Extended megadroughts in the southwestern United States during Pleistocene interglacials
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The potential for increased drought frequency and severity linkedto anthropogenic climate change in the semi-arid regions of thesouthwestern United States (US) is a serious concern1. Multi-yeardroughts during the instrumental period2 and decadal-lengthdroughts of the past two millennia1,3 were shorter and climaticallydifferent from the future permanent, ‘dust-bowl-like’ megadroughtconditions, lasting decades to a century, that are predictedas a consequence of warming4. So far, it has been unclear whetheror not such megadroughts occurred in the southwestern US, and, ifso, with what regularity and intensity. Here we show that periods ofaridity lasting centuries to millennia occurred in the southwesternUS during mid-Pleistocene interglacials. Using molecular palaeotemperatureproxies5 to reconstruct the mean annual temperature(MAT) in mid-Pleistocene lacustrine sediment from the VallesCaldera, New Mexico, we found that the driest conditions occurredduring the warmest phases of interglacials, when the MAT wascomparable to or higher than the modern MAT. A collapse ofdrought-tolerant C4 plant communities during these warm, dryintervals indicates a significant reduction in summer precipitation,possibly in response to a poleward migration of the subtropical dryzone. Three MAT cycles 2 6C in amplitude occurred withinMarine Isotope Stage (MIS) 11 and seem to correspond to themuted precessional cycles within this interglacial. In comparisonwith MIS 11, MIS 13 experienced higher precessional-cycle amplitudes,larger variations in MAT (4–6 6C) and a longer period ofextended warmth, suggesting that local insolation variations wereimportant to interglacial climatic variability in the southwesternUS. Comparison of the early MIS 11 climate record with theHolocene record shows many similarities and implies that, in theabsence of anthropogenic forcing, the region should be entering acooler and wetter phase
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