Shifting Tides: Sea level rise in US
August 04 2011 | Coastal Preservation,
by Chad Nelsen
There is southwest ground swell in the water with 6 to 8 foot waves, a period of 18 seconds, and 10-wave sets hitting the coast every 20 minutes. It’s sunny and the water is glassy. Frog is standing on the bluff overlooking his favorite local surf spot. He turns and says to his friend, “It should be good in a couple of hours.” What? Why isn’t he dashing out to surf what seems like ideal conditions? Because the tide isn’t right. This spot, like so many of our favorite waves, is very tide sensitive. Too high and the waves don’t break on the outer reef. Too low and the inside reef is exposed and dangerous. A few feet of water makes a big difference.
Now imagine if we added four feet of water on top of the normal tides. Permanently. That’s what future sea level rise predictions suggest could happen.
Coastlines throughout the world are very sensitive to changes in sea level. Sea level changes where waves break, how coasts erode or accrete, the tidal flow in estuaries, and the location and extent of wetlands and coral reefs. And small changes, on the order of inches, can have profound impacts on the coasts we love and work so hard to protect.
The history of sea level rise:
On a geologic time scale, the Earth has seen major changes in sea level throughout history. The oceans rose and fell hundreds of meters, alternately flooding and exposing vast extents of the land. That’s why you can find shells in the rocks in Wyoming. About 20,000 years ago, during the last glacial age sea level was about 400 feet below current levels and it has been rising gradually every since.
Changes in sea level are controlled by the volume of water in the sea (eustatic change) and the relative movement of the land (tectonic change). The volume of ocean water is controlled by thermal expansion and by the amount of water trapped as ice. Thermal expansion occurs when global temperatures increase and the water expands. Even the smallest expansion of the seawater can lead to significant changes when it occurs across entire oceans. In additional to thermal expansion, sea level also rises in response to the melting of mountain glaciers and the world’s massive ice sheets in Greenland and Antarctica.
Tectonic changes are more local in effect. Depending on the geologic setting, in some parts of the world the land is rising and in other parts it is sinking. For example, in many areas on the north latitudes such as Canada, Alaska and Scandinavia, the land is actually rebounding from the weight of ancient glaciers and is rising. If the land rises more quickly than global sea level is rising, then the local effect is actually a relative sea level decline. In other places, like the Gulf coast where the land is sinking from resource extraction, sea level rise is exacerbated.
For these reasons, sea level rise will be different at different locations.
Check out this map to see what sea level rise looks near you:
Today’s sea level changes
According to the Intergovernmental Panel on Climate Change (IPCC), the average rise in global sea levels has been 1.7 mm/year over the last 100 years .
At a local scale, sea level changes every hour due to many factors including the tides, winds, surf conditions, water temperature and atmospheric pressure. This “noise” in combination with the fact that annual sea level rise in on the order of millimeters make overall changes in sea level impossible to observe with the naked eye. That said, sea level changes over time can be measured using tide gauges and satellite altimetry. For example, this historic tide chart for Honolulu, HI shows annual and decadal fluctuations but also a general trend of sea level rise over the last 100 years.
Figure 1: Monthly mean sea level in Honolulu, Hawaii (NOAA)
This historical information, based on everyday observations, makes it pretty clear that sea level is rising. Predicting future sea level increases or a rate of acceleration of future sea level rise is more challenging.
Future Sea Level Rise:
Given the dynamic and complex nature of fluctuations in the earth’s global temperature, it is much more difficult to predict future sea level rise than to simply measure the sea level rise that we have observed in the past. The big question is how will global climate change affect sea level rise in the future. The IPCC, which is a scientific body designed to bring together scientists from around the world and synthesize information, has provided a range of scenarios for future sea level rise based on climate models. As you can see in Figure 2, one commonly-cited prediction of future global sea level rise ranges from approximately 0.5 to 1.4 meters (1.5 to 4.5 feet) above 1990 levels by 2100 . The variation is dependent on how much the earth warms. It is notable that under all circumstances sea level rise is anticipated to accelerate.
Figure 2: Past sea level and sea-level projections from 1990 to 2100 based on global mean temperature projections of the IPCC
Sea level has been changing throughout the earth’s history and has been rising on most parts of the U.S. coastline over the last 100 years. With the predicted increased emissions of greenhouse gases and resultant global warming, sea level rise will increase and the only question that remains is how much and how fast it will rise.
In the United States, almost 90% of the coast is already vulnerable to coastal erosion , which is creating challenges to protect coastal development without destroying our beaches and coastal habitats. Accelerating sea level rise will make those challenges even greater.
Check out NOAA’s coastal vulnerability map here:
What is clear is that we can’t afford to wait for things to get worse before we begin long term planning for our beaches and coastlines. So next time you are checking the tide before a surf, walk on the beach or a visit to the tide pools, think about the highest of the high tides you see and what it would be like if the tide was another 4 feet higher. That is part of the challenge we will face over the next 100 years of coastal protection.