at least in part by the lowering of carbon dioxide during colder times in response to changes in ocean chemistry. We currently live in one of the warmer, or “high,” times of these orbital cycles. Previously, the coolest “low” times brought glaciation to nearly one-third of the modern land area.

Recent examination of high-time-resolution records has shown that much of the climate variability occurred with spacings of one to a few thousand years. Changes within high times have been large, widespread (hemispheric to global, with cold, dry, and windy conditions typically observed together), and rapid (over periods as short as a single year to decades).

The changes have been especially large in the North Atlantic basin. In the modern climate, the warm and salty surface waters of the Gulf Stream heat the overlying atmosphere during winter, becoming dense enough to sink into and flow through the deep ocean before upwelling and returning in a millennium or so. Numerical models and paleoclimatic data agree that changes in this “conveyor belt” circulation can explain at least much of the observed millennial variability, although the reconstructed changes may be more dramatic than those modeled. Sinking can be stopped by North Atlantic freshening associated with increased precipitation or with melting of ice sheets on land and a resulting surge into the ocean. North Atlantic sinking also might be stopped by changes in the tropical ocean or elsewhere.

Of concern to Alley et al. is that some global warming models project North Atlantic freshening and possible collapse of this conveyor circulation, perhaps with attendant large, rapid climate changes. At least one model indicates that slowing the rate of greenhouse gas emissions might stabilize the modern circulation.

Fluorescence Sensing

After a long induction period, say de Silva et al., fluorescent molecular sensors are showing several signs of wide-ranging development.20 The clarification of the underlying photophysics, the discovery of several biocompatible systems, and the demonstration of their usefulness in cellular environments are key indicators. Another sign is that the beneficiaries of the field are multiplying and have come to include medical diagnostics through physiological imaging, biochemical investigations, environmental monitoring, chemical analysis, and aeronautical engineering.

The design of fluorescent molecular sensors for chemical species combines a receptor and a fluorophore for a “catch-and-tell” operation. The receptor module engages exclusively in transactions of chemical species, while the fluorophore is

20  

Frontiers of Science/1998. A. Prasanna de Silva, Jens Eilers, and Gregor Zlokarnik, at <http://www.pnas.org/cgi/content/full/96/15/8336>.



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