PERSPECTIVES CLIMATE CHANGE What caused atmospheric westerly winds Shifting Westerlies to shift after the last glacial period? J. R. Toggweiler he westerlies are the prevailing ica-rich deep water must be drawn to the sur- winds in the middle latitudes of face. As mentioned above, silica-rich deep Earth’s atmosphere, blowing water tends to be high in CO . It is also T 2 from west to east between the high- warmer than the near-freezing surface pressure areas of the subtropics and waters around Antarctica. the low-pressure areas over the poles. A shift of the westerlies that draws They have strengthened and shifted more warm, silica-rich deep water to poleward over the past 50 years, pos- the surface is thus a simple way to sibly in response to warming from explain the CO2 steps, the silica pulses, rising concentrations of atmospheric and the fact that Antarctica warmed carbon dioxide (CO2) (1–4). Some- along with higher CO2 during the two thing similar appears to have happened steps. Anderson et al.’s two silica pulses 17,000 years ago at the end of the last ice occur right along with the two CO2 steps, age: Earth warmed, atmospheric CO2 in- which implies that the westerlies shifted creased, and the Southern Hemisphere wester- early as the level of CO2 in the atmosphere lies seem to have shifted toward Antarctica began to rise. Had the westerlies shifted in (5, 6). Data reported by Anderson et al. on response to higher CO , one would expect to Southward movement. At the end of the last ice 2 page 1443 of this issue (7) suggest that the see more upwelling and more silica accumula- on October 23, 2009 age, the ITCZ and the Southern Hemisphere wester- shift 17,000 years ago occurred before the lies winds moved southward in response to a flatter tion after the second CO2 step when the level of warming and that it caused the CO2 increase. temperature contrast between the hemispheres (5, CO2 is highest, but instead the silica accumula- The CO2 that appeared in the atmosphere 6, 11). According to Anderson et al., the northern tion drops back down. 17,000 years ago came from the oceans rather westerlies may have also shifted to the south; this What made the westerlies shift when they than from anthropogenic emissions. It was shift is not depicted in the figure. did? The answer seems obvious empirically vented from the deep ocean up to the atmo- but may be difficult to understand theoreti- sphere in the vicinity of Antarctica. The south- the ACC to the surface around Antarctica. Over cally. The Northern Hemisphere is systemati- ern westerlies are important in this context the past 50 years, the westerlies have shifted cally warmer than the Southern Hemisphere, because they can alter the oceanic circulation southward so that they are better aligned with especially near the Atlantic Ocean, where the www.sciencemag.org in a way that vents CO2 from the ocean inte- the ACC and draw more mid-depth water to the overturning circulation transports heat across rior up to the atmosphere. The prevailing view surface than they did before (8, 9). At the peak the equator from south to north. As a result, has been that the westerlies shifted 17,000 of the last ice age, the opposite situation pre- Earth’s thermal equator—the Intertropical years ago as part of a feedback: A small CO2 vailed: The westerlies were so far north of Convergence Zone (ITCZ)—is north of the increase or small warming initiated a shift of today’s position that they were no longer equator. The easterly trade winds that flank the the westerlies toward Antarctica; the shifted aligned with the ACC and could not draw much ITCZ to the north and south are also skewed westerlies then caused more CO2 to be vented mid-depth water to the surface. toward the Northern Hemisphere. Downloaded from up to the atmosphere, which led to more The mid-depth water upwelled by the Anderson et al.’s two pulses of sediment warming, a greater poleward shift of the west- westerlies is rich in CO2 and in silica, a nutri- accumulation took place along with Heinrich erlies, more CO2, and still more warming (5). ent that fuels biological production in the sur- Event 1 and the Younger Dryas—events in But Anderson et al. show that the westerlies face waters around Antarctica. Siliceous which icebergs and melting glaciers flooded did not shift in response to an initial CO2 remains of the organisms settle to the sea floor the North Atlantic with fresh water, thereby increase; rather, they shifted early in the cli- and accumulate in the sediments. Anderson et weakening the overturning. The weakened mate transition and were probably the main al. show that the accumulation of siliceous overturning cooled the Northern Hemisphere cause of the initial CO2 increase. sediment increased dramatically during the and warmed the Southern Hemisphere, The strongest southern westerlies are found transition out of the last ice age. They attribute thus reducing the temperature asymmetry. several hundred kilometers to the north of a this increase to a poleward shift of the wester- Sediment records from the southern Caribbean broad oceanic channel that circles the globe lies that drew more CO2- and silica-rich water Sea show that the trade winds shifted to the around Antarctica. The stress from the wester- up to the surface. south during the two pulses (11, 12). Thus, the lies on the ocean drives the Antarctic Circum- A detailed analysis of the ice-core records ITCZ shifted closer to the equator and the polar Current (ACC) through the channel. This from Antarctica shows that atmospheric CO2 southern westerlies apparently shifted toward stress also draws mid-depth water from north of concentrations rose in two steps along with the Antarctica along with the southward move- air temperatures over Antarctica (10). The silica ment of the trade winds (see the figure). accumulation in Anderson et al.’s best resolved The sediment accumulation rate during Geophysical Fluid Dynamics Laboratory/National Oceanic record also shows two pulses that correspond in Anderson et al.’s two pulses was five times as and Atmospheric Administration, Princeton University, Princeton, NJ 08540, USA. E-mail: robbie.toggweiler@ time to the two steps (7). To create such a pulse high as it was at the Last Glacial Maximum noaa.gov in silica accumulation, larger quantities of sil- (just before the two pulses), and twice as high J. TOGGWEILER/CARNEGIE MELLON UNIVERSITY CREDIT: 1434 13 MARCH 2009 VOL 323 SCIENCE www.sciencemag.org Published by AAAS PERSPECTIVES as it is today. This points to massive changes in in the Southern Hemisphere than in the References 1. J. W. Hurrell, H. van Loon, Tellus Ser. A 46, 325 (1994). the wind-driven upwelling around Antarctica north (3). The results of Anderson et al. (7) 2. G. J. McCabe et al., J. Climate 14, 2763 (2001). during the transition out of the last ice age and suggest that in the past, the westerlies 3. N. P. Gillett et al., Nature 422, 292 (2003). suggests that the westerlies were closer to (or shifted asymmetrically toward the south in 4. D. T. Shindell, G. A. Schmidt, Geophys. Res. Lett. 31, L18209 (2004). stronger next to) Antarctica during the transi- response to a flatter temperature contrast 5. J. R. Toggweiler, J. L. Russell, S. R. Carson, tion than they are now. between the hemispheres. The magnitude of Paleoceanography 21, PA 2005 (2006). Climate scientists have attributed changes the shift seems to have been very large. If 6. F. Lamy et al., Earth Planet. Sci. Lett. 259, 400 (2007). 7. R. F. Anderson et al., Science 323, 1443 (2009). in the westerlies over the past 50 years to the there was a response to higher CO2 back 6. O. A. Saenko et al., Climate Dyn. 25, 415 (2005). 9. J. L. Russell et al., J. Climate 19, 6382 (2006). warming from higher CO2. The changes pre- then, it paled in comparison. Changes in the dicted by climate models in response to north-south temperature contrast today are 10. E. Monnin et al., Science 291, 112 (2001). 11. G. H. Haug et al., Science 293, 1304 (2001). higher CO2 are fairly small, however, and not going to be as large as they were at the 12. D. W. Lea, D. K. Pak, L. C. Peterson, K. A. Hughen, tend to be symmetric with respect to the end of the last ice age, but even small Science 301, 1361 (2003). equator. The observed changes have been changes could be an additional source of quite asymmetric, with much larger changes modern climate variability. 10.1126/science.1169823 CHEMISTRY Inducing Chirality with Circularly An unexpected difference in electron transfer rates for right- and left-handed versions of a Polarized Light molecule is caused by quantum interference. Robert J. Cave on October 23, 2009 e have an intuitive understanding the rate will be faster the more the donor and bridge (the solid line). In this tight-binding of how the shape and symmetry of acceptor states delocalize onto the bridge low- model, reversing the helicity of the bridge Wobjects affects their use from our ering the effective mass of the electron and while preserving the bonding strength and rel- hands. Each hand is chiral: Its mirror image leading to more efficient tunneling. ative energetics would yield equivalent elec- (the right versus the left hand) is different This delocalization is enhanced when tron transfer rates through the two possible from it. When we extend right hands, they can bonding is strong between the donor, accep- bridge helicities.