NEWS & VIEWS that were imprinted on the seafl oor, occurs when the lake levels are about 90% and triggered the climate cooling event much like traces left behind at a crime of the ice thickness. At that point, the lake 8,200 years ago6. But climate may not only scene. Taken together, these suggest that water is not held back by the dam any longer have been aff ected by the disturbance of the when Lake Agassiz-Ojibway ended in a and may eventually lift the ice dam up North Atlantic ocean. Large ice-dammed catstrophic outburst, the water made its like a giant iceberg: catastrophic fl ooding lakes act as regional climate ‘coolers’ way underneath the damming Laurentide is unavoidable. during summer and thereby reduce ice Ice Sheet to the Tyrell Sea — the prehistoric Th is type of fl ooding scenario is evident melting6, and have been shown to stabilize sea occupying Hudson Bay at this time. from the seafl oor morphology of Hudson their damming ice sheets. Th erefore, even It is not that the morphology of the Bay2. Th e fl ooding lake water from Lake though the fi nal outburst probably triggered Hudson Bay seafl oor was unknown4, but Agassiz-Ojibway interacted with the bottom the cooling event 8,200 years ago, the the multibeam data provide a new level of and generated sandwaves while squeezing disappearance of Lake Agassiz-Ojibway detail. And the inferred subglacial fl ood its way underneath the ice sheet with high may nevertheless have accelerated the fi nal is compatible with one model experiment velocities (Fig, 1). Without the overlying disintegration of the Laurentide Ice Sheet. that suggested a complex multiple fl ooding ice sheet constraining the water passage, More extensive multibeam mapping scenario3. Such a scenario is also supported Lajeunesse and St-Onge point out that it of Hudson Bay similar to those presented by additional sediment core studies2. is diffi cult to create sandwaves over such by Lajeunesse and St-Onge may further But what causes an ice-dammed lake an extensive and relatively fl at area as the constrain the fi nal stampede of Lake to burst underneath a huge ice sheet? As southern Hudson Bay. Th e multibeam images Agassiz-Ojibway and provide refi ned long as the damming ice sheet is thicker show another critical detail: the sandwaves boundary conditions for modelling than the lake is deep, there is little risk of are not cross cut by the characteristic the outburst. Th is, in turn, is the key to an outburst. Cracks in the ice can even curved or arc-shaped features resulting deciphering how the outburst could trigger a become sealed if the pressure from the from scouring of the seafl oor by icebergs. global cooling event. overlaying ice is higher than the pressure Th is scour zone is mapped northward of the from the water in the dammed glacial lake3. sandwaves (Fig. 1), suggesting that it was the References 1. Lajeunesse, P. & St-Onge, G. Nature Geosci. 1, 184–188 (2008). However, as the climate becomes warmer, ice sheet margin distal to the dammed Lake 2. Upham, W. US Geol. Surv. Monograph 25, 1–658 (1895). the ice sheet shrinks and the lake expands. Agassiz that collapsed and disintegrated fi rst 3. Clarke, G. K. C., Leverington, D. W., Teller, J. T. & Dyke, A. S. Th e self-stabilization mechanisms fail as in response to the fl ooding. Quaternary Sci. Rev. 23, 389–407 (2004). 92, water pressure approaches the pressure Th e staggering amount of cold 4. Josenhans, H. W. & Zevenhuizen, J. Mar. Geol. 1–26 (1990). 5. Teller, J. T., Leverington, D. W. & Mann, J. D. Quaternary Sci. of the damming ice sheet. Lajeunesse and freshwater bursting into the North Atlantic Rev. 21, 879–887 (2002). St-Onge point out that the break point Ocean could have aff ected ocean circulation 6. Krinner, G. et al. Nature 427, 429–432 (2004).

CLIMATE SCIENCE Predicting dry lands Cacti and tumble weeds have long been Th ese regions are expected to experience the photogenic icons of dry desert more droughts lasting four to climates, far removed from the lush six months by the middle of the century. green areas many call home. But more- In the model simulations, it takes frequent extended periods of drought several more decades before statistically are one of the most worrying potential signifi cant changes in drought extent impacts of anthropogenic climate and frequency occur in other regions, change. Will increasing greenhouse including most of North America. gases mean that these prickly plants will By contrast, northern Europe, soon replace our forests and gardens? Australia, the Tibetan Plateau, the Aft er analysing simulations from easternmost part of central Africa and eight climate models that formed the the island nations of Southeast Asia basis of the Fourth Assessment Report should escape relatively unscathed, of the Intergovernmental Panel of as changes in their drought patterns Climate Change, Justin Sheffi eld and are not predicted to stand out against Eric Wood of Princeton University, past climate variability by the end of USA, suggest that it may be some time the century. before greenhouse-gas-related changes in Th e exact timing of these impending DOUG DOLDE drought will stand out against the noisy hydrological changes will depend on the climate background of the past decades rates of future emissions of greenhouse budget, such as water diversion, irrigation (Clim. Dyn. doi:10.1007/s00382-007- gases. In the scenario with the highest and land-use changes, all of which may 0340-z; 2008). emissions (SRES A2), the models predict spur greater drought responses. Th e People in the Mediterranean, that statistically signifi cant changes in potential dire consequences of increasing southern Africa and most of drought for some regions will occur in less droughts underscore the need for careful South America (as well as wine lovers than twenty years. water resources management and elsewhere) may notice the ill-eff ects of None of the models account for other planning in the coming century. greenhouse-gas-induced aridity fi rst. anthropogenic changes to the water Alicia Newton

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