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 changes 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. Changes in land ice cover also contribute to this (melting or forming of sea ice does not change global sea level appreciably, but melting land ice increases the amount of water in the oceans).
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.
Both the Kings Point and Bridgeport Datasets show sea level rising over the last century. The rate of increase at both stations over the entire time period (Kings Point from 1932 and Bridgeport from 1965) to present is about 1/10 of an inch per year (a little less than 1 foot per 100 years). However, the actual data hint at the possibility that this rate has been increasing over the last few decades (since most recent data points are above the line). Bridgeport shows slightly faster long term rise than Kings Point, though this may be because this dataset is shorter (and excludes the earlier period where sea levels were rising more slowly). The rates of sea-level rise observed in Long Island Sound is 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 Long Island Sound and the surrounding communities 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.