Moisture amplification of the high-altitude deglacial warming

Abstract

In response to anthropogenic warming, glaciers are shrinking almost everywhere, endangering water accessibility in areas located downstream. Glacier fluctuations are at first order controlled by local precipitation and temperature, but large uncertainties persist on the potential role of local moisture in amplifying or dampening temperature changes at high-elevation. Here, we combine glacier extents and Sea Surface Temperature (SST) during the Last Glacial Maximum (LGM) to quantify altitudinal thermal gradients (lapse rate) from 40°N to 40°S along the American Cordillera. We also constrain modern lapse rates based on present day temperature and SST database to explore how the lapse rate has changed since the LGM along this North South transect. Based on proxy-based quantitative paleo-precipitation estimations above 2000 m, we investigate how these lapse rate changes compare with moisture modifications around the Cordillera and discuss the mechanisms that potentially controlled lapse rate changes during the post-LGM deglacial warming.We find that lapse rate changes are linearly related to changes in precipitation and derive a quantitative relationship between these two parameters. To further explore the processes involved in controlling lapse rate variations, we use the IPSL global climate model outputs, for the LGM and pre-industrial states in this region. The IPSL model also yields a shallower modern lapse rate in the wetter tropical region, confirming the observed correlation between precipitation changes and lapse rate variations. The IPSL model also supports a close coupling of continental relative moisture and mean annual precipitation in the studied area, indicating that moisture is involved in the precipitation – lapse rate relationship. Our results suggest that future warming may be enhanced in high altitude regions where precipitation is expected to increase. Using our most reliable relationship linking precipitation and lapse rate changes, we conclude that, assuming a 1 °C warming at sea level, a mean annual precipitation increases of 500 mm.a−1 could lead to a warming amplification of 4.1 ± 0.8 °C at 4000 m asl (mean elevation of modern glaciers). In this case, a 2 °C warming at sea level would yield >6 °C degrees warming at 4000 m asl. This study therefore confirms that special attention should be given to the climate projections of glacier melting in tropical and mid latitude regions.