Ecosystem Targets and Supporting Indicators
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The Sea Level Rise indicator shows the average sea level at a given point, over a year. This removes the influence of short term fluctuations in sea level caused by things like tides, storms, or seasons, and allows us to focus on the long term trend.
Long term changes in sea level are primarily caused by water from melting glaciers and ice sheets as well as increases in the average temperature of water in the ocean. As the water warms, it expands. The change in volume for each gallon of water is very small, but the amount of water is very large (on the order of 350 quintillion gallons) so increasing ocean temperature by one degree can result in a substantial increase in the volume of the ocean, and therefore, the sea level. Melting land ice, glaciers and ice shelves, are now the dominant cause of sea level rise, but formation or melting of sea ice has no effect on sea level.
There are many other shorter-term factors that can influence the annual mean sea level, such as fluctuations in tidal cycles, particularly stormy years, or annual wind patterns, and these factors contribute to the annual variability in this indicator, but long term patterns are generally caused by changes in regional climate.
It is important to understand regional changes in sea level (as opposed to simply going by global averages) for a number of reasons:
1) Because sea level is influenced by many factors, regional climate and weather patterns can result in more or less severe changes in sea level rise in different regions, even nearby ones. For example, if regional wind patterns over Long Island Sound shifted from a pattern of predominantly westerly (from west to east) winds to easterly (east to west), we might expect the region to experience higher sea levels. A shift from southerly (south to north) winds to northerly (north to south) winds might produce an increase in sea level on Long Island, but a decrease in Connecticut.
2) When we measure sea level, we’re actually measuring the relative sea level or the level of the water as compared to the land at a given location. But the land actually doesn’t remain at the same height. For thousands of years, most of the Northeastern USA was covered by glaciers a mile thick or more. The weight of this ice pushed down on the land and compacted it. When the ice melted, the land slowly rebounded (like a couch cushion after you stand up). Scientists call this process isostatic rebound. Some types of land compacted more than others, and some types of land bounce back faster than others. Isostatic rebound can reduce the relative sea-level rise because the land is also rising, but as this process nears completion, the rate of relative sea-level increase will accelerate. Relative sea-level rise can also be impacted by erosion or deposition of land in different parts of the Sound.
Kings Point, Bridgeport, New London, and The Battery datasets show sea level rising over the last century. Although not in Long Island Sound, The Battery is included to compare the Long Island Sound stations (Kings Point, Bridgeport, and New London) with the longest dataset in the region. All stations are experiencing a similar linear increase in SLR over the entire time period (Kings Point from 1932, Bridgeport from 1965, New London from 1939, and The Battery from 1856 to present), which is about 1/10 of an inch per year (a little less than 1 foot per 100 years). Bridgeport shows a slightly faster long term rise than Kings Point, New London, and The Battery. We show a linear increasing trend here, however the increasing number of recent data points above the line at most sites show an acceleration of sea level rise, consistent with global trends since 1970. The rates of sea-level rise observed in Long Island Sound are more than 50 percent higher than the global average during the same time period, which was about 6.5 inches per century. This shows that the coastal communities surrounding Long Island Sound must pay particular attention to rising sea levels, as we should anticipate a continued above-average response in our region compared to global average changes in sea level.