Sea Level Rise

Sea level rise, one of the many consequences of a warming Earth, will likely prove to be one of the greatest challenges humanity will face over the next century. We know that the Earth is warming, and that warming in the past has led to sea level rise. Not only do higher temperatures cause polar ice to melt, increasing the amount water in the oceans, but warmer water expands (known as thermal expansion), causing additional sea level rise. While we don’t know exactly how much, or how fast, sea levels will rise, researchers are continuously collecting new data and developing better-informed models.

Sea level rise since the late 19th century. Source: Kemp et al. 2011.

In a study published in the Proceedings of the National Academy of Sciences, researchers at the University of Pennsylvania found that sea levels along U.S. Atlantic coast are rising faster now than any other time in the past 2,000 years. Furthermore, sea levels have risen consistently over the last 2,000 years in tandem with rises in temperature.

“Scenarios of future rise are dependent upon understanding the response of sea level to climate changes. Accurate estimates of past sea-level variability provide a context for such projections,” said Andrew Kemp, a postdoctoral fellow at the Yale University Climate and Energy Institute.

Sea levels were relatively stable between about 200 B.C. and 1,000 A.D., but began to rise by a millimeter a year during the 400-year period known as the Medieval Climate Anomaly. Sea levels were once again stable up until the late 19th century, when global temperature and sea levels were once again on the rise. Over the last 100 years, sea level has risen an average of 2-3 mm per year. Evidence suggests that this rate of sea level rise will only accelerate over the coming years and decades as we continue to release ever more CO2, and other greenhouse gases, into the atmosphere.

The Greenland ice sheet and western Antarctica have been losing mass, contributing to sea level rise. Source: Velicogna et al. (2014)

There is consensus now among the scientific community that humanity is altering the global climate, primarily by burning fossil fuels and releasing billions of tons of greenhouse gases into the atmosphere. Greenhouse gases act to raise the mean global temperature by trapping some of the heat that would have otherwise radiated out into space. As a result of increasing temperatures, large amounts of polar ice have already begun to melt. Because ice is great at reflecting light (and heat) back into space, the reduction in polar ice acts as a positive feedback cycle; as more ice melts, less light and heat is reflected back into space, further raising temperatures and leading to faster ice melt.

The two largest ice reserves on Earth, the Antarctic and Greenland ice sheets, have been losing mass in recent decades due to melting and the calving (breaking off) of large icebergs along their peripheries. It is estimated that Greenland is shedding about 239 cubic kilometers (57 cubic miles) of ice per year, while Antarctica has been losing a little more than 100 cubic kilometers (24 cubic miles) of ice since 2002. Greenland lost 260 square kilometers (100 square miles) at one time when a large ice sheet broke off from the Petermann galcier in 2010.

How fast will sea level rise?

Most predictions suggest that sea level rise will increase significantly in the near future; well beyond the 3 mm annual rise seen today. In 2013, the Intergovernmental Panel on Climate Change (IPCC) released their Fifth Assessment Report in which they outlined several probable sea level rise scenarios. According to the models, sea level rise could be expected anywhere between 0.2 to 2 meters (0.66 to 6.6 feet) by the final decade of the 21st century.

In 2010, the released a summary of current literature on sea-level rise, which included numerous predictions based on a variety of models. There was a good deal of variability among the predictions, with anticipated sea level rise between 0.56 and 2 meters (1.8 to 6.5 feet) by 2100. In one recent on sea level rise, Rignot et al. (2011) estimated a 12.6 inch increase in sea level by 2050. Even if sea level rises only 1 -2 feet over the next 100 years, it will likely continue to rise  faster in the centuries that follow.

Estimate of sea level rise to 2100. IPCC 2013.

What are the likely consequences of sea level rise?

Nearly half the world’s 7.5 billion people live in coastal communities that could be affected by even a modest rise in sea level. Many of the world’s great cities, such as New York, London, San Francisco, Dhaka, Shanghai and Mumbai, lie partially at or just above sea level. A few cities, including New Orleans and London, have already had to build gates, levees and other physical barriers to keep water out during storm surges. The 8.5 meter (28 feet) storm surge that accompanied Hurricane Katrina in 2005 was enough to flood most of New Orleans, large portions of which lie below sea level. The new flood gates built since 2005 were designed to protect the city given the current sea level, not the 2-6 feet of additional water expected over the coming century.

The Maldives government erected a seawall around Male, the capital, to protect the city against the rising sea.

While richer, developed nations will likely opt to protect their coastal cities by constructing or upgrading flood barriers, cities in developing countries will have little defense against the rising water. Millions of coastal inhabitants will lose their homes and have to resettle to higher ground. Several low-lying areas and islands have already been affected by the 20 cm (8 inches) of sea level rise over the last 100 years. The Maldive island chain in the Indian Ocean, which at no point is no more than 2 meters above sea level, will likely be one of the first nations significantly affected by sea level rise.

Affected areas of the southeast U.S. coast with 1 meter of sea level rise. Source: NOAA.
Coastal inundation in southern Louisiana assuming 1 meter of sea level rise. Source: NOAA.
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