May 6, 2008

Slower Sea Level Rise

Filed under: Sea Level Rise

One of the major pillars of the greenhouse scare is that sea level is rising due to global warming, coastlines will be inundated, and disasters will occur in coastal areas throughout the world. Who could ever forget Al Gore’s documentary showing us the World Trade Center Memorial under water due to sea level rise? A year ago, climate change hero James Hansen warned the world that non-linearities in the ocean-atmosphere system could lead to a whopping 5 meter or more sea level rise over this century.

As we have covered many times in the past, sea level is certainly rising – of course, it has been rising for the past 10,000 years. During the last glacial period, sea level dropped 400 feet as water was tied up in ice, and as we have moved out of the cold glacial period, sea level has recovered. The question for climate change experts is not “Is sea level rising” but rather “Is sea level rise accelerating?” In 2001, the Intergovernmental Panel on Climate Change (IPCC) wrote “No significant acceleration in the rate of sea level rise during the 20th century has been detected”, while in 2007, IPCC wrote “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.” To say the least, the IPCC has been very cautious on the issue of accelerated sea level rise.

Several articles have been published recently on sea level rise that caught our eye at World Climate Report. The first appeared recently in Global and Planetary Change and was written by a pair of scientists with India’s National Institute of Oceanography. Unnikrishnan and Shankar begin their article noting “Apart from changes in the atmospheric variables, global sea-level rise is one of the good indicators of climate change. Increase in global atmospheric temperature has a direct effect on the ocean by causing a rise in ocean temperature and melting of glaciers. Both these processes lead to a rise in global sea level.” Furthermore, they state “The Intergovernmental Panel on Climate Change (IPCC) reported values between 1–2 mm yr−1 for the 20th century sea-level rise based on tide-gauge data.”

Unnikrishnan and Shankar collected tide gauge data for a variety of stations located at coastal locations around the Indian Ocean (see Figure 1). They conducted a series of tests for inter-station consistency and they also adjusted the sea level measurements for vertical land movements. At the end of the day, they found that the corrected sea level rise in the region over the past five decades was indeed between 1–2 mm yr−1. However, some of the trends were suspect, so they reduced the number of stations for conducting the analyses. They state “In conclusion, therefore, we use the estimated trends for Aden, Karachi, Mumbai, and Kochi in the Arabian Sea and for Vishakhapatnam in the Bay of Bengal. The sea-level rise estimated from these stations is between 1.06– 1.75 mm yr−1, with an average of 1.29 mm yr−1. Given the problems noted above with some of the records, the average estimate for the basin is likely to be towards the lower end of this range.” When compared to global records, they write “The present study indicates that the estimates for the north Indian Ocean are consistent with global estimates, though somewhat lower.” Imagine that — once someone collects data in their part of the world, they seem to conclude that sea level is rising at a rate slower than the rate reported by the IPCC.


Figure 1. Location of tide gauges with records longer than 20 years in the north Indian Ocean (from Unnikrishnan and Shankar, 2007).

The second article also appears in Global and Planetary Change and was prepared by a team of French oceanographers. As their title suggests, Berge-Nguyen et al. collected thermosteric sea level data based on temperatures in the top 700 meters of the ocean, tide gauge, satellite altimetry, and ocean reanalysis data. They used a series of sophisticated multivariate statistical techniques and ultimately produced the graphic below (Figure 2). Like you, we look at this graphic and see a global increase in sea level of approximately 80 mm over the 54-year time period. The math is simple – the graph shows a rise of 1.48 mm yr−1.


Figure 2. Global sea level curves over the period 1955-2003 (from Berge-Nguyen et al., 2008)

This team from France collected a variety of global sea level datasets, they conducted a complicated set of analyses, and they produced their best-guess of global sea level for each year from 1955 to 2003. We see a rise in sea level that is below the estimate of the IPCC and we see no acceleration through the past five decades. Basically, nothing seems to be happening with sea level that is remotely out of the ordinary. IPCC certainly seems to be exaggerating the best estimate of sea level rise, and it make us wonder what else they might be exaggerating.

References:

Berge-Nguyen, M., A. Cazenave, A. Lombard, W. Llovel, J. Viarre, and J.F. Cretaux. 2008. Reconstruction of past decades sea level using thermosteric sea level, tide gauge, satellite altimetry and ocean reanalysis data. Global and Planetary Change, 62, 1–13.

Unnikrishnan, A.S., and D. Shankar. 2007. Are sea-level-rise trends along the coasts of the north Indian Ocean consistent with global estimates? Global and Planetary Change, 57, 301–307.




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