Evidence for general instability of past climate from a 250-kyr ice-core record

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Moisture variability in the southwestern United States linked to abrupt glacial climate change

Many regions of the world experienced abrupt climate variability during the last glacial period (75-15 thousand years ago1,2). These changes probably arose from interactions between Northern Hemisphere ice sheets and circulation in the North Atlantic Ocean, but the rapid and widespread propagation of these changes requires a large-scale atmospheric response whose details remain unclear4-7. Here we use an oxygen isotope record from a speleothem collected from the Cave of the Bells, Arizona, USA, to reconstruct aridity in the southwestern United States during the last glacial period and deglaciation. We find that, during this period, aridity in the southwestern United States and climate in the North Atlantic region show similar patterns of variability. Periods of warmth in the North Atlantic Ocean3, such as interstadials and the Bølling-Allerød warming, correspond to drier conditions in the southwestern United States. Conversely, cooler temperatures in the high latitudes are associated with increased regional moisture. We propose that interstadial warming of the North Atlantic Ocean diverted the westerly storm track northward, perhaps through weakening of the Aleutian Low, and thereby reduced moisture delivery to southwestern North America. A similar response to future warming would exacerbate aridity in this already very dry region. © 2010 Macmillan Publishers Limited. All rights reserved

Fennoscandian freshwater control on Greenland hydroclimate shifts at the onset of the Younger Dryas

Sources and timing of freshwater forcing relative to hydroclimate shifts recorded in Greenland ice cores at the onset of Younger Dryas, ∼12,800 years ago, remain speculative. Here we show that progressive Fennoscandian Ice Sheet (FIS) melting 13,100–12,880 years ago generates a hydroclimate dipole with drier–colder conditions in Northern Europe and wetter–warmer conditions in Greenland. FIS melting culminates 12,880 years ago synchronously with the start of Greenland Stadial 1 and a large-scale hydroclimate transition lasting ∼180 years. Transient climate model simulations forced with FIS freshwater reproduce the initial hydroclimate dipole through sea-ice feedbacks in the Nordic Seas. The transition is attributed to the export of excess sea ice to the subpolar North Atlantic and a subsequent southward shift of the westerly winds. We suggest that North Atlantic hydroclimate sensitivity to FIS freshwater can explain the pace and sign of shifts recorded in Greenland at the climate transition into the Younger Dryas

Multiple causes of the Younger Dryas cold period

The Younger Dryas cooling event disrupted the overall warming trend in the North Atlantic region during the last deglaciation1, 2, 3, 4, 5, 6. Climate change during the Younger Dryas was abrupt7, 8, 9, and thus provides insights into the sensitivity of the climate system to perturbations. The sudden Younger Dryas cooling has traditionally been attributed to a shutdown of the Atlantic Meridional Overturning Circulation by meltwater discharges10, 11, 12, 13. However, alternative explanations such as strong negative radiative forcing14 and a shift in atmospheric circulation15 have also been offered. Here we investigate the importance of these different forcings in coupled climate model experiments constrained by data assimilation. We find that the Younger Dryas climate signal as registered in proxy evidence is best simulated using a combination of processes: a weakened Atlantic Meridional Overturning Circulation, moderate negative radiative forcing and an altered atmospheric circulation. We conclude that none of the individual mechanisms alone provide a plausible explanation for the Younger Dryas cold period. We suggest that the triggers for abrupt climate changes such as the Younger Dryas are more complex than suggested so far, and that studies on the response of the climate system to perturbations should account for this complexity