Rising Seas On Tap for the West Coast?
The late House Speaker Tip O'Neill once observed famously that “all politics is local.” The same, it turns out, goes for sea level. Overall, the world's oceans have been rising over the past century — at about 2 millimeters (mm) per year until the 1990's, and at about 3 mm per year thereafter — mostly as a consequence of global warming.
But that's just the global average. If you live by the ocean, the increase you've actually seen could be more or less than that. In some places, the land happens to be sinking while the water rises, exaggerating sea-level rise. In others, the rise is masked because the land is rising too — due, for example, to what's known as post-glacial rebound, which began when the massive ice sheets of the last Ice Age melted away thousands of years ago, and which is still going on.
Ocean currents can affect local sea level too, and that's been the case along the West Coast for the past 30 years or so. “It's been a well-kept secret,” says Peter Bromirski of the Scripps Institution of Oceanography, part of the University of California, San Diego.
But as Bromirski and three colleagues report in the Journal of Geophysical Research-Oceans, the West's sea-level holiday may be about to end, because of shifting prevailing winds. Since the 1970s, when sea-level rise along the West Coast slowed to a stop, the winds helped drive ocean currents that brought cold water from the depths up to the surface in that part of the Pacific. Cold water is denser than warm water so it takes up less space, and masks any overall rise in sea level.
Those winds appear to be changing, though, possibly indicating a shift in the natural, decades-long climate fluctuation known as the Pacific Decadal Oscillation, or PDO (the PDO has been invoked by a small handful of scientists as the main cause of global warming, but the vast majority of climate experts are confident that greenhouse gases are the culprit).
If the PDO really is entering a new regime, ocean currents are likely to shift, warming the waters off California, Oregon and Washington and restarting sea-level rise up and down the coast.
Even then, however, the rise in sea level won't necessarily be uniform, because the land is rising in some places and falling in others. It's not always glacier-related, either. In the Los Angeles basin, says Bromirski, where a lot of oil has been pumped out of the ground, the land is literally deflating, so the sea-level rise could be exaggerated. North of Cape Mendocino, California, by contrast, he explains that the Gorda tectonic plate is sliding underneath the American Plate, forcing it, and the coast along with it, upward. Even if sea level rises, its effect may not be as dramatic here.
In short, sea level is indeed rising, the rate has accelerated in recent decades, and the acceleration is likely to continue.
Results, however, may vary.