Sea Level Rise Explained

Global sea levels are rising.  Impacts on coastal regions are already being felt by increased flooding, larger storm surges, and more.  This page briefly explains the causes, impacts, and actions some communities are taking to combat this serious issue.

What is it and what are the causes?

“Sea level rise” refers to the way in which, on average, the world’s oceans are becoming higher, relative to land.

How can oceans rise?

There are primarily two ways in which sea level can rise or fall over long periods of time:

The first is known as thermal expansion and is based on the simple fact that water takes up more volume as it becomes warmer. If the Earth’s oceans become colder, the total volume will decrease, whereas as oceans become warmer (as they are now), the volume increases.

The second method is melting arctic ice from the world’s massive glaciers and ice sheets, primarily in Greenland and Antarctica.  When ice that has been trapped on land melts or slides into the ocean, sea levels rise.

 

For More:

NASA Sea Level Rise Portal : Get the latest data and learn about the techniques scientists use to measure sea level rise, ocean temperatures, and more.

Sea Level Rise Portal at the Smithsonian Institution : An in-depth historical overview of sea level rise.

Going Deeper:

Sea level rise – fact & fiction: John Englander at TEDxBocaRaton (11 minute video)

Is sea level rising?: Benjamin P. Horton at TEDxNavesink (19 minute video)

How much are the oceans rising?

Sea levels have varied greatly throughout geologic history, but approximately 3,000 years ago, average levels stabilized close to the present level.  But during the 20th century, a rise of 0.7 inches per decade was detected, and over the last 20 years, the rise per decade has been about 1.3 inches.  In other words: The seas are rising again, and at an accelerating rate.  (Source)

Global Average Absolute Sea Level Change 1880-2015

Chart: EPA.gov

Because of local factors such as terrain, ocean currents, and subsidence (sinking land), sea level rise is not uniform around the globe.  This map shows how different coastal regions have changed over the last half century:

Relative Sea Level Change Along U.S. Coasts 1960-2015

Map: EPA.gov

 

 

What is causing recent and accelerated sea level rise?

As more greenhouse gases such as carbon dioxide and methane are emitted into the atmosphere, largely by burning fossil fuels, heat is trapped, which raises the Earth’s temperature.  Much of that excess heat is absorbed into the ocean. In fact, according to the Intergovernmental Panel on Climate Change (IPCC) “about 93% of the excess heat energy stored by the Earth over the last 50 years is found in the ocean” As the oceans warm, they increase in volume, raising global sea levels. More recently, melting arctic ice has become an additional contributor.

 

 

 

 

 

For More:

Antarctica is melting faster than scientists expected : This video from PBS NewsHour examines recent arctic melting and its possible impacts.

Abrupt Sea Level Rise Looms As Increasingly Realistic Threat : Reporting from the Yale School of Forestry & Environmental Studies about sea level rise.

 

What are Potential Impacts of Sea Level Rise?

Increased Flooding

Because of global sea level rise, frequency of flooding has increased worldwide. Research from Climate Central has shown that it is very likely that half of the flooding events in the 1950s would not have occurred, while the percentage has increased to three-quarters in the last decade, if not for sea level rise. New Jersey in particular has seen a dramatic rise in the number of flooding events in the last century, with Sandy Hook going from 108 events between 1955 and 1984 to 420 between 1985 and 2014.  Similarly, Atlantic City went from 85 events between 1955-1984, rising to 414 in 1985-2014.

For more:

When Rising Seas Hit Home: An Analysis by the Union of Concerned Scientists:  An interactive site that offers in-depth analysis of chronic inundation in the United States.

New York Times: Flooding of Coast, Caused by Global Warming, Has Already Begun: The New York Times takes a closer look at coastal flooding and global warming.

 

Storm Surges and Intensity of Hurricanes

Because hurricanes are rare compared to events like storms and coastal flooding, scientists have a more difficult time determining the level to which global warming may be a contributing factor in their frequency. But due to a combination of higher surface ocean temperatures (which can add energy to a hurricane), increased rainfalls (air can hold 7% more water for every degree Celsius increase), and higher sea levels, an increase in Category 4 and 5 hurricanes and associated storm surges is expected to increase over the next century.

 

For more:

Fuel for the Storm

Explaining Storm Surges at climate.gov

Surging Seas at Climate Central

 

Erosion

As the IPCC has stated, sea level rise

…usually causes the shoreline to recede inland due to coastal erosion. Increasing wave heights can cause coastal sand bars to move away from the shore and out to sea. High storm surges (sea levels raised by storm winds and atmospheric pressure) also tend to move coastal sand offshore. Higher waves and surges increase the probability that coastal sand barriers and dunes will be over-washed or breached. More energetic and/or frequent storms exacerbate all these effects.

For more:

Rising Sea Levels and Moving Shorelines

USGS National Assessment of Coastal Change Hazards Portal

Population and Economy

In the United States, over 123 million people (39 percent) of the population lives in counties directly on the shoreline (as of 2010) with 25 million living in areas currently vulnerable to coastal flooding. A 2016 study published in Nature Climate Change estimated that if sea levels rise three feet by 2100, 4.2 million people would be at risk of inundation, and if the oceans rise closer to six feet, the number of people affected increases to 13.1 million.

Meanwhile, the economic impact of extreme weather events has been increasing. The National Oceanic and Atmospheric Administration tracks weather and climate related events “where overall damages/costs reached or exceeded $1 billion”  Adjusted for inflation, the U.S. averaged fewer than three billion-dollar natural disasters per year from 1980 to 1990, but since 2010, the average is more than 10 per year– and these figures do not yet include Hurricanes Harvey, Irma and Maria.

For more:

New York Times: The Cost of Hurricane Harvey (article includes a chart of all $1 billion+ weather events since 1980)

NOAA: Billion-Dollar Weather and Climate Disasters

 

Other Impacts

Among other areas of concern that scientists are continuing to examine are contamination of freshwater aquifers and agricultural soils, as well as potential habitat loss for coastal animals (such as sea turtles).

 

Future Sea Level Rise: Where and When?

The National Oceanic Atmospheric Administration (NOAA) has created an interactive tool that allows users to examine potential sea level rise, flooding, and unique local conditions by simply typing in your address or locality.

Visit the NOAA.gov Sea Level Rise Viewer

What Can Be Done?

Coastal communities around the country are responding to the dangers of sea level rise, often in starkly different ways.

Here’s a report from PBS NewsHour, comparing the challenges and responses of New York City with those of Charleston, South Carolina:

And here’s a report from The Guardian UK that compares Miami and Atlantic City’s efforts in the fight against rising seas:

Ultimately, in order to reduce the most severe effects of sea level rise, fewer greenhouse gasses must be emitted.  As Climate Central’s Mapping Choices report states:

Carbon emissions causing 4 degrees Celsius of warming (7.2 degrees Fahrenheit) — a business-as usual scenario — could lock in enough eventual sea level rise to submerge land currently home to 470 to 760 million people globally. Carbon cuts resulting in the proposed international target of 2 °C warming (3.6 °F) would reduce the rise locked in so that it would threaten areas now occupied by as few as 130 million people.