The study points out that very large and severe wildfires, bark-beetle outbreaks and a doubling of the proportion of dead trees in response to early 21st-century warmth and drought conditions are evidence that a transition of southwestern forest landscapes toward more open and drought-tolerant ecosystems may already be under way.

And while 2000s drought conditions have been severe, the regional tree-ring record indicates there have been substantially stronger megadrought events during the past 1,000 years.

The strongest megadrought occurred during the second half of the 1200s and is believed to have played an important role in the abandonment of ancient Puebloan cultural centers throughout the Southwest. The most recent megadrought occurred in the late 1500s and appears to have been strong enough to kill many trees in the Southwest.

"When we look at our tree-ring record, we see this huge dip in the 1580s when all the tree rings are really tiny," Williams said. "Following the 1500s megadrought, tree rings get wider, and there was a major boom in new trees. Nearly all trees we see in the Southwest today were established after the late-1500s drought, even though the species we evaluated can easily live longer than 400 years. So that event is a benchmark for us today. If forest drought stress exceeds late 1500 levels, we expect that a lot of trees are going to be dying."

Will future forest drought-stress levels reach or exceed those of the megadroughts of the 1200s and 1500s?

Using climate-model projections, the team projected that such megadrought-type forest drought-stress conditions will be exceeded regularly by the 2050s. If climate-model projections are correct, forest drought-stress levels during even the wettest and coolest years of the late 21st century will be more severe than the driest, warmest years of the previous megadroughts.

The study forecasts that during the second half of this century, about 80 percent of years will exceed megadrought levels.

The current drought, which began in 2000, is a natural case study about what to expect from projected climate scenarios. While average winter precipitation totals in the Southwest have not been exceptionally low, average summer-fall evaporative demand is the highest on record.

And trees, Williams says, are paying the price.

The team concluded forest drought stress during more than 30 percent of the past 13 years, including 2011 and 2012, matched or exceeded the megadrought-type levels of the 1200s and 1500s. The only other 13-year periods when megadrought-type conditions were reached with such frequencies in the past 1,000 years were during the megadroughts themselves.

UA co-author Daniel Griffin said, "This research is distinctly different from work done in a similar vein in two ways: One, it puts these projections for the future in a concrete historical context, and two, it shows that the impacts on the forests will not be restricted to one species or one site at low elevation, but in fact will take place at forests across the landscape."

Griffin is a doctoral candidate in the UA School of Geography and Development.

Co-author Craig D. Allen, a research ecologist with the U.S. Geological Survey, said, "Consistent with many other recent studies, these findings provide compelling additional evidence of emerging global risks of amplified drought-induced tree mortality and extensive forest die-off as the planet warms."

The article, "Temperature as a potent driver of regional forest drought stress and tree mortality," is written by A. Park Williams (Los Alamos National Laboratory), Craig D. Allen (U.S. Geological Survey), Alison K. Macalady (University of Arizona), Daniel Griffin (UA), Connie A. Woodhouse (UA), David M. Meko (UA), Thomas W. Swetnam (UA), Sara A. Rauscher (LANL), Richard Seager (Columbia Univ.), Henri D. Grissino-Mayer (Univ. of Tennessee), Jeffrey S. Dean (UA), Edward R. Cook (Columbia Univ.), Chandana Gangodagamage (LANL), Michael Cai (LANL) and Nate G. McDowell (LANL).

Los Alamos National Laboratory, the U.S. Department of Energy and the National Science Foundation funded the research.