July 22, 2009

Sea Level Rise: An Update Shows a Slowdown

Filed under: Sea Level Rise

Of all the potential woes bandied about with regards to “global warming,” the only one which really is in uncharted territory is a large and rapid rise in sea level. Otherwise we are rather routinely exposed to all nature of weather extremes as they are a part of the natural climate. Droughts, floods, heat waves, tornadoes, hurricanes, etc. have impacted human societies in the past and they will continue to do so into the future—with or without “global warming.”

The degree to which we are affected by extreme weather depends on a number of factors, especially preparedness.

One of the driving forces motivating us to “be prepared” (aside from our scout leader), is the frequency with which extreme weather occurs. Generally, the more frequent something negative occurs, the more we act to prepare ourselves. So, in this sense, if weather extremes do be more common in the future, we’ll more than likely quickly become better prepared to deal with them—lessening their negative impacts and probably boosting the economy along the way as we build the necessary precautions into our way of life (consider the improved building codes in Florida after Hurricane Andrew, or the response in Chicago (and France) in the aftermath of killer heat waves). In truth, the impact of climate change is very low compared to the impact of climate itself.

However, one exception to this could be sea level rise. As an organized society, we have pretty much experienced the global ocean level being pretty close (within a couple of feet) to where they are now. True, the global sea level has been creeping upwards for the past 10,000 years (after the big jump up at the end of last ice age) and true, we have had to make some adjustments to a few of our particularly vulnerable coastal cities, but by and large the sea level hasn’t been rising at a rate so great as to cause large societal disruptions or reorganizations.

America’s Cassandra, NASA astrophysicist James Hansen (among others), continues to make alarming statements that this about to change if we don’t immediately take enormous measures to reduce our emissions of carbon dioxide (and other greenhouse gases) that result from human activities (primary among them being energy production from fossil fuels). But, no matter what convoluted plans that Congress, the EPA, or other international congregations eventually come up with to try to limit and reduce our CO2 emissions, the overall impact of “global warming” will be small and the earth’s temperature will continue to rise over the long term (at least over the course of the coming century). Consequently, sea levels will be driven upwards and we’ll have to increasingly turn our attention to dealing with them. So the big questions are “How much?” and “How fast?”

First, rest assured that sea level rise this century will not come anywhere close to the extreme projections (10-20 feet) hawked by Hansen. There is simply no empirical evidence or model forecasts that support such claims. In fact, the more folks study the issue, the more support builds for modest sea level rise—something on the order or a foot or two by century’s end—an amount ten times less than alarmist forecasts.

Here’s evidence.

In 2007, Simon Holgate of the U.K.’s Proudman Oceanographic Laboratory produced a history of global sea levels rise from 1904 to 2003 based upon a set of reliable, long-term observations from 9 tide gauge stations scattered around the world. The overall average rate of sea level rise in Holgate’s study period was found to be 1.74 ± 0.16mm/yr (about 0.07 in/yr, or 7 inches per century). In addition, he made two other notable findings, 1) the rate of sea level rise was, on average, greater in the first half of his record than the second, and 2) that there is a large degree of decadal variability in the rate of sea level rise. Figure 1 illustrates the latter of these findings. The blue curve is the rate of sea level rise during overlapping 10-yr periods as derived from his 9-station long-term record, and the magenta curve is the same quantity derived from a more globally complete 177 tide gauge network covering the period 1948-2002. Notice that the decadal rate of sea level rise fluctuates semi-regularly from rates exceeding 4 mm/yr to rates that are sometimes even less than zero (i.e. falling sea levels).


Figure 1. Decadal rate of global sea level rise as determined from a 9-station tide gauge network for the period 1904-2003 (blue curve) and from a 177-station tide gauge network for the period 1948-2002 (magenta curve) (Holgate, 2007).

Now keep in mind that for that for the sea level to rise 10 feet above current levels by the year 2100, it needs to get going. Across the remaining 91 years between now and then, the rise in global sea level needs to average about 1.32 inches per year (33.5mm/yr)—about 20 times the average rate of rise during the 20th century. Even to get a three-foot rise by 2100 would require the average rate of sea level rise to be about 0.4 inches/yr (10 mm/yr)—about 5-6 times that 20th century average.

Are there any signs that sea level rise is speeding up?

Hardly.

There are, of course, claims that the rate of sea level rise is accelerating. These largely come about because sea level rise estimates made by satellites (over the past decade and a half) average about 3.1 mm/yr (0.12 in/yr) (Figure 2) compared with sea level rise estimates derived from tide gauges which average only about 1.8 mm/yr (0.07 in/yr). However, as mentioned, satellites have only been measuring sea level since about 1993, while the tide gauge history, as shown by Holgate, can be extended back to at least the early 20th century. Therefore, comparing the short-term trend from satellites to a long-term trend from tide gauges is hardly a robust comparison given the large degree of short-term variability as shown by Holgate (Figure1).


Figure 2. Sea level history as recorded by satellite-borne instrumentation, 1993-present (source: http://sealevel.colorado.edu/).

The veracity of this satellite/tide-gauge comparison, or rather lack thereof, has not been lost on everybody. The IPCC (AR4, p. 5) cautioned:

The global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961-2003. 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.

Well, the very latest sea level rise data is helping to make things less “unclear.”

Notice that the in the past couple of years (the righthand portion of Figure 2) the rate of sea level rise has apparently slowed a bit. So it would be interesting to place the changes in the rate of sea level rise from the satellite data in the context of the historical changes in the rate of sea level rise derived by Holgate (in Figure 1).

Voila, Figure 3.


Figure 3. Decadal rate of sea level rise from satellites (red curve) appended to the decadal rate of global sea level rise as determined from a 9-station tide gauge network for the period 1904-2003 (blue curve) and from a 177-station tide gauge network for the period 1948-2002 (adapted from Holgate, 2007).

We calculate the running 10-year trends in sea level as observed from satellites, and append it to the running 10-yr trends in sea level derived by Holgate from the tide gauge network. In this context, the satellite trends (red curve in Figure 3) don’t look unusual at all—they seem to fit squarely into the pattern of long-term fluctuations. And further more, they have been declining!

So rather than evidence of accelerating sea level rise in recent years, what we have is nothing more than the same type of variation that has been going on for at least 100 years. It was merely a coincidence that the satellites began observing the sea level rise during a natural upswing in the rate of sea level rise, that has now turned into a downswing—a behavior that has repeated itself a good half-dozen times during the past century.

So, there is now good reason to believe that the IPCC’s “faster rate [of sea level rise] for 1993 to 2003” better “reflects decadal variability” rather than much of “an increase in the longer-term behavior.” Hopefully this fact will find its way into the next IPCC report.

And for global warming alarmists, a declining rate of sea level rise certainly doesn’t do much for “cause.” Couple the slowing rate of sea level rise with the slowing rate of temperature rise and you should have a slowing rate of public concern. An inconvenient situation as Congress considers “global warming” legislation that will certainly raise energy process while uncertainly mitigate climate change—something that natural forces seem to be doing already, and for free.

References:

Holgate, S. J., 2007. On the decadal rates of sea level change during the twentieth century, Geophysical Research Letters, 34, L01602, doi:10.1029/2006GL028492.

IPCC, 2007. Climate Change 2007: The Physical Basis. Cambridge University Press, Cambridge, U.K., pp. 996.




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