January 4, 2008

Lowering Sea Level Rise

Filed under: Sea Level Rise

Have you seen the latest on sea level rise? If not, you will find over one million websites on the topic and according to almost all of them, global sea level is rising ever faster, the acceleration will increase into the future (by some estimates resulting in a rise of several meters by century’s end) and the entire mess is caused by burning fossils fuels and increasing the atmospheric concentration of greenhouse gases. But is what you find on these sites really the latest on sea level rise? Are sea levels really rising at the pace that they are so often made out to be?

We have written about sea level rise many times in the past, and there is no doubt that the sea is currently rising worldwide. However, the sea level rise has been taking place almost monotonically over the past 8,000 years, with substantial decadal variability embedded in the trend. In 2001, the Intergovernmental Panel on Climate Change (IPCC) concluded that “No significant acceleration in the rate of sea level rise during the 20th century has been detected.” In 2007, IPCC notes “Global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961 to 2003. The rate was faster over 1993 to 2003: about 3.1 [2.4 to 3.8] mm per year. Whether the faster rate for 1993 to 2003 reflects decadal variability or an increase in the longer-term trend is unclear.” A key question is not whether sea level is rising, but rather, has there been any acceleration in the rise – the jury is still very much out on that issue.

Yet another major article has appeared on the subject of sea level rise, and the results are not going to be popular with the developers of the million or so websites on the topic. The article is published in Geophysical Research Letters, the authors are from Tulane University and the State University of New York at Stony Brook, and the work was not funded by any horrible industry group. Kolker and Hameed begin their article stating “Determining the rate of global sea level rise (GSLR) during the past century is critical to understanding recent changes to the global climate system. However, this is complicated by non-tidal, short-term, local sea-level variability that is orders of magnitude greater than the trend.” Once again, some element of the greenhouse scare that might seem simple at first glance appears to be a lot more complicated than we originally may have thought. The authors further note that “Estimates of recent rates of global sea level rise (GSLR) vary considerably” noting that many scientists have calculated rates of 1.5 to 2.0 mm per year over the 20th century. They also show that other very credible approaches have led to a 1.1 mm per year result, and they note that “the IPCC [2007] calls for higher rates for the period 1993–2003: 3.1 ± 0.7.” They state that “Debate has centered on the relative contribution of fresh water fluxes, thermal expansion and anomalies in Earth’s rotation.”

We thought the “debate” regarding anything related to global warming was over, but yet another article suggests to us that some debate still exists – very interesting. They further state that “Determining GSLR rates is complicated by non-tidal, year-to-year variability in local mean sea level that is one to two orders of magnitude greater than the long-term trend, potentially masking changes in the rate of rise. The cause of this variability is largely unknown, although it has been linked to storms, winds and floods, wind driven Rossby waves, shifts in major ocean currents such as the Gulf Stream, volcanically induced ocean heat content variations, and in the Pacific Ocean, the El Nino Southern Oscillation.”

Alright, enough about all these unknowns and debates regarding sea level rise. Kolker and Hameed gathered long-term data regarding the Icelandic Low and the Azores High to capture variation and trend in atmospheric “Centers of Action” associated with the North Atlantic Oscillation which is regarded as “One potential driver of Atlantic Ocean sea level.” As seen in Figure 1, these large-scale features of atmospheric circulation vary considerably from year-to-year and appear to change through time in terms of latitude and longitude.


Figure 1. Long term changes in the state of the COAs for months November through March (from Kolker and Hameed, 2007)

You see where this is heading? Next up, the authors used to “Centers of Action” data to predict sea levels around the North Atlantic. They used the raw sea level measurements made around the basin, and the also used sea level data that were corrected to account for the “glacial isostatic adjustment” (GIA) and the mysterious “inverse barometer” (IB) effect. Very simply stated, the IB is related to deviations in sea-surface elevation in response to deviations in atmospheric pressure.

As seen below (Figure 2), the COA data do a wonderful job predicting sea levels at Halifax, New York City, Charleston, Stockholm, and Cascais (coastal town near Lisbon). Kolker and Hameed note that “Overall, our multiple regressions can account for 59–79% of the variability in the RAW sea level data and 24–79% of the IB + GIA adjusted data. These findings suggest that meteorological processes drive coastal sea-level variability by redistributing water, heat, and the response of the ocean to atmospheric pressure across the ocean basin.”


Figure 2. Multiple regressions of sea level and COAs, which predict a major fraction of the variability in sea level and a majority of the long-term trends in sea level. (left) In red are the raw sea level data and in black are the regression outputs. (right) The corrected data are in red and while the regression outputs of this data are in black. Y axis units are in mm (from Kolker and Hameed, 2007)

Kolker and Hameed used these relationships to statistically control for variations and trends in atmospheric circulation. They find that the “residual” sea level rise (that not explained by COA variability) in the North Atlantic lies somewhere between 0.49±0.25mm/yr and 0.93±0.39mm/yr depending on the assumptions they employ, which is substantially less than the 1.40 to 2.15 mm per year rise found in the data corrected for the glacial isostatic adjustment. This “residual” sea level rise includes both local processes such as sedimentation changes, as well as larger-scale processes such as rising global temperatures.

Whether or not the influences of rising global temperature lie completely within the residual, or whether they also are co-mingled with the COA variation is a question that seems to be not sufficiently answered by the described methodology, but, nevertheless, the authors feel certain that they have adequately isolated the global influence in the residuals of their analysis. When extrapolating from their North Atlantic results to the globe at large, they suggest that their research yields a global sea level rise near 1.1 mm per year which is well below IPCC and other estimates.

Had they discovered anything that would inflate the rate of sea level rise, we’d have seen it on the front page of every newspaper around the world. But discovering anything that lowers the estimate means coverage at World Climate Report, but likely no where else.

Reference:

Kolker, A. S., and S. Hameed. 2007. Meteorologically driven trends in sea level rise. Geophysical Research Letters, 34, L23616, doi:10.1029/2007GL031814.




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