Global warming questions.

Stuart Jordan's excellently comprehensive article on global warming ("The Global Warming Crisis," November/December 2005) nevertheless left me wishing he had been a bit more thorough on one point. He mentioned global warming as a possible benefit to agriculture: "It seems probable that large areas of Russia and Canada would become more productive," but he didn't mention that a temperature rise of just a few degrees would destroy the winter wheat crop in Kansas. For the wheat to grow, there must be frost on the ground after planting.

James Rogers 

Hollywood, California

Concerning Stuart Jordan's article, "The Global Warming Crisis," I heard someone on TV say that there is a thirty-year cycle the ocean conveyer belt goes through which is the primary reason for the recent increase in storms. The speaker said that global warming would add to this effect. Jordan doesn't mention this cycle. Is there anything to it?

Jim Jones 

Colonial Heights, Virginia 

Jordan Replies

James Rogers' point is well taken. The fact that climate change may increase agricultural productivity in some parts of the world is no guarantee it will do so in others. In addition, one thing all climate modelers agree on is that not all parts of the world change in the same way. For example, regional effects are often strong enough to produce cooling in some places when the average global temperature is rising. If, as seems likely, climate change continues, someone's ox is sure to be gored somewhere, and there may be beneficiaries as well. But overall, the odds are against it.

In answer to Jim Jones' question, it has been well established that the main driving force behind the meridional circulation of the North Atlantic conveyor belt, with ultimate worldwide impacts, is the descent of cold high-density saline water into the abyssal ocean at high northern latitudes. If too much fresh water from the observed increase in sea-ice melting reduces the salinity (and thus the density) of this water, the volume of water that descends is reduced and so is the driving force.

Observations show that since the mid 1960s a huge increase in the volume of fresh water has been introduced into the North Atlantic due to sea-ice melting. Nevertheless, the same study reviewing these results claims "Patterns of freshwater accumulation observed in the Nordic Seas suggest a century timescale (italics mine) to reach freshening thresholds critical to that portion of the Atlantic meridional overturning circulation (see the article by Ruth Curry and Cecilie Mauritzen in the June 17, 2005, issue of Science). Yet a seemingly contradictory report by Deflef Quadfasel appears in the December 1, 2005, issue of Nature, arguing that the velocity of water in the conveyor belt, which includes the northward surface movement of the Gulf Stream, has slowed down 30 percent in the past fifty years. Such changes, if real, might well be associated with the current increase in hurricane intensity over, say, the past thirty years. But even this wouldn't necessarily be a cyclic phenomenon.

The best statement science can make today is that we still don't know if a thirty-year cyclic process is at work, but it is almost certain that global warming of the surface ocean layers is at least partly responsible for the recent enhancement of hurricane intensity.

Stuart Jordan

Greenbelt, Maryland