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Historical Sea Levels

Sea Level Rise

Well over 70 percent of our planet is currently covered by water. This was not always the case. During the last ice age about 22,000 years ago, ice covered large portions of the northern and southern hemispheres. Ice then covered about 32 percent of the land and 30 percent of the oceans. When these massive ice sheets melted, they released the water inside them which was enough to make seas rise by about 430 feet (130 meters). See the chart to the left.

So rising seas are not a new phenomenon, they have been rising for a very long time. However, they have been rising very modestly for the last 5,000 years. Data from 1901 to 2000, shows that global sea levels rose by only 7.5 inches (19 cm). which is 0.075 inches (1.9 mm) per year. While the average temperature increase over this same period was 1.1°F (0.6°C).

But, from 1993 to 2014, the seas have increased by 0.13 inches (3.2 mm per Colorado University) per year almost a 70% increase. Sea level rise is expected to continue for centuries.  Top

How Are Sea Levels Measured?

Tide Gauge

The Mean Sea Level (MSL) is the average height of the ocean's surface at the halfway point between the mean high tide and the mean low tide. But there are many things changing the water level at any given location and time:

Sea levels change by the second (waves), by the hour (tides) and by the week (planetary and solar orbit changes). To get around these issues, scientists use tide gauges. A tide gauge is a long pipe with a small hole below the water line. This pipe is often called a stilling well. Even though waves are changing the water level outside the tide gauge constantly, they have little effect inside the gauge. The sea level can be read relatively accurately inside the pipe. If read on a regular basis over a time span of a year and then averaged, one can get a reasonably accurate measure of the local Mean Sea Level. Over the last 20 years, satellites have played an increasingly significant role in measuring sea levels.  Top

Sea Level Rise Is Accelerating

Sea Levels

The graph to the left depicts the mean sea level rise from 1870 to May of 2014. The measurements in blue from 1870 to 2001 are based on tide gauge measurements by Church and White. The data from 1993 to 2014, shown in red, are satellite recordings compiled by the University of Colorado. Recurrent seasonal variations have been removed from all data.

As one can see, the rate of sea level change has increased by a factor of 4 (3.2 mm divided by 0.8 mm) over the last 144 years. If we assume that the 0.8 mm rise of the late 1800s, (which was due to nature) continues until today; then the other 0.24 mm, 75% of the current rise, is human induced.

In addition to variability over time, sea level heights are not the same across the globe. Also, changes in sea levels are not uniform either. Some regions have levels rising faster than others. Despite the worldwide trend of rising sea levels, in some regions the mean sea levels are currently falling. See the next section.  Top

Sea Surface Height Changes

Global Sea Heights

The map to the left shows global patterns of "changes" in sea surface heights as measured by satellites from January 1993 through the end of 2012. Places where the sea surface height has increased by up to 9 millimeters per year are shown in deep red and purple. The places where sea levels dropped some are in blue; while the areas with no change are in white.

Off hand, one would expect the biggest sea increases to be next to the melting glaciers near the poles. However, the deepest red/purple area is in the western Pacific. That is also where the normal height (not change in height) is about 18 inches higher than the eastern Pacific, which shows very little change. High trade winds and currents normally push water higher in the eastern Pacific except in El Niño years. If there is extra water flowing into the oceans (from Glacier run off), it follows that some of it will be pushed to the eastern Pacific and raise sea levels there.

Thus we see that overall sea level changes are not very uniform and using the average increase across the board would be very misleading. Some areas, like Indonesia, will see increases larger than average, while some places, like the western parts of the US and Canada, will see very little sea rise.  Top

Factors Influencing Sea Level Rise

Measuring

There are two main factors that contribute to sea level rise: thermal expansion - ocean water expands as it warms; and melting of large land glaciers and ice sheets.  

Thermal expansion – Heat in the upper layer of the ocean is released quickly into the atmosphere. However, heat absorbed by the deeper layers of the ocean will take much longer to be released and will have a significant impact on ocean warming. Global mean ocean temperature has been rising because about 10% of the heat produced by greenhouse gases has been trapped in the oceans.

The impact of thermal expansion on sea level rise has increased dramatically. During the past 50 years, thermal expansion accounted for only about a quarter of the observed sea level rise. Yet during the last 10 years, its impact on sea levels has increased to the point where it accounted for half (1.6 mm/year) of all observed sea level rise.

Melting - Loss of mass from glaciers world-wide, as well as from the ice sheets of Greenland and Antarctica contributes 1.2 mm to sea level rise per year. This is almost all of the sea level rise not caused by thermal expansion. Roughly 60% of ice loss contributing to sea level rise is from "in-land" glaciers and not from the two ice sheets of Greenland and Antarctica. Melt from these smaller glaciers has accelerated over the past decade.

Together, thermal expansion and melting ice calculations contributed only 2.8 mm to the sea level rise of 3.2 mm per year between 1993 and 2013, about a 13% shortfall. What is causing the shortfall? Ocean temperatures, and therefore estimates of thermal expansion, are not available for parts of the ocean in the Southern Hemisphere, where significant fresh water input from Antarctica is likely. In addition, ocean temperature measurements only extend 2000 to 3000 meters deep. Much of the unexplained sea level rise could be due to either thermal expansion that is unaccounted for and/or the melt contribution of glaciers may be understated.  Top

Future Sea Level Rises

Future Sea Levels

There is nothing mankind can do to prevent future sea level rises (SLRs). At this point in time, rising seas are part of nature. The seas have been rising for thousands of years and will continue for centuries into the future. But, it is most likely true that humans have accelerated the rate of sea level rise starting roughly 100 years ago.

The dark area in the illustration to the left depicts the most likely range of sea rise from 2010 to 2100. An optimistic outcome would be an increase of only one foot. This would result from a very aggressive plan to control carbon dioxide and other gases that make up the greenhouse effect. On the other hand, the most likely worse case would be a rise of four feet which would represent some control of greenhouse gases, but offset by significant melting of the polar ice sheets in the Arctic and Antarctica. The author thinks the most likely SLR outcome is about three feet, in between the two limits as indicated by the orange bar.

The models used to create the chart predict the "very best" case to be 0.66 feet (8 inches) or the amount of sea rise from the last century. This is considered implausible given the increased melting of the polar glaciers. The absolute "worst case" scenario is a rise of 6.6 feet also considered implausible. This would require runaway melting of both sets of polar ice sheets which is considered unlikely because eastern Antarctica has increasing annual snow accumulation on their high interior mountains which is being offset by slow glacier travel to the sea. The net is a very minute decrease in overall eastern Antarctica glacier mass, not a runaway melt. Western Antarctica, on the other hand, is much lower than eastern Antarctica and is seeing very significant glacier melting making a major contribution to sea rise. See the Polar Ice Page for more detail.  Top

Vulnerable International Regions

Vulnerable Regions

Regions around the world which are particularly vulnerable to flooding are often found in delta systems. These are low-lying coastal regions where rivers enter the ocean in developing countries such as Bangladesh, Vietnam, India, and China, and have especially large at-risk populations. Sea level rise increases the risk of both temporary and permanent flooding of these coastal lands. About 23% of the world's population lives in these coastal zones. Unfortunately, many of these countries that are most vulnerable to sea level rise do not have the resources to combat a dangerous sea rise. It is not only small island states that need to worry about sea rise, large island nations such as the Philippines and Indonesia are at severe risk because they do not have enough land at higher elevations to rescue possible coastal populations.

Another possibility for some island nations is the danger of losing their fresh-water supplies as sea level rises push saltwater into their aquifers. For these reasons, those living on several small island nations, like the Maldives in the Indian Ocean and the Marshall Islands in the Pacific, could be forced to completely evacuate at some point in the 21st century.

Even comparatively modest sea level rise projections will wreak havoc. Coastal flooding, storm damage, eroding shorelines, salt water contamination of fresh water supplies, flooding of coastal wetlands, and an increase in the salinity of estuaries are all realities of even a small amount of sea level rise. Low lying coastal cities and villages will be affected. Resources critical to island and coastal populations such as beaches, freshwater, fisheries, coral reefs, and wildlife habitat will be at risk.

It's worth keeping in mind, however, that average changes in sea level do not occur uniformly around the globe. There is actually a fair amount of difference in sea level rise in different parts of the world due to ocean circulation and wind pressure patterns.  Top

Vulnerable US Regions

Coastal Vulnerability Index (CVI)

The coastal vulnerability index (CVI), developed by the U.S. Geological Survey (USGS) shown above in colors along the coasts of the US, incorporates six local physical variables that have been quantified. These are: a local geographical index, the coastal slope, the average sea level rise, the amount of shoreline erosion, the average local tide range and the average local wave height. This methodology highlights those regions where the various effects of sea level rise may be the greatest.

First, lets dismiss the the US regions that are "not" vulnerable to sea level rise. Alaska and some areas along the Pacific Northwest coast are at a low risk of sea level rise because the sea level is actually "falling". Land in these regions is on the "rise" because of the loss of weight from the great ice sheets that once covered much of North America during the last Ice Age. The rate of land levels increasing exceeds the sea level increases in these regions. The New England coastline from Maine down to New York City is also rated as low risk. This is due to rugged, rocky shorelines that rise quickly from the sea. Exceptions are the Cape Cod area and Long Island; these are low level islands and beach areas.

Eastern US SLR

CVI values show numerous areas of very high vulnerability along the the mid-Atlantic coast (Maryland to North Carolina) and northern Florida. These high vulnerability areas are typically coastlines where the regional coastal slope is low and where the major shore types are barrier islands. Also very vulnerable is the lower Chesapeake Bay. Here, the low coastal slope, vulnerable salt marshes and high rate of sea-level rise combine for a high CVI value.

The region around New Orleans is the most vulnerable region of the entire US coast. An amazing 55% of the gulf coast falls in the "very high" or "high" CVI range. The highest rates of "sea level rise" in the United States are in the Mississippi delta region - 10 mm per year, or 1 inch every 2.5 years. This region is very high risk because in addition to rising sea levels, the land around the mouth of the Mississippi River is actually "sinking".

The Florida Panhandle, as well as the West Florida coast, are low to moderate risk because the land is not sinking very much, wave heights are lower, and the slope of the land is relatively steep near the coast.

The Texas coast is considered to be a high to very high risk because of the relatively high mean wave height, sinking land, and very shallow coastal slope.

The Pacific Coast also has 50% of its length in the "very high" or "high" vulnerability range. Areas of very high vulnerability include the San Francisco - Monterey Bay coast and in southern California from San Luis Obispo to San Diego, where the coast is highly populated. The highest vulnerability areas are typically lower-lying beach areas. A majority of the high risk areas face directly west and as a result, are highly impacted by large Pacific Ocean waves.  Top

Some Consequences Of Sea Rise By The Year 2100

There is some hope, though. If we act rapidly to reduce emissions we can still prevent the "worst effects" of climate change. Switching reasonably quickly to renewable energy sources is our only option to avoid a "disastrous" sea level rise.

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