Here are 10 things you should know about sea level rise, what causes it and how bad it might get.
1. There is enough water stored as ice to raise sea level 230 feet.
Most of this ice is located in Antarctica and Greenland. Antarctica, with an area 40 percent greater than the United States, is covered by an ice sheet almost a mile thick that holds about 200 feet equivalent of sea level. Most of that ice is — for now — stable, but scientists are concerned that the massive West Antarctic Ice Sheet, which holds about 11 feet of potential sea level rise, has reached a tipping point and will collapse. Another 23 feet of equivalent sea level is stored as ice in Greenland, and it is melting at an increasing rate. The rest is in glaciers and ice caps spread around the world, and they, too, are generally melting.
2. Sea levels have changed by hundreds of feet in the past.
We generally think of sea level as stable, but sea level has varied a lot over time as we have gone from ice age to ice age at about 100,000-year intervals. At the height of the last ice age, when much of North America was covered in ice, sea level was about 400 feet lower. We are now in a warm period between ice ages; sea level should be peaking and then eventually starting to go down again. But human-caused climate change is altering this cycle.
3. We are changing sea level at a very rapid rate.
While sea level has varied greatly in the past, it has generally changed slowly, over many thousands of years — except when ice sheets collapse. We will explain more on ice sheet collapse later, but prior to about 1900, we know sea level was stable for several thousand years. A warming world is now pushing sea levels higher, and the rate of rise is accelerating.
Sea level rose about seven inches during the 20th century. That may not seem like much, but the rate of sea level rise has almost doubled in recent years, and scientists expect that rate to continue to go up. About a third of the current rate of sea level rise is from thermal expansion of the oceans (the water expands like mercury in a thermometer), because they are absorbing about 90 percent of the increased heat from climate change. But in the future, melting ice will play a greater role. Predictions for the year 2100 are in the range of two to three feet, excluding any potential contributions from ice sheet collapse.
The Earth has already warmed by 1 degree Celsius (1.8 degrees Fahrenheit) above pre-industrial levels. Scientists estimate that if it warms by about 4 to 5 degrees Celsius (7.2 to 9 degrees Fahrenheit), which is projected to happen by the end of the century if we don’t act on climate change, then all the ice will eventually melt. That’s 230 feet of sea level rise.
5. Scientists are racing to better understand how sea level will rise as temperatures climb higher and higher.
The curve, or rate at which ice melts, is not directly proportional to time and temperature. In other words, at one-fifth of the temperature rise needed to melt all the ice (about where we are today), it is not likely that exactly one-fifth of all the ice will melt, leading eventually to 46 feet of sea level rise — could be more, could be less. The same is true with melting over time.
Ice sheets and glaciers are currently melting slowly, but there have been times in the past when they have collapsed rapidly, causing sea level to rise by a foot or more per decade — for decades at a time. We may be facing one of those periods in the near future.
Scientists are working to understand how unstable the ice sheets are, and how much additional sea level rise we could see over the current projections, particularly for the short-term (like in the time frame your kids will see). But with 230 feet of potential sea level rise on the table, it doesn’t take a whole lot more melting for a coastal city to be okay in one set of assumptions, and under water in another.
6. The last time sea levels changed significantly, there weren’t a lot of people around.
When the last ice age ended around 10,000 years ago and sea level started to rise to its present level, there were only about 5 million people on Earth, and they didn’t live in expensive cities right on the coast. Today, in the United States alone, nearly 5 million people live within 4 feet of high tide. So, not only are sea levels changing at a rapid clip, but we have also built a tremendous amount of infrastructure (such as houses, highways, utilities and nuclear power plants) right on yesterday’s coastline.
Not only will the sea level rise itself cause issues, but it is expected to greatly increase the damage from storm surges which will reach farther inland, or overtop natural or man-made barriers that would not have otherwise been breached. This is likely how one would first “get wet” from sea level rise.
7. Sea level rise is not going to stop anytime soon.
With the greenhouse gases currently in the atmosphere, we are already committed to further temperature increases, and further melting of the ice and increasing sea levels. As far into the future as you want to plan for, sea level will probably still be rising. It is something we, and future generations, are going to have to live with. Our climate change actions at this point can slow down the rate of sea level rise and limit the damage, but we cannot stop it.
8. Sea level rise will not be the same everywhere.
There are many reasons why sea level rise might be greater in some places than in others, but in general, when ice melts, sea level rise is less closer to the location of the ice and greater farther away. Differences in the regional heating of the oceans, changes in ocean currents, and a variety of other factors can contribute to regional variations in sea level rise.
In addition, land subsidence, like that occurring along the Gulf Coast of the United States, can worsen the impacts of sea level rise. Land uplift, like that occurring in Scandinavia, can buffer some against future sea level rise.
9. Melting Arctic sea ice does not contribute directly to sea level rise.
The Arctic has a lot of floating sea ice, and it is in the news a lot because it is decreasing dramatically, but sea ice loss in the Arctic does not directly contribute to sea level rise. That’s because it is already floating — it is not on land, so its melting does not directly impact sea level rise. If this sounds strange, you can verify this by watching a glass of ice water and checking the water level as the ice melts — it will not rise or fall. However, if you add more ice to the glass, the water level will rise. (Loss of Arctic ice does directly impact climate change because dark blue ocean absorbs more energy than white ice does, so it adds a bit to global warming).
10. The cost of sea level rise will go up faster than sea level itself.
In other words, the damage caused by going from 1 foot of sea level rise to 2 feet will be much greater than the damage from the first foot of sea level rise. In addition, the cost of conventional berms or levees to protect the shoreline also increases much faster than the height. For example, doubling the height of a levee makes the cost goes up by a factor of almost 4.
Sea level is already rising fast, but nobody knows exactly how much this could speed up if we continue to warm the earth at the rate we are doing now. One thing we do know is the more greenhouse gases and heat we put in, the faster sea level will rise.
That is why tackling climate change is so important. We have to ask ourselves how much of a chance we want to take. The difference between an aggressive stance on climate change and a slower path could mean the difference between an orderly approach to sea level rise and a chaotic retreat from the coasts.
Motta has worked in the energy and environment field as a program manager at the National Renewable Energy Laboratory. He is currently an affiliate at the Institute of Arctic and Alpine Research working on the communication of sea level rise risks and impacts.
White is professor of geological sciences, professor in the environmental studies program, and fellow and director of the Institute of Arctic and Alpine Research at the University of Colorado in Boulder. He is a Web of Science most highly cited scientist (one of the top 1 percent most highly cited authors in his field).
Nerem is a professor of aerospace engineering Sciences at the University of Colorado in Boulder, and leader of NASA’s sea level change team.