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Impact of Anthropogenic Warming on an Emerging North American Megadrought

Event Date
May 13, 2020
Event Time
2:00 pm - 3:00 pm

Severe and persistent 21st-century drought in southwestern North America motivates comparisons to medieval megadroughts and questions about the role of anthropogenic (human-caused) climate change. This webinar is based on research that used hydrological modeling and new 1,200-year tree-ring reconstructions of summer soil moisture to demonstrate that the 2000–2018 southwestern North American drought was the second driest 19-year period since 800 CE, exceeded only by a late-1500s megadrought.

The megadrought-like trajectory of 2000–2018 soil moisture was driven by natural variability superimposed on drying due to anthropogenic warming. Anthropogenic trends in temperature, relative humidity, and precipitation estimated from 31 climate models account for 47% of the 2000–2018 drought severity, pushing an otherwise moderate drought onto a trajectory comparable to the worst southwestern North American megadroughts since 800 CE.



Introduction & Welcome

Speaker: Elizabeth Weight, NOAA's National Integrated Drought Information System (NIDIS), Cooperative Institute for Research in Environmental Sciences (CIRES)

  • Introduction of today's speaker: A. Park Williams, Lamont Associate Research Professor at Lamont-Doherty Earth Observatory, Columbia University
  • The National Integrated Drought Information System (NIDIS) was created by Congressional law in 2006 with a mandate to help the nation prepare for, mitigate, and respond to the effects of drought.



Large Contribution from Anthropogenic Warming to an Emerging North American Megadrought

Speaker: A. Park Williams, Lamont-Doherty Earth Observatory, Columbia University

  • Read the article in Science, "Large contribution from anthropogenic warming to an emerging North American megadrought." Study co-authors: Ed Cook, Jason Smerdon, Ben Cook, John Abatzoglou, Kasey Bolles, Hun Baek, Andrew Badger, Ben Livneh
  • In western North America, recent decades have seen reduced river flow and lake levels, unsustainable depletion of groundwater reserves, increased wildfire activity, and massive bark beetle outbreaks.
  • Key takeaways:
    • 2000–2018 had the driest 19-year average summer soil moisture on record for the southwestern North American study region.
    • 2000–2018 soil moisture followed a trajectory similar to the onset of the medieval megadroughts, but the megadroughts were longer.
    • The researchers estimate anthropogenic climate trends account for 47% of the 2000–2018 drought severity. Uncertainty is high due to poorly understood precipitation and vegetation processes.



Questions & Answers

Moderator: Elizabeth Weight, NOAA/NIDIS, CIRES