Island of Doubt blog by James Hrynyshyn on ScienceBlogs.com

There's a good reason why of all the consequences of anthropogenic global warming, nothing garners as much attention as sea level rise — with the possible exception of those darn charismatic polar bears, that is.

It's the same reason Al Gore devoted half a dozen slides in his climate change presentation to animations depicting the flooding that would come with a six- or seven-metre rise. While we can't predict just how much the oceans will rise if the world's glaciers and the Western Antarctic and Greenlandic ice sheets were to melt, everyone knows, without having to take a course in radiative forcing or carbon cycles, that it would be disastrous for hundreds of millions of people who live in low-lying areas.

And it's why amateurs who would rather we don't worry about climate change, like Bjorn Lomborg, also focus on the subject. He and an actual climatologist, Stefan Rahmstorf, are having it out on the pages of the Guardian, for example. It's scary and, no matter what Lomborg thinks, it's a genuine threat. But in a world of finite and ever-shrinking news cycles, is it the best illustration of what climate change really means?

This week the still ridiculously sparse coverage afforded to matters of climate change was consumed by comments on sea level rise coming out of the International Scientific Congress on Climate Change underway in Copenhagen.

"Unless we undertake urgent and significant mitigation actions, the climate could cross a threshold during the 21st century committing the world to a sea level rise of metres", said John Church [of the Centre for Australian Weather and Climate Research, Hobart, Tasmania, Australia].... In the lower end of the spectrum it looks increasingly unlikely that sea level rise will be much less than 50 cm by 2100.

This isn't really news, in the sense that such numbers have been bandied about for months. Back in 2007, when the IPCC released its Fourth Assessment, the official word was we can expect to see a rise of just 18-59 cm by century's end,. This was misinterpreted by dilettante reporters, many of whom probably still had visions of Gore's seven-metres-in-10-seconds flood slides in their heads, to mean that expectations had actually fallen. Of course, even then the IPCC numbers were out of date, since the cut-off for considering new data was more a year previous. By 2008, glaciologists were falling over themselves trying to come up with better prediction ranges based on what has been happening at either pole.

Based on what happened to the Laurentide ice sheet at the end of the last ice 10,000 years ago, a study published in Nature Geoscience suggested that "current projections of GIS melt rates for the coming century may be only minimum estimates, even without considering positive feedbacks from ice-sheet dynamics." The authors came up with a sea level increase of about 1.3 meters, from Greenland melt alone.

Almost simultaneously, the authors of a paper in Science produced a low-end estimate of sea level rise in the same time frame of between 0.8 and 2.0 metres. So Church's numbers in Copenhagen hint at a firming agreement that we're looking around of a metre or so by the end of the century. Which would be, as they say, disastrous. The environmental refugee problem will be beyond anything in human experience. But it won't happen overnight. It will take decades even if we trigger some climate tipping points. And it will be possible to adapt.

There are more worrisome consequences than sea level rise. Acidification of the oceans will eventually cause the collapse of every fishery on the planet, leaving a good portion of humanity without a source of protein. Nature Geoscience just this past week published findings that suggest things are already going to hell in a handbasket for foraminifera, single-celled organisms "that represent between 25 and 50% of the total open-ocean marine carbonate flux and influence the transport of organic carbon to the ocean interior."

We find that modern shell weights are 30-35% lower than those from the sediments, consistent with reduced calcification today induced by ocean acidification.

What that means is, if we lose the forams, entire ecosystems could collapse and with them the ability of the oceans to sequester a lot of the carbon that we're still spewing out.

And then there's loss of agricultural production. New Scientist's Gaia Vince recently laid out a vision for what might happen should the Earth warm by 4° C, which is toward the lower end of business-as-usual forecasts for 2100 and possible, though not as likely, by 2050.

India, Bangladesh and Pakistan, for example, will feel the force of a shorter but fiercer Asian monsoon, which will probably cause even more devastating floods than the area suffers now. Yet because the land will be hotter, this water will evaporate faster, leaving drought across Asia. Bangladesh stands to lose a third of its land area - including its main bread basket.

The African monsoon, although less well understood, is expected to become more intense, possibly leading to a greening of the semi-arid Sahel region, which stretches across the continent south of the Sahara desert. Other models, however, predict a worsening of drought all over Africa. A lack of fresh water will be felt elsewhere in the world, too, with warmer temperatures reducing soil moisture across China, the south-west U.S., Central America, most of South America and Australia. All of the world's major deserts are predicted to expand, with the Sahara reaching right into central Europe.

Glacial retreat will dry Europe's rivers from the Danube to the Rhine, with similar effects in mountainous regions including the Peruvian Andes, and the Himalayan and Karakoram ranges, which as result will no longer supply water to Afghanistan, Pakistan, China, Bhutan, India and Vietnam.

Along with the exhaustion of aquifers, all this will lead to two latitudinal dry belts where human habitation will be impossible, say Syukuro Manabe of Tokyo University, Japan, and his colleagues. One will stretch across Central America, southern Europe and north Africa, south Asia and Japan; while the other will cover Madagascar, southern Africa, the Pacific Islands, and most of Australia and Chile (Climatic Change, Vol. 64, p. 59).

It gets bleaker from there. At best, the only tracts of land still capable of growing significant quantities of food will be in northern Canada and Siberia. The need to evacuate huge portions of the tropical regions and resettle the affected populations in milder regions will effectively put an end to the modern concept of the nation state.

As with all predictions, Vince's story represents just one of a wide range of possible scenarios. But it's not a worst-case scenario. I didn't even excerpt the quotes from James Lovelock. My point is, we've plenty of things to worry about that will prove much harder to adjust to, without getting tied up in knots over exactly how much the oceans will rise. In many ways, the relatively slow pace of sea level rise might be among the least of our worries.

Link to blog post: http://scienceblogs.com/islandofdoubt/2009/03/sea_level_rise_a_red_herring.php