Re: naturalSCIENCE Cover Story of November 1, 1997
From: S. Fred Singer
Date: Tue, 17 Mar 1998 15:59:44 -0500
Reply To: firstname.lastname@example.org
Subject: A New European Ice Age?
A New European Ice Age?
Your cover story of November 1 describes a "new European ice age," based on the idea that greenhouse warming will increase precipitation in the North Atlantic to such an extent as to disrupt the thermohaline circulation. Thomas Stocker and Andreas Schmittner ("Influence of carbon dioxide emission rates on the stability of the thermohaline circulation," Nature, 388:862--865, 1997) have done an instructive calculation based on a rather simple ocean-atmosphere model. They indicate that sudden irreversible changes in circulation are possible in response to a rapid rise in greenhouse gases.
But the important factor here is not the rise in carbon dioxide emission rate, as their title suggests, but the rate of rise in temperature. If one compares their Figure 2 with their Figure 3, the critical rate of rise seems to be 0.3 °C per decade. An obvious way to validate the model results is to search for events in the geological record that show similar rapid temperature increases. Changes of as much as 0.3 °C per decade have indeed been measured in ocean sediment cores, where the resolution was good enough to resolve rapid variations (Keigwin, L.D., Science 274:1504-1508, 1996). Some events even occurred as recently as 2500 to 3000 years ago, i.e., within historic times, but seem not to have caused irreversible changes in climate. Nevertheless, it would be important to search written and other records for evidence of major, long-lasting climate disturbances.
It might even be worthwhile to search for other mechanisms that could upset the North Atlantic thermohaline circulation. Rising salt concentrations in the Mediterranean could pose such a threat. A middle layer of water formed south of Greece, called the Levantine Intermediate Water, spreads uniformly throughout the Mediterranean and forms around 80% of the water that exits through the lower levels at the Straits of Gibraltar. As reported by Nigel Williams in Science (279:483-484, 1998), Robert Johnson of the University of Minnesota believes that the saltier Mediterranean water might deflect the Gulf Stream westward toward the Labrador Sea, drastically cooling northern Europe. Eelco Rohling of the University of Southampton in the U.K. thinks the Mediterranean water could have the opposite effect, pushing the Gulf Stream further toward Europe and turning up the heat there.
Added to all of this uncertainty are the human activities that can affect salinity. As pointed out over a decade ago by Hugh Ellsaesser and colleagues (Rev. Geophys., 24:745-792, 1986), the construction of the Aswan Dam and other irrigation projects around the Mediterranean are reducing the influx of fresh water and contributing to the rise in salinity. Johnson ("Climate Control Requires a Dam at the Straits of Gibraltar," Trans. AGU 78:227--281, 1997) has even suggested a grandiose engineering scheme to counteract the effects.
S. Fred Singer
1 Dr. Singer, an atmospheric physicist, is the president of the Science & Environmental Policy Project based in Fairfax, Virginia. He is emeritus professor of environmental sciences at the University of Virginia and former director of the U.S. Weather Satellite Service.