A Hot Topic

From the archives:

(19 January 2006) Today's issue of Nature features several interesting articles about the effects of global warming. Two are research articles, with one revising estimates of the expected increase in sea level due to global warming and the other demonstrating how certain important marine ecosystems could be vulnerable to changes in ocean currents due to global warming. The journal also contains an editorial and a news feature about the need to monitor ocean currents more closely to better assess the consequences of global warming and to warn us of impending climate shifts.

One of the most obvious effects of global warming will be an increase in sea levels, both from melting snow and ice and the natural tendency of water to expand slightly as it warms up. In "Low sea level rise projections from mountain glaciers and icecaps under global warming," scientists Sarah Raper and Roger Braithwaite provide a more conservative prediction for the expected increase in sea level over the next century. According to the sources used in the paper, current models vary, but the average predicted increase over the next century is 0.387 meters (just over 15 inches). Although about 60% of the increase is expected to come from water expansion, this study focuses specifically on sea level rise due to melting mountain glaciers and icecaps.

Although previous models predict a contribution of 0.106 meters in sea level increase from mountain glaciers and icecaps, this paper cuts that estimate in half. The rationale for this is that the authors in the current study accounted for changing patterns in snowfall, with increased snowfall in some areas expected to slow the melting process. The study is interesting and rigorous, but even if the results are correct, this would only reduce the total expected sea level increase by roughly 10%, and an increase in the sea level of 12 or 13 inches instead of 15 inches would still be very significant.

An ongoing themes in climate change research is that global warming can manifest itself in a variety of ways (see my previous post on this topic). In "Reduced mixing generates oscillations and chaos in the oceanic deep chlorophyll maximum," scientists Jef Huisman, Nga Pham Thi, David Karl, and Ben Sommeijer model variation in the deep cholophyll maximum (DCM), an important layer of plankton around 100 meters below the surface of the ocean, and they find that it is much less stable than originally thought, a finding which has important implications for global warming:

Climate models predict that global warming will increase the stability of the vertical stratification in large parts of the oceans. This will reduce vertical mixing and suppress the upward flux of nutrients, leading to a decline in oceanic primary production. Our model predicts that the same process of reduced vertical mixing may induce oscillations and chaos in the phytoplankton of the DCM, generated by the difference in timescale between the sinking flux of phytoplankton and the upward flux of nutrients. Thus, counter-intuitively, increased stability of the water column due to global warming may destabilize the phytoplankton dynamics in the DCM, with implications for oceanic primary production, species composition and carbon export.

What exactly the broader implications of a change in the DCM would be remain unknown, but such jarring disruptions to natural ecosystems are rarely good.

The current issue of Nature also features an editorial and news piece on the potential for global warming to cause drastic weather and climate changes by shutting down thermohaline circulation, the scenerio that was recently dramatized into the film The Day After Tomorrow. It is thermohaline circulation in the Atlantic Ocean, for example, that carries warm water northward, allowing Europe to enjoy a relatively mild climate for its latitude. Although the editorial is lukewarm to the potential for major climate changes, it calls for governments and scientists to step up efforts to monitor this phenomenon more closely worldwide. The news feature by Quirin Schiermeier, called "Climate change: a sea change" goes into much more detail on the issue.

Other possible effects of a shutdown predicted by models include warming in the tropics, or, rather surprisingly, over Alaska and Antarctica. Rainfall patterns might change, too. A southern shift of the thermal equator--which has accompanied thermohaline circulation shutdowns during ice ages--could lead to monsoon failures, and droughts in Asia and the Sahel region, says Severinghaus, and these effects seem to be independent of sea ice. Such shifts could have severe consequences for poor farmers in many parts of the world, consequences that may be considerably more disruptive than colder winters in affluent northern Europe, says Severinghaus. And, as Schlesinger points out, a weakening or stopping of the thermohaline circulation would reduce the carbon dioxide uptake of the ocean, which would mean a positive feedback on global warming. The oceans currently absorb about a third of the carbon dioxide released from fossil fuels, although the proportion is set to decrease as emissions climb.

Clearly, we do not understand all of the effects global warming could have on our planet, which is a reason to be even more cautious than we already are, not less. Although global warming is a very timely topic and a source of numerous ongoing research projects, this week was particularly interesting. Hopefully the pace of research will continue to increase, giving us a better idea of just what kind of an effect we are having on our environment and how we can change to counteract this.

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