Volume 6, Number 17: 23 April 2003
In our Editorial of 2 April 2003, we challenged the contention of Alley et al. (2003) that "human forcing of climate change is increasing the probability of large, abrupt events," basing our case primarily on real-world observations that clearly suggest otherwise, but additionally citing the computer modeling study of Rind et al. (2001) that also suggests otherwise. Here, we review still more modeling and observational studies that challenge Alley et al.'s contention.
To recapitulate Alley et al.'s position on this subject, they write in their U.S. National Research Council report (Alley et al., 2002) of "large, abrupt climate changes" of "as much as 10°C change in 10 years," stating that these changes "can occur when gradual causes push the earth system across a threshold [our italics]." They further contend that "human activities could trigger abrupt climate change," stating that "warming and the associated changes in the hydrological cycle constitute a threshold [our italics] for the THC [thermohaline circulation]" of the world's oceans. "Once reduced, the THC is more susceptible to perturbations," they claim," also contending that "very close to a threshold [our italics], the evolution of the THC loses predictability altogether."
The THC threshold that has been most discussed within this context is the amount of freshwater delivered to the North Atlantic Ocean (supplied by increased Arctic river discharge resulting from the intensified hydrologic cycle presumed to accompany CO2-induced global warming) plus meltwater from Greenland (presumed to result directly from rising temperatures) that has the presumed power to slow, or even stop, North Atlantic Deep Water (NADW) formation, which is believed by many to be one of the main driving forces of the THC that redistributes heat around the world and is thought to bring considerable warmth to Europe. If this "threshold hypothesis" and the more basic THC concept are correct, some as-yet-unspecified degree of global warming could conceivably induce a rapid regional or hemispheric cooling, which according to Alley et al. may have the potential to plunge much of the world into a global deep freeze ten times worse than the Little Ice Age that preceded the Modern Warm Period.
In a significant theoretical refutation of this tortured thinking, Rind et al. (2001) conducted several computer-model sensitivity analyses of this scenario, concluding that (1) NADW formation "decreases linearly with the volume of fresh water added," (2) the decrease occurs "without any obvious threshold effects," and (3) "the effect is not rapid," all of which findings fly in the face of Alley et al.'s prognostications.
The newest modeling study of relevance to Alley et al.'s human-induced abrupt climate change threshold hypothesis is that of Seidov and Haupt (2003), who begin their analysis by noting that the "asymmetry of the Atlantic and Pacific sea surface salinity (SSS) is recognized as an important element of the global ocean thermohaline circulation." Nevertheless, they report that this aspect of the THC has received little attention from the climate-science research community. Therefore, building upon their own earlier work on the subject (Seidov and Haupt, 2002), which focuses on the role of Atlantic and Pacific sea surface salinity, they performed a number of sensitivity experiments with the ocean model of the Geophysical Fluid Dynamics Laboratory. These experiments showed, in their words, that "Atlantic-Pacific SSS asymmetry is one of the most critical elements for maintaining the global ocean conveyor," and, hence, that "high-latitudinal freshwater impacts, as a mechanism of altering global thermohaline circulation [i.e., the hypothesis of Alley et al.], may be less effective than inter-basin freshwater communications."
Other studies conclude much the same thing, but they arrive at this conclusion from a very different conceptual direction. Munk and Wunsch (1998), Wunsch (2000) and Wunsch (2002), for example, all conclude, on the basis of fundamental theoretical considerations, that the THC is sustained primarily by the work of the wind and secondarily by tidal forcing. So basic are these considerations, in fact, that Wunsch (2000) categorically states that "there cannot be a primarily convectively driven circulation of any significance."
In further explaining this fact, Wunsch (2002) notes that "both in models and the real ocean, surface buoyancy boundary conditions strongly influence the transport of heat and salt," acknowledging the matters upon which Alley et al. and Seidov and Haupt have focused their attention, but he emphasizes that "these boundary conditions do not actually drive the circulation," noting again that "for past or future climates, the quantity of first-order importance is the nature of the wind field."
Although Munk and Wunsch's thoughts may seem a bit esoteric, the energy requirements of their more basic view of the subject have been observationally verified by the work of Egbert and Ray (2000), while other supporting evidence has been supplied by Berger and von Rad (2002). Hence, in view of the fact that the mass flux of the THC is primarily a creature of wind and tide, there is no valid reason to even entertain the climate-alarmist speculations that have been spawned by the reviews of Alley et al., much less succumb to the cacophony of highly irrational calls to forsake the energy source that has produced the marvels of the modern world.
| Sherwood, Keith and Craig Idso |
References
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