As our climate warms, one of the more under-recognized consequences for future generations is global sea level rise (GSLR), especially considering that nearly 4 billion people on Earth live within 100 miles of the coast. In this and the following column, I'll try to answer some questions I keep hearing, starting with the obvious ...
How much will sea level rise?
The consensus is between 3 and 6 feet by the year 2100. The US Army Corps of Engineers foresees a GSLR of more than 5 feet. The Intergovernmental Panel on Climate Change (IPCC) predicts more than 3 feet (unrealistically) assuming that contributions from polar ice sheets remain stable. Last January, the National Oceanic and Atmospheric Administration (NOAA) hedged its bets, predicting anywhere between 1 and 8 feet. Meanwhile, many geologists predict a rise between 10 and 30 feet.
Why can't they agree on a number?
Even for the smartest climatologists using the fastest computers in the world, the problem is insanely tough because of so many uncertainties, which include: sketchy sea level data prior to 1993 (when altimetry data from satellites became available); the inconsistency of sea level rise across the globe; and — the biggest elephant in the room — the future of the West Antarctic ice sheet.
But right now the sea level is rising?
Yes. Satellite data shows an average GSLR of about 3 inches over the course of the last 24 years and the trend is accelerating. Using less accurate data from old local records and from core samples, GSLR has risen by about 8 inches since 1880, when we started pumping great volumes of carbon dioxide into the atmosphere.
I thought sea levels varied wildly long before the Industrial Revolution?
Back in the last ice age, which peaked about 18,000 years ago, oceans were almost 400 feet below their present levels. But the oceans have been pretty stable for the last 2,000 years, changing their levels by a mere couple of feet. More than the actual levels, though, it's the unprecedented speed of the current change that is so alarming.
How do rising air temperatures affect the oceans?
As the atmosphere warms, oceans absorb 80-90 percent of the extra heat.
So? Why does this lead to the rise of sea levels?
1. Warm water, being less dense than cold, takes up more volume, thus accounting for between 30 and 50 percent of the current rise (the range demonstrating the uncertainties of the science). The good news is that the warming of water in the ocean depths happens very slowly; the bad news is that it's unstoppable. Even if greenhouse gas emission ceased right now, the surface-to-depth exchange of ocean water would continue for centuries.
2a. Fresh water melting from Earth's two "big ice buckets," the Antarctica and Greenland ice sheets, adds to the volume of the oceans. Two thirds of Earth's fresh water is trapped in Antarctic ice.
2b. Fresh water from mountain glaciers and smaller polar ice caps also adds to the oceans, although to a lesser extent than 2a.
Next time, we'll look at more issues related to rising sea levels, in particular the role of Antarctica and Greenland.