Rising Sea Level and the Future
Video: Sea Rise (2:26)
Coastal flooding is getting worse. Let's look at some of the reasons why. We will start with this simple diagram. The ocean itself is rising mostly because of humanhuman-causedal warming. One reason is that the warming melts ice in mountain glaciers and in the Antarctic and Greenland ice sheets, releasing water that reaches the ocean to make the ocean bigger. Warming is also causing the ocean water itself to expand and take up more space. In addition, not directly related to warming, we pump a little more water out of the ground than nature replaces and that extra water is reaching the ocean to make the ocean bigger. The global sea level rise is causing flooding on most coasts but, there's much variation. In some places, the land is going up or down because of mountain-building processes and because of the response to the end of the Ice Age that had pushed some land down and other land up and now it's going back to where it had been. The land may be going down in many places because of compaction of mud in Deltas or because we're pumping water or oil out of the land and causing it to compact. The land may be building out because extra sediment is arriving as in a Delta that's growing, but the land may be eroding because extra sediment is being removed. The ocean itself may be rising or falling locally. The ocean surface is not perfectly flat. In some places the winds pile water against the land, other places they move it away, and as we change the winds and the currents that can change how much water is piled up or moved away. The warming may be making the strongest storms even stronger so that they can blow more water inland. And you might think of some other complexities. So global warming is flooding most coasts, but there's real need for experts who really understand and can help.
Coasts change for many different reasons, and in many different ways. But, recently, most of the U.S. (and world) coasts have been retreating because sea level is rising, and that rise is accelerating. The total size of the ocean is increasing, as water that had been stored on land in glaciers and ice sheets and in the ground is transferred to the ocean, and as warming of the ocean causes the water already there to expand and take up more space.
The rate of rise is now more than 3 mm/year (a bit over an inch per decade), and has been accelerating. That isn’t much if you’re at the top of a cliff in Acadia, but if you are on a sandy beach that slopes very gradually, the inch of sea-level rise may cause the coast to retreat by many feet or even a few tens of feet. That in turn means that a whole lot of houses and property can be lost in a single human lifetime.
This ongoing sea-level rise is being caused primarily by the rising temperature of the Earth’s climate (“global warming”), which is being driven primarily by human activities. (We’ll return to this later in the course, but we have very high scientific confidence that it is correct.) Most of the world’s small glaciers have been melting, Greenland’s ice sheet has been melting and flowing faster into the ocean, and Antarctica’s ice sheet has been flowing faster into the ocean, adding water to the oceans. Also, as the ocean itself warms, the water expands and takes up more room.
We also build dams on rivers, and the water that fills the reservoirs is taken out of the ocean and stored on land, causing sea-level fall. But, we “mine” groundwater by pumping it out of the ground faster than nature puts it back, and that water eventually reaches the ocean to raise sea level. Today, groundwater pumping is probably more important than dam building, contributing a little to sea-level rise.
The polar ice sheets contain a huge amount of water—if they melted, they would raise sea level nearly 250 feet (roughly 70-80 m). Philadelphia and the other great port cities of the world would become undersea hazards to shipping but really great places for fish to hide out, and the southern coast of Florida would be somewhere up in Georgia. We do not expect such a fate, but we cannot rule out the possibility that a dynamic collapse of the West Antarctic ice sheet could raise sea level more than 10 feet (3 m) in a human lifetime or two. If we don’t change our behavior, Greenland and its 24 feet of sea level is also looking shaky, although it would take centries or longer to melt completely. (Melting all of the remaining mountain glaciers would raise sea level only 1 foot or so, less than 0.5 m.) Drs. Anandakrishnan and Alley spend a lot of their research trying to reduce our uncertainty about the future of the great ice sheets, a fascinating and important topic.
Policy Implications of Sea-level Rise
Even if we humans stopped warming the climate, sea level will rise at least somewhat more, because much of the ocean has not yet fully warmed from the atmospheric warming we have already caused but will continue warming to “catch up” with the warmer air, and more ice will melt before reaching equilibrium with the warming of the air we have already caused. (If you come into a cold house in the winter and turn up the heat, it will take a while until everything feels warm; we have turned up the heat in the air, and it will take centuries for the ocean and the glaciers to fully catch up.) The near-certainty of continuing sea-level rise has some policy implications. For example, disaster aid following hurricanes that allows people to rebuild in vulnerable places will simply create the need for more disaster aid in the future. Many people believe that those who wish to build on the coasts should be required to carry insurance or to otherwise demonstrate that they have sufficient resources to cover their coming losses. Similar arguments apply to those who wish to build on earthquake faults, landslide deposits, and floodplains. Many people living in relatively safe but less-scenic places object to paying for others to live in dangerous but beautiful places.
Engineering Solutions
Because people love the coasts so much, and wish to live near them, all sorts of engineering solutions have been tried. These have had some success, but many failures, and they often lead to legal and political difficulties.
One approach is to build “dams” that stick out into the water and block the longshore transport of sand. These dams are usually called “groins” if they are small, and “jetties” if they are larger. (See the figure above). By making the coast rougher, and slowing waves, the plan is to trap sediment along the coast in much the same way that a dam traps sediment along a river. This plan sometimes works. However, recall that when a dam is built on a sand-bedded river, sediment is trapped upstream of the dam but eroded downstream. The same often happens along a coast; sediment is trapped “upstream” of the groin (on the side from which the longshore drift comes), but sediment is eroded on the “downstream” side (the side to which the longshore drift goes), where the sand-free waves attack the beach to pick up more sand. Saving someone’s beach while destroying the beach of a neighbor is a good way to generate lawsuits.
Video: Groins (3:14)
Most of our coasts really are eroding. Here’s one way we sometimes try to deal with it and how this might go wrong. You can build dams sticking out from the shore, and these can interrupt the longshore drift of sediment, trapping sediment where you are and reducing erosion. But, it can go wrong. Recall first what we learned about rivers and sediment transport. Here’s a river with a canoeist going down under the sun. If you build a dam and fill a reservoir with water behind the dam, recall sediment is trapped, to build a delta. The sediment is then not released from the dam; clear water is. If the bed of the river is sandy, it’s easily eroded. Then the river is eroded there as the clean water picks up more sediment and carries it away.
Now imagine that this same diagram is an ocean next to a coast, next to land, viewed from above from your drone. This coast is eroding, like most of them on Earth, and that is endangering houses there. The longshore drift is carrying a lot of sediment past and is removing slightly more than it brings. So, recalling what you learned from the river, you could build a dam to trap the sediment. This dam is called a groin; if it’s small, it’s called a jetty if it’s large. But whatever you call it, it captures sediment to help save your house. But if the coast is easily eroded, the clean water that goes past your groin erodes your neighbor’s house, and then your neighbor gets a lawyer and sues you and there are all sorts of problems.
Here’s a version of the same diagram from the Maine Geological Survey: Longshore transport is going there. A couple of groins have been built; sediment is building up on one side to save houses; sediment is eroding on the other side to endanger houses. In between some houses, you might not be sure which one happens: erosion or sediment.
Here’s a picture of a groin. This one happens to be an old one on Waikiki Beach in Hawaii that they were replacing. In this case, the eroded side has a wall that’s protecting it, so it isn’t cutting back into the luxury hotel. And the sediment buildup side has a nice beach that people love. If we look along this out to sea, it’s a beautiful view. Some places are building these, some places have outlawed them because of the problems.
People also spend millions of dollars to go out to sea, find sand that has fallen off into deep water, and bring the sand back to the beach. This sand usually lasts a single year or a very few years before being washed back to deep water, and in one recent case was mostly washed away in less than a week, but in some especially popular tourist destinations the investment may pay off.
We saw earlier that people do other things, such as building giant walls to keep the sea out. In the case of New Orleans, huge amounts of money were spent building a levee system, and then people built their houses and businesses in the supposed safety behind the levees. When hurricane Katrina broke the walls, the losses included the cost of building the levee system, the valuable things that the levees were built to protect, and the new valuable things that people built after the levees were erected, as well as the lives of so many people.
Geologists often look at such past events and then take a “natural” view of the coasts—we should figure out where the coast wants to go and build there rather than trying to stop the coast. But many people just don’t like that, and a lot of construction is likely to occur—and be destroyed—over the coming years, especially if we drive more warming.