Monday, April 30, 2012

The Gravity of Sea Level Rise

Did you know that "sea level" really isn't level at all? If you were sitting in a sailboat off the Dingle Peninsula in western Ireland, you would be about 160 meters "higher" than if you were in that same sailboat around Sri Lanka. In addition to steric factors (temperature, salinity) and meteorological factors (wind, barometric pressure), the global distribution of sea water has a lot to do with gravity, which is not uniform across the globe. Gravity is stronger around Ireland, for example, than around Sri Lanka, and that leads to more water being attracted to the area around Ireland; hence, the "higher sea level" there.

One of the points I hope to impress upon readers is that, whatever you may believe about carbon dioxide levels and global warming, prediction of any resulting sea level change is very tricky stuff. Many factors are involved, not the least of which is gravity. And gravity itself can be enigmatic. There are natural spatial variations in gravity across the globe, and even the ice caps exert their own individually distinguishable gravitational pull on the surrounding sea. In other words, right now sea level is higher around Greenland and Antarctica than it would be if there were no ice caps there. The gravitational pull of the ice caps "piles up" sea water next to them. Melt the ice caps, and their gravitational influence melts along with it. The sea level around Greenland and Antarctica would likely fall, even before considering isostatic rebound.

According to the GOCE readings, gravity in our part of the world is somewhat lower than average, suggesting we won't be strongly attracting gravitationally redistributed sea water resulting from melting ice caps. In fact, analysis of the absolute gravity stations from our area suggest sea level is rising very slowly here, much less than suggested by GPS and tide gauges.

Global and local gravity variations, steric factors, meteorological factors, isostasy, and tectonic movements, to name just a few, all serve to influence the global distribution of sea water (and sea level). There can't be one "sea level rise forecast" for the globe any more than there can be one weather forecast for the globe. And our ability to predict the weather is almost certainly better than our ability to predict sea level rise.

When you hear "experts" make sea level predictions for 50 or 100 years from now, imagine they are forecasting the weather for a particular afternoon that far into the future. What will sea level be around here 100 years from now? You might as well ask if we'll need an umbrella for May 1, 2112.

GOCE geoid showing areas of stronger gravity (red) and weaker gravity (blue)

1 comment:

  1. This just in from Ecology--we'll all be shoreline property owners soon.