SPECIAL SECTION: Trends in sea level extremes in the USAPI

by Dr. Rashed Chowdhury

The following three paragraphs are taken from US National Assessment of the Potential Consequences of Climate Variability and Change Educational Resources Regional Paper: US-Affiliated Islands of the Pacific and Caribbean—Sea Level Variability available at http://www.usgcrp.gov/usgcrp/nacc/education/islands/islands-edu-6.htm

"Sea-level rise and the associated erosion and inundation problems are currently extremely important issues for many of the US-Affiliated islands. There are three factors that affect the impacts of sea-level rise on islands: the natural amount of sinking and rising of the individual islands (due to plate tectonics); the rate and extent of global sea-level rise; and the occurrence of periodic events, such as extreme lunar tides, ENSO related changes, and storm-related wave conditions.

Many of the projected consequences of long-term sea-level rise, such as salt water intrusion into freshwater lenses and coastal erosion are already problems in some if not most island jurisdictions. Climate-related changes of these conditions are seen, therefore, as magnifying existing problems rather than as problems in isolation from other stresses. Island communities must deal with these problems today and, in so doing, can develop important insights into how they might most effectively respond to climate-related changes in sea level over both the short- and long-term.

Long-term global rates of sea-level rise are projected to be 2 to 5 times faster in the 21st century than during the 20th century. In addition to considering only the consequences of a gradual, long-term rise in sea level, island communities will continue to face short-term sea level changes as well. In some locations in the Pacific, temporary rises in sea level from storms, lunar tides, and ENSO events raise the sea level even higher than is projected for the next century. Future sea-level rise, both global and periodic (because increasing global sea level will also raise the level from which temporary events occur), will increasingly contribute to negative consequences for populations and ecosystems."

In Figure 4 (below), the annual maximum sea level (HSL) shows a significant increasing trend, the increase being concentrated in the later half of 20th century. Annual variability shows an apparent trend with the most pronounced increase occurring in the 1960s and 1970s. The linear regression fitted to the data of the annual maximum show an increase in the maximum on the USAPI during the last 50 years. The last 60 years’ data suggest that the maxima has increased by about fourteen centimeters in Guam, sixteen centimeters in Marshalls, and ten centimeters in American Samoa. However, according to the aforementioned paper, it should be mentioned that the relative sea level (RSL) at some tide gauges has been observed to be falling. This is due of a) the short time series used to compute the trend (interannual and decadal fluctuations can mask longer term trends), and b) land motion at the tide gauge. The uncertainty of these values is reflected in their high error bars and in many cases is larger than the trend estimate itself.

Figure 4 . Annual maxima/minima of sea levels for Guam, Majuro (Marshalls) and Pago Pago, (American Samoa). HSL is the highest sea level; LSL is the lowest sea level