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Most of the news we hear today about sea level concerns the rise which we expect will accompany global warming of planet earth.  This is actually a two-pronged effect:  On a warming planet, sea level would rise as a result of melting ice at higher latitudes, but would also rise simply because a column of warm water sits slightly higher than the same column of cold water (This is called “steric height” and results from thermal expansion and contraction). On a global basis sea level is rising at a current rate of 3 mm per year (Figure 1). Recent analyses of some exciting long-term data sets yield consistent and startling results: 1) The rate of change of sea level is not at all uniform across the globe; and 2) that sea level off the central coast of California has actually been falling since 1997. 

The first clear evidence comes from space-borne satellite altimeters, which can observe sea level changes over the entire globe.  The satellites do indeed show sea level rising globally (Figure 1).


Figure 1. Mean global sea level as measured from space from 1992 to the present. The mean rate of increase in ~3 mm per year (from the AVISO altimetry product).

However, in some areas including the central California coast, it is actually falling (Figure 2). 


Figure 2. Trends in sea level as measured from space with yellow and red increasing and blue decreasing. Note the extensive blue region of California. (From the AVISO altimetry product)

Can we believe this satellite? Observations on the ground suggest that indeed we can. Using a new analysis technique called singular spectrum analysis, researchers at the Moss Landing Marine Laboratory took a second look at the sea level record at San Francisco, which at 153 years is the longest in-water record of coastal sea level in the country. If you simply fit a straight line between 1900 and 1980, sea level has risen at a rate of 1.8 mm/year at this location, close to estimates of sea level rise averaged globally. The new analysis however (Figure 3) shows that sea level at San Francisco has actually been falling since 1997. Note that this is not unique: There was also a period in the late 1800s when sea level was decreasing at San Francisco.

Figure 3. Long-term trends in sea level at San Francisco, 1855 to 2008. The lower panel is an expansion from the upper panel as shown in red. (Figure contributed by Larry Breaker, MLML).

So what gives?  These data sets and analyses show that the sea level issue is a lot more complicated than it first appears to be.  We know that the El Niño / Southern Oscillation (ENSO) cycle has a profound impact on coastal sea level, but generally at time scales much shorter than this.  Scientists have recently come to appreciate a second, longer-term cycle in the ocean called the Pacific Decadal Oscillation (PDO).  While not strictly decadal as the name implies, the PDO also causes large-scale shifts in the temperature patterns around the ocean.  The year 1997 corresponds to a major shift towards the cold side of the PDO in the eastern Pacific, which was accompanied by lower coastal sea levels in a pattern very similar to Figure 2.   While more research is needed, this shift in the PDO appears to be the most likely cause of the trends off California at this time.  Importantly, when the PDO shifts back to the warm side, it is reasonable to expect that sea level will rise much faster than the historical mean value of 1.8 mm/yr.  The historical mean also may not persist: The observed rate, which translates to half a foot by the end of the century, is much lower than the United Nations projection (1.9 ft) and several estimates from the academic community (4.6 ft).  This is because these projections call for an accelerated sea level rise due to accelerating ice melt at high latitudes. 

Figure 4. Temperature (A) and chlorophyll (B) anomalies with respect to the long-term mean and dissolved oxygen (C) at buoy M1 in the Monterey Bay. (Figure contributed by Francisco Chavez, MBARI).

Why should people care about this level of complexity?   The consensus opinion among scientists these days is that over the long term, sea level can be expected to rise.  The data presented here however show that the rise is neither uniform across the globe nor constant at any given location.  Coastal planners should be aware that the rate of sea level rise will likely not be constant, and plan accordingly.  We should not be lulled into a false sense of security because sea level off central California is presently falling.  In addition to coastal planning, these changes in the ocean impact the biology as well.  Time series off Monterey, CA since 1988 show that temperature has been cooler and chlorophyll, an indicator of primary productivity, has been higher since 1997 while dissolved oxygen has been decreasing (Figure 4).  Cold water off the California coast is generally good for the food chain. 

Unfortunately, observations such as these are rare.  One of the goals of CeNCOOS, and IOOS in general, is to obtain many more such time series along the coast to help us understand the space/time variability of these processes. New physics-based numerical models will allow forecasting of the trends in these major ocean indicators, to help environmental planners and managers at state and local levels as they proceed to implement steps to mitigate rising sea level based on long-term expectations.

Steve Ramp, CeNCOOS Program Director
Francisco Chavez, MBARI
Larry Breaker, MLML