The glacial pace of sea-level rise

“Flow velocities of ocean-ending outlet glaciers would have to be about 49 km/yr, 70 times faster than those glaciers move today” for Greenland to raise sea level 2 m, says Tad Pfeffer about his new research in Science. That’s three times faster than he and his colleagues have ever observed an outlet glacier to move. This doesn’t mean sea level isn’t rising due to glacier melt. Actually, the oceans could rise more and faster than International Panel on Climate Change (IPCC) scientists believed possible.

Image from Free Geography Tools

There is a nice write-up on the latest and greatest thought-exercise on global warming and sea level rise (SLR) at Scientific American online, complete with links to stunning pictures of glacier flows. The basic premise of the story is that Greenland, the world’s largest island, holds enough ice to raise global sea levels by 7 meters. That’s enough to sink parts of Manhattan. Try it yourself. But the thrust of the research is that Greenland’s ice melt won’t sink Manhattan because the glaciers are moving too slowly for a global SLR of more than 2 little ol’ meters. Low range scenarios predict SLR < 1 m by year 2100, including thermal expansion. Global warming denialists may claim that the results of the study downplay the effects of sea level rise, but they do not. One to two meters is a significant rise for low-lying coastal communities. Furthermore, the new estimate is higher than the high-end IPCC estimate of 0.16- 0.60 m SLR by 2100. Coastal communities, beware of this state of denial. Seawalls will not help you. They’ll just drain your municipal budget. Focus instead on outbound highways and bridges.

Now, I am no expert on global warming. I am skeptical of Doomsday scenarios (the world was supposed end 25 years ago). But, I study oceanography, listen to my professors, and choose to defer to experts when possible. Now that so many climate scientists agree that Earth is warming and the ice is melting, the pressing question for coastal communities like mine becomes not “if or if not” but “how, when, and where” will sea level rise?

At its core, the research by Pfeffer et al. (2008) studies the question “what is a glacial pace?”, finding it’s not enough raise global sea level more than 2m. However, the global warming model is complicated. Subarctic glaciers in Alaska, Argentina, Canada and Russia contribute 60 percent of sea level rise from glacial melt. And, sudden movements of large Antarctic ice sheets could, shall we say, overcompensate for Greenland’s glacial pace. If the relatively fresh meltwater is buoyant, it will travel in a wave, with peaks and troughs, in the surface layer across the ocean basin until the energy balance is restored. So, as I see it, “average sea level rise” is less important than wave amplitude. One 7 m wave will flood Manhattan, as illustrated above.

However, if the meltwater is cold and relatively dense, it will sink or “downwell” to become part of the North Atlantic bottom water, and begin to travel along the global conveyor belt of thermohaline circulation. In this scenario, the effect of meltwater would be delayed as bottom water travels the globe to upwellings zones in other parts of the world. But warm water expands, too, and there are feedback mechanisms, so really, it all just makes my head hurt.

To me, the real problem is that the trend in SLR estimates is increasing, and the uncertainty, aka the known unknowns are also increasing. One big question remains. What is the glacial pace where it matters the most?

Reference:

Pfeffer, W.T., J. T. Harper, S. O’Neel. 2008. Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise. Science Vol. 321. no. 5894, pp. 1340 – 1343. DOI: 10.1126/science.1159099