During most of the past 100,000 years, temperatures on Earth were much colder than they are now and climate was very unstable. About 21,000 years ago that climatic period culminated in the Last Glacial Maximum (LGM), when about 50 million km3 of water was locked in huge ice sheets, lowering sea level by more than 120 metres. Climate during the LGM was clearly very different from what it is today. But how different? This is the subject of the paper by Ganopolski et al. on page 351 of this issue1.
Using a properly tuned, simplified, coupled ocean-atmosphere climate model, they first verify the model's ability to simulate modern climate on the global scale. After adapting insolation, concentrations of atmospheric greenhouse gases and ice-sheet distribution to values typical for the LGM, they find that the same model yields a stable climatic state with other atmospheric and oceanic characteristics that are reminiscent of the LGM. The simulated changes are broadly consistent with what we know from decades of invaluable analyses of marine sediments, polar and tropical ice cores, tree rings and groundwater — all of which are archives that record past climatic changes.
Why should we be interested in simulating the climate of the LGM, when that state is unlikely to return for another 50,000 years2 and Earth's climate will instead most probably warm considerably? There are four reasons. First, climate models must be able to simulate the full range of dynamical behaviour of the climate system, and so the LGM or transient and extreme climatic periods are the most critical tests they can be exposed to. Second, models that estimate future changes must be consistent with the sensitivity of the climate system to altered forcing parameters. Third, these models, if correct, can provide a more detailed picture of past changes for regions or parameters for which no suitable palaeoclimate archives are available. Finally, this will eventually contribute to a quantitative, model-based interpretation of palaeoclimatic proxy data.
Ganopolski et al. abandon the classical approach of using comprehensive three-dimensional general circulation models of one of the two main components of the climate system (the atmosphere or the ocean), while using a simplified representation of the other. Instead, they build on the progress made in developing zonally averaged climate models3,4, which have become important tools in palaeoclimaticesearch, and combine it with a new atmospheric model component. Such models permit the numbers of runs required for optimal tuning. This means that loosely constrained model parameters or incompletely known boundary conditions can be varied in such a way that the relevant atmosphere-ocean exchange fluxes of momentum, heat and fresh water, simulated by the respective model components, are brought into adequate agreement.
Because of limitations on computer time, such systematic fine-tuning is not yet possible for coupled three-dimensional models. So most of these models drift to unrealistic states, and flux corrections must be applied. This remedy is undesirable but permissible for small and linear excursions from a well-defined climate state. But caution is called for when modelling climates as different from today's as the LGM.
The crucial component for a successful simulation is the hydrological cycle, which is notoriously difficult to simulate. Water vapour is the most important greenhouse gas, and its reduction in level contributes significantly to the global cooling of 6.2 °C in Ganopolski and colleagues' model. This is colder than the usually accepted estimates of about 4 °C, but it is consistent with recent reconstructions of significantly lower temperatures for the LGM at high and low latitudes5,6,7.
The hydrological cycle also influences the circulation of the deep ocean. Changes in patterns of evaporation and precipitation determine the location of deep-water formation and the mix of waters from northern and southern origin in the North Atlantic8. Reduction in high-latitude precipitation, and an even bigger decrease in evaporation due to a larger extent of the ice cover in the northern North Atlantic (the ice margin moves from 75° N to 55° N in the model), mean that a surface freshwater anomaly can develop; in consequence, the area of deep-water formation in the North Atlantic also moves about 20° south. The density of the sinking waters is reduced, with the consequence that they penetrate less deeply. In this way, water masses of southern origin can extend far north in the Atlantic basin and fill much of the deep Atlantic. This meridional shift in the distribution of water mass below 1,000 m, seen in the model, is consistent with palaeoceanographic evidence from stable carbon isotope measurements on benthic foraminifera9.
As well as strengthening our confidence in reconstruction of proxy data, coupled climate models are also crucial in assessing hypotheses on which less complete models are based. Ganopolski et al. find that, during the LGM, the oceanic meridional heat flux in the Atlantic was very different from today. Although this is not surprising (sea-surface temperatures were lower10, and sites of deep-water formation and sea-ice margins moved south), it contradicts recent assumptions used for atmosphere-only models11. The reduction of heat delivery into the North Atlantic by northward-flowing waters is also expected, given the extremely cold temperatures on Greenland during the LGM (ref. 5). This leads to a pronounced asymmetry in cooling for the LGM, with a fall of more than 20 °C in northern polar regions but only 5 °C in the south.
Much remains for palaeoclimatologists and palaeomodellers to do. We need longer data sets with higher time resolution; careful (re-)calibration of proxy indicators; better dating of records, and improved synchronization of records that are located far apart; new analytical techniques; and, not least, coupled low-order and three-dimensional climate models with well-tested parametrizations12.
Figure 1 outlines four areas of the globe where important issues have to be resolved. The time is clearly ripe for another look at CLIMAP10, the last large-scale integrated project to tackle the LGM, one which incorporates the vast amount of palaeoclimatological data accumulated over the past 20 years. Moreover, the ocean interior requires continued attention, and initiatives such as IMAGES (International Marine Global Change Study)13, or the more encompassing PAGES (Past Global Changes)14, are invaluable in this respect. After all, research into Earth's climatic history is a cornerstone of building the better computer models that are so urgently needed to assess what the future may hold.
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