September 10, 2012

Sea Level Acceleration: Not so Fast

Filed under: Sea Level Rise

Sea level rise is a topic that we frequently focus on because of all the gross environmental alterations which may result from anthropogenic greenhouse gas emissions, it is perhaps the only one which could lead to conditions unexperienced by modern societies. A swift (or accelerating) sea level rise sustained for multiple decades and/or centuries would pose challenges for many coastal locations, including major cities around the world—challenges that would have to be met in some manner to avoid inundation of valuable assets. However, as we often point out, observational evidence on the rate of sea level rise is reassuring, because the current rate of sea level rise from global warming lies far beneath the rates associated with catastrophe. While some alarmists project sea level rise of between 1 to 6 meters (3 to 20 feet) by the end of this century, currently sea level is only inching up at a rate of about 20 to 30 centimeters per hundred years (or about 7 to 11 inches of additional rise by the year 2100)—a rate some 3-4 times below the low end of the alarmist spectrum, and a whopping 20 to 30 times beneath the high end.

To get from here to catastrophe surely requires a significant acceleration in sea level. And, because disasters pay scientists handsomely, a lot of people have been looking. Here is how the Intergovernmental Panel on Climate Change (IPCC) in its Fourth Assessment Report summed up its investigation:

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. There is high confidence that the rate of observed sea level rise increased from th3 19th to the 20th century, the total 20th-century rise is estimated to be 0.17 [0.12 to 0.22] m.

Since 2003—the last data assessed by the IPCC—the rate of sea level rise has slowed (Figure 1).

Figure 1. Decadal (overlapping) rates for sea level rise as determined from the satellite sea level rise observations, 1993-2011 (data available from http://sealevel.colorado.edu/).

This observation seems to tip the scale to “decadal variability” rather than an “increase in the longer-term trend” in explaining the 1993 to 2003 behavior.

But there is much more evidence that no anthropogenic global warming-related acceleration of sea level rise is taking place.

A couple of months ago, an important paper was published that examined the changing historical contribution of ground water removal (for human water needs, primarily irrigation) to global sea level. A primary finding was that this non-climate component of sea level rise was both significant and rapidly increasing, currently making up between 15 and 25 percent of the current observed rate of sea level rise. Further, the rate of ground water extraction has been increasing over time, which imparts a slight acceleration to the rate of sea level rise over the past half-century or so. Once this non-climate signal is removed, there remains no evidence for a climate-related acceleration. We covered that finding here.

Another paper has just been accepted in the journal Geophysical Research Letters that identified multidecadal cycles in the historical mean sea level observations from many ocean basins. A research team led by Don Chambers from the University of South Florida examined tide gauge records from across the globe and found oscillations with a period of about 60 years in all ocean basins except the Central/Eastern North Pacific. Chambers et al., note that a 60-yr quasi oscillation has previously been identified in other earth/climate systems including ocean circulation, global mean surface temperatures, large-scale precipitation patterns, and atmospheric pressure, among other things. Many of these cycles can be traced back hundreds of years—an indication of a natural (rather than manmade) origin.

Chambers and colleagues note that given the strong possibility for such cycles in the global sea level data, that care must be taken when attempting to identify accelerations, as they, in fact, simply be upswings in the natural oscillatory behavior. For instance, in most ocean basins, the bottom of the cycle was reached in the 1980s and an upswing has been occurring since then—precisely when the IPCC notes that the rate of sea level rise has been increasing. For this reason, Chambers et al. note:

The 60-year oscillation will, however, change our interpretation of the trends when estimated over periods less than 1-cycle of the oscillation. Although several studies have suggested the recent change in trends of global [e.g., Merrifield et al., 2009] or regional [e.g., Sallenger et al., 2012] sea level rise reflects an acceleration, this must be re-examined in light of a possible 60-year fluctuation. While technically correct that the sea level is accelerating in the sense that recent rates are higher than the long-term rate, there have been previous periods were the rate was decelerating, and the rates along the Northeast U.S. coast have what appears to be a 60-year period [Figure 4 of Sallenger et al., 2012], which is consistent with our observations of sea level variability at New York City and Baltimore. Until we understand whether the multi decadal variations in sea level reflect distinct inflexion points or a 60-year oscillation and whether there is a [Global Mean Sea Level, GMSL] signature, one should be cautious about computations of acceleration in sea level records unless they are longer than two cycles of the oscillation or at least account for the possibility of a 60-year oscillation in their model. This especially applies to interpretation of acceleration in GMSL using only the 20-year record of from satellite altimetry and to evaluations of short records of mean sea level from individual gauges. [emphasis added –eds.]

The bottom line is this: the more people look for the anticipated acceleration in the rate of sea level rise, the less evidence they seem to find in support of it. All the while, we eat into the 21st century with a rate of sea level rise not much different from that experienced during the 20th century—and one which was hardly catastrophic, readily proven by a simple look around.

References:

Chambers, D., M.A. Merrifield, and R. S. Nerem, 2012. Is there a 60-year oscillation in global mean sea level? Geophysical Research Letters, doi:1029/2012GL052885, in press.

Wada, Y., et al., 2012. Past and future contribution of global groundwater depletion to sea-level rise. Geophysical Research Letters, 39, L09402, doi:10.1029/2012GL051230.




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