Keeping Track of Sea Level

By Gary Griggs, Distinguished Professor of Earth and Planetary Sciences and Director of the Institute of Marine Sciences at UCSC

Sea level rose fairly rapidly when the last Ice Age ended about 18,000 years ago. Ice sheets melted, glaciers retreated, and all of that melt water flowed into the oceans. Most of the nearly 400 feet of sea-level rise the planet experienced following the end of the Ice Age time took place over the next 10,000 years. Averaging this out (400 feet of rise over 10,000 years) gives us a rate of rise of about a half an inch or 12 mm per year for the entire ocean. There is also evidence that some of this rise occurred in pulses as large masses of the Antarctic ice sheets collapsed rapidly, driving sea level up at perhaps an inch per year.

Somewhere around 8,000 years ago, however, sea level rise slowed, and rose relatively little until the late 1800s. The industrial revolution began at about that time, as coal and then oil and gas were exploited for fuels that were utilized to produce heat and energy for factories and transportation. These fossil fuels, or the preserved remains of terrestrial plants (coal) and marine plants (petroleum), represented highly concentrated sources of carbon that generated heat when burned. Burning these fossil fuels, however, also generated large amounts of carbon dioxide.

About one-half of all the carbon dioxide generated then and now ends up in the atmosphere, about one-fourth is absorbed by the oceans, and the other one-fourth is taken up by the marine and terrestrial plants of the world. Human activity in 2010 generated about 40 billion tons of carbon dioxide, or 4.5 million tons every hour. Nearly 90% of this was from fossil fuel combustion and the remainder from cement production and land use changes (burning tropical rain forests, for example).

What does all of this have to do with sea level? There is now a clearly established relationship between carbon dioxide content of the atmosphere and the Earth’s temperature, and a close connection between global temperatures and sea level. The warmer the Earth, the more ice melts and the more seawater expands, both raising sea level.

After about 8000 or so years of a relatively constant sea level, as best we can tell from the fossil record of ancient shorelines, sea level began to rise again.

As the carbon dioxide content of the atmosphere began to increase in the late 1800s, global temperatures slowly rose, and sea level followed.

How do we know how fast sea level has risen? Well, about 200 years ago, the first tide gages were set up in Europe, one on the coast of France and another in Poland. A tide gage is an instrument that records water levels. These are essentially large pipes that extend down into the ocean and have a float inside that moves up and down with the tide. As the tide rises and falls each day, these gages record the changes in water level, day after day, year after year.

These instruments were installed to provide accurate information on water depths and how these changed over time so ships could enter and leave ports safely. Each gage is anchored on some structure, a concrete breakwater or wharf, a rock outcrop, or some other solid foundation that is stable over long periods of time.

As time went on, however, and the sea level records were studied, two things became clear: 1] Not only was the level of the ocean changing daily as the tide went in and out, but sea level was also rising globally; and 2] sea level was not changing uniformly everywhere.

Each of these official gages, and there are now hundreds of these along coastlines around the world (go to: http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml and you can look at sea level records for any tide gage) that track sea level at specific coastal locations. Many coastal areas are not stable, however; some are sinking (New Orleans and Venice, for example), and some are rising (Alaska and Scandinavia, for example). You can check the website listed above for these and other locations. Each tide gage keeps track of how sea level is changing relative to the particular area of land it is anchored on.

Sea level rose globally about seven inches from the late 1800s to the late 1900s, or at an average rate of about 1.7 millimeters/year (the thickness of a quarter). This is a global average, however, because some coastlines are rising and some are sinking. These geographic variations were resolved in 1993 when two satellites were launched into orbits high above the Earth. They use radar and GPS technology to measure the level of the ocean from space very precisely. This high-tech approach eliminates the problem of land motion on Earth and has given us a new and higher global sea-level rise rate since 1993 of 3.2 millimeters per year, or 12 inches per century.