Climate variations in the semiarid Southwest US are closely tied to changes in water, upon which ecosystems and societies depend. The current drought (16 years and counting) highlights the vulnerability of these systems and the role of human and natural causes. Paleoclimate records from across the Southwest point to the occurrence of multidecadal “megadroughts” long before the anthropogenic climate change of the last century. The relationship of these extreme intervals to external forcing and internal modes (e.g. El Niño) is not well established and is critical for understanding future drought risk. Equally critical is identifying how the distinct summer (monsoon) and winter (Pacific storm) regimes contribute to drought. Paleoclimate records are critical for understanding natural drought variability over multidecadal-multicentury time scales. Results from cave records in Arizona complement other sources of information about dry (and wet) excursions over the past several thousand years. Stable isotope records from caves can be ambiguous about total moisture, but provide information about the relative contributions of summer monsoon and winter storms. We complement this approach with new methods based on the abundance of trace and minor elements to assess overall wet-dry fluctuations.
Results/Conclusions
Our records from Arizona caves show clearly that the summer monsoon system has weakened substantially since about 7000 years ago. Elemental results suggest a trend towards drier conditions during this interval. Within the past few millennia, our data suggest significant multidecadal-multicentury variability in both total moisture and its seasonal distribution. Such extremes would have substantial impacts on managed and natural ecosystems if they were to happen today, particularly when superimposed on hotter background conditions. The Southwest already faces a higher risk of drought from a warming climate; this risk is exacerbated by the naturally occurring persistent droughts that our work documents.