Extended megadroughts in the southwestern United States during Pleistocene interglacials

Abstract

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, 'dust-bowl-like' megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C(4) plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ∼2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4-6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.

The global monsoon across timescales: coherent variability of regional monsoons

“…Although the climate impact of modern-day ITCZ shifts in the Americas has been a topic of research for decades, paleomonsoon studies began only in recent years. In this work, it has been shown through the analyses of lacustrine deposits in New Mexico that periods of enhanced summer monsoon precipitation in MIS 11 and 13 are related to precessional forcing (Fawcett et al, 2011), while the Holocene drying trend observed in sediment cores from a lake in Nevada suggested a weakening of the North American monsoon, in conjunction with a weakening of monsoons across the Northern Hemisphere (Benson et al, 2002). A period of increased monsoon precipitation during the Holocene thermal maximum followed by a trend toward drier conditions since ∼ 5 ka has also been inferred from marine sediments in the Cariaco Basin (Haug et al, 2001) and the Gulf of Mexico (Poore et al, 2003).…”

Bacterial GDGTs in Holocene sediments and catchment soils of a high Alpine lake: application of the MBT/CBT-paleothermometer

“…Several attempts have recently been made to apply the GDGT-based proxies to lacustrine sediment records Tierney and Russell, 2009;Blaga et al, 2010;Bechtel et al, 2010;Tierney et al, 2010a, b;Tyler et al, 2010;Zink et al, 2010;Fawcett et al, 2011;Loomis et al, 2011;Pearson et al, 2011). All studies so far (mostly targeting surface sediments) revealed that branched GDGTs occur ubiquitously in lake sediments and often represent the most abundant sedimentary GDGTs Tierney and Russell, 2009;Tyler et al, 2010;Blaga et al, 2010;Bechtel et al, 2010;Tierney et al, 2010a;Loomis et al, 2011;Pearson et al, 2011).…”

Imprint of the Atlantic multi-decadal oscillation and Pacific decadal oscillation on southwestern US climate: past, present, and future

“…The region has experienced several severe droughts in the recent and distant past Cook et al 2010;Touchan et al 2011;Fawcett et al 2011;Oglesby et al 2012). Current climate models forecast an imminent transition to a more arid climate (e.g.…”