Why is the sea level rise so insensitive to decadal changes? - DB2Our planet's oceans constitute an enormous mass. Given that, I'd be surprised to see rapid changes in overall ocean heat content as a consequence of atmospheric temperatures. Having said that, though, we are seeing changes.Here's a link to a good general summary:http://www.wunderground.com/climate/SeaLevelRise.aspSince the beginning of the 20th century, the seas have continued to rise at an average rate of 1.7 ± 0.5 mm per year, according to the IPCC (Bindoff et al., 2007). This increase, however, has not happened at a constant rate. The first noted increase was over the period of 1961 to 2003, when the average rate of sea level rise was 1.8 ± 0.5 mm per year (Church et al, 2001; Church and White, 2006; Bindoff et al., 2007). Global mean sea level measurements have become even larger since 1993. According to the IPCC, "For the period 1993 to 2003, the rate of sea level rise is estimated from observations with satellite altimetry as 3.1 ± 0.7 mm yr–1, significantly higher than the average rate."Thermal expansion from current and future greenhouse gas sequestration and increasing mass losses from the world's glaciers and ice sheets due to warming oceans and surface temperatures represent the two primary ways that anthropogenic climate change and its associated feedbacks do and could continue to affect sea level. But what proportion of sea level rise is attributable to each? Also, where exactly is the ocean expanding the most? Where is all this melting ice coming from? The table in Figure 3 offers a quick summary of the sources of sea level rise and their related input.Since the beginning of the century, the impact of thermal expansion on sea level rise has increased dramatically. During the previous half century, thermal expansion accounted for only about a quarter of the observed sea level rise. Yet during the last decade of that period, its impact on sea level increased to the point where it accounted for roughly half of all observed sea level rise. The IPCC states, "For the period 1961 to 2003, the average contribution of thermal expansion to sea level rise was 0.4 ± 0.1 mm yr–1, for the period 1993 to 2003, the contributions from thermal expansion (1.6 ± 0.5 mm yr–1)." In other words, the oceans have risen roughly 25mm since 1961 due to thermal expansion.There is, of course, variability. The different layers of the ocean trap different amounts of heat, and therefore warm at various rates. Input from melting snow and ice also effects ocean warming.Loss of mass from glaciers world-wide, as well as from the ice sheets of Greenland and Antarctica contributes another 1.2 ± 0.64 mm to sea level rise per year. This is practically all of the sea level rise not attributed to thermal expansion.According to Meier et al. (2007), roughly 60% of ice loss contributing to sea level rise is from glaciers and ice caps and not from the two ice sheets of Greenland and Antarctica. Melt from these smaller glaciers has accelerated over the past decade, and may cause 0.1 to 0.25 m of additional sea-level rise by 2100. The authors of the 2007 study report, "At the very least, our projections indicate that future sea-level rise may be larger than anticipated and that the component due to [glaciers and ice caps] will continue to be substantial."I just provided short snips. The entire article is worth the read, particularly the "Predictions for Future Sea Level Rise" section.Speaking of predictions, here's another link with analyses worth pondering:http://tinyurl.com/az4sdmsSea Level Rise: Faster than ProjectedWe have over a century of global sea level data based on tide gauge measurements, and about 20 years of data from satellite altimetry. Both tide gauges and satellite data indicate that the present rate of sea level rise is about 3 mm/yr. But the IPCC models center around a projection of 2 mm/yr over recent decades. Even the high end of IPCC projections for sea level rise rate fall short of what’s been observed. Simply put, sea level is rising faster than projected.The IPCC projections don’t include dynamical processes which contribute to sea level rise, specifically the complexities inherent in the wasting of ice sheets. That’s why most researchers already considered the projections to be too conservative. The most recent report gave an upper limit to sea level rise by the year 2100 of about 59 cm, but if you were to poll those who are actively researching this issue you’d probably get a consensus nearer to 1 full meter increase by that time.
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