster rate than at any other time during the past 2,000 years.

According to a new study, in coastal North Carolina the sea level has been rising at an unprecedented rate during the past 100 years. Credit: iStock. 

2) This new 2,000-year timeline of sea level rise shows a connection between global temperatures and ocean levels; during warm periods on Earth, sea level rose and during cooler spans, seas either remained level, or even fell slightly. 

3) Scientists anticipate that average global temperatures and sea levels will keep increasing, and some now estimate that 20 million Americans will be affected by rising seas by 2030.

What the new science says:

This week, the Proceedings of the National Academy of Sciences published a new study showing that along part of the U.S. Atlantic Coast, sea level rise has been occuring at a faster rate during the past 100 years than at any other time in the past 2,000 years. More importantly, the study's continuous record of change over two millennia shows a distinct connection between global temperatures and sea level:

Historically, when global temperatures have been on the rise, sea level has increased. During cooler times on Earth, sea level has remained stable or has even decreased slightly.

To be fair, scientists have known for years that warmer temperatures cause sea level to increase (there are several reasons why this happens, nicely explained here). And scientists also already knew that sea levels have been rising along the Atlantic Coast, because they’ve been collecting water level measurements all along the coast for more than a century. But 100 years of data isn’t enough to fully understand what the relationship is between sea level and global temperatures. In other words, just because temperatures and sea level are both rising now, how do we know they are really connected?

In this new study, researchers from the University of Pennsylvania (along with colleagues from Finland, Germany, and Pensylvania State University) have reconstructed a 2,000-year history of sea level changes along the North Carolina coast by studying markers that tide-sensitive creatures left behind in coastal sediments. The team found that, “for the last 2,000 years, the relationship between sea level and temperature is very, very strong,” says University of Pennsylvania geology professor Benjamin Horton, who led the study.

Between 100 B.C. and A.D. 950, for example, average global temperatures were relatively stable, and during this time there was little fluctuation in sea level at the study site. Then, during a warm period between A.D. 950 and 1400, sea level at this location slowly climbed higher. In the midst of a cooler period that followed, sea levels stabilized, and then around 1880, they began to rise again as temperatures picked up.

The rate of sea level rise during the past 130 years, however, has been like nothing else recorded during the past 2,000 years, the study states. The warming that has taken place during the last century has caused seas to rise in North Carolina by an average of 0.83 inches per decade, which is more than twice as fast as during any other recorded time period.

Why the science is important:

This new study of sea level rise is all about the past, describing how global temperature changes relate closely to sea level rise during the past 2,000 years. Key implications of the study, however, point firmly toward future sea level rise. 

This particular research doesn’t make any predictions about how sea level will change (in North Carolina or anywhere else) in the future, but it does confirm that when global temperatures rise, so does the sea. Climate scientists project that average temperatures around the planet are going to keep climbing as greenhouse gas concentrations increase, so now this study adds more evidence to show that sea level rise will likely continue too, possibly at an accelerated rate.

If you live along the Atlantic Coast (or any coast), however, you’re probably more interested in hearing about how much sea level rise you need to prepare for (already, some researchers estimate that 20 million Americans will be affected by sea level rise by 2030). As DotEarth blogger Andy Revkin deftly pointed out yesterday, if sea level rise continues at the same observed rate it has during the past century, it won’t pose much of a threat, since there has already been seven or eight inches of sea level rise since the beginning of the 20th century. Most climate models project that global temperature will rise more quickly throughout the coming decades, though, so if sea levels follow suit — and this study suggests they will — we should expect sea level rise to be measured in feet, rather than inches, by the end of the century.

Models devised specifically to project sea level rise also show that changes will speed up in the next century, rather than remaining steady. This new 2,000-year reconstruction, and the pattern of sea level rise it shows in North Carolina, in fact, is very consistent with a model proposed by a 2009 study, which shows that global sea levels could rise at least 3 feet by 2100 compared to current levels.