An unprecedented combination of heat plus decades of drought could be in store for the Southwest sometime this century, suggests new research from a University of Arizona-led team….
“The bottom line is, we could have a Medieval-style drought with even warmer temperatures,” [lead author Connie] Woodhouse said.
In 2007, Science (subs. req’d) published research that “predicted a permanent drought by 2050 throughout the Southwest” “” levels of aridity comparable to the 1930s Dust Bowl would stretch from Kansas to California.
In October, a National Center for Atmospheric Research (NCAR) study warned, “The United States and many other heavily populated countries face a growing threat of severe and prolonged drought in coming decades … possibly reaching a scale in some regions by the end of the century that has rarely, if ever, been observed in modern times.”
UPDATE: A new Environmental Research Letters article, “Characterizing changes in drought risk for the United States from climate change,” comes to a similar conclusion as the NCAR study, “Drought frequencies and uncertainties in their projection tend to increase considerably over time and show a strong worsening trend along higher greenhouse gas emissions scenarios, suggesting substantial benefits for greenhouse gas emissions reductions.” See especially Figure 4C.
Now a new Proceedings of the National Academy of Sciences study, “A 1,200-year perspective of 21st century drought in southwestern North America” looks at the paleoclimate record to see the kind of drought the southwest — and other regions — might experience. It concludes:
Instrumental records and paleoclimatic evidence for past prolonged drought in the Southwest that coincide with elevated temperatures can be assessed to provide insights on temperature- drought relations and to develop worst-case scenarios for the future. In particular, during the medieval period, ˆ¼AD 900- 1300, the Northern Hemisphere experienced temperatures warmer than all but the most recent decades. Paleoclimatic and model data indicate increased temperatures in western North America of approximately 1 °C over the long-term mean. This was a period of extensive and persistent aridity over western North America. Paleoclimatic evidence suggests drought in the mid-12th century far exceeded the severity, duration, and extent of subsequent droughts. The driest decade of this drought was anomalously warm, though not as warm as the late 20th and early 21st centuries. The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future. The severity, extent, and persistence of the 12th century drought that occurred under natural climate variability, have important implications for water resource management. The causes of past and future drought will not be identical but warm droughts, inferred from paleoclimatic records, demonstrate the plausibility of extensive, severe droughts, provide a long-term perspective on the ongoing drought conditions in the Southwest, and suggest the need for regional sustainability planning for the future.
Future droughts will be fundamentally different from all previous droughts that humanity has experienced because they will be very hot weather droughts, as I have written (see Must-have PPT: The “global-change-type drought” and the future of extreme weather).
The University of Arizona news release makes a similar point:
Droughts that are accompanied by warm temperatures have more severe impacts on ecosystems, said Meko, an associate research professor in the UA’s Laboratory of Tree-Ring Research.
During the Medieval period, temperatures were about 1.8 degrees Fahrenheit (1 C) above the long-term average. Average temperatures in the Southwest have been warmer than that since 1990 and are projected to increase at least another 3.6 F (2 C) by 2100, Woodhouse said.
Well, holding at 2C warming for the SW requires the world to take action immediately and stay below 450 ppm. Not bloody likely at this point. On our current emissions path, the SW is facing 6C (11F) warming or more.
The most severe warm-climate drought in the Southwest within the last 1,200 years was 60 years long and occurred during the mid-12th century, according to research by Meko and others. That drought covered most of the western U.S. and northern Mexico.
For a 25-year period during that drought, Colorado River flow averaged 15 percent below normal, according to the tree-ring-based reconstruction of stream flow at Lees Ferry.
For every 1.8 degree Fahrenheit (1 C) of warming in the future, Colorado River flow is projected to decrease between two and eight percent, Woodhouse and her co-authors wrote.
Thus, in the worst-case, the Colorado River flow could drop by half by 2100.
The Colorado River supplies water for cities and agriculture in seven western states in the U.S. and two states in northwestern Mexico. Los Angeles, Las Vegas, Denver, Phoenix, Tucson and Albuquerque are among the many cities dependent on Colorado River water.
“Even without warming, if you had one of those medieval droughts now, the impact would be devastating,” she said. “Our water systems are not built to sustain us through that length of drought.”
… In recent decades, temperatures have been higher than during the previous 1,200 years, and future temperatures are predicted to be even warmer, Woodhouse said.In addition, other research predicts that changes in atmospheric circulation will reduce the amount of winter precipitation the Southwest receives in the future, she said.
“The bottom line is, we could have a Medieval-style drought with even warmer temperatures,” Woodhouse said.
Can’t say we weren’t warned.
Moreover, unlike medieval times, things will just get worse and worse for a long, long time — see NOAA stunner: Climate change “largely irreversible for 1000 years,” with permanent Dust Bowls in Southwest and around the globe.
The NCAR study makes clear what other regions face Dust-Bowlification (click figure to enlarge):
- Much of Latin America, including large sections of Mexico and Brazil
- Regions bordering the Mediterranean Sea, which could become especially dry
- Large parts of Southwest Asia
- Most of Africa and Australia, with particularly dry conditions in regions of Africa
- Southeast Asia, including parts of China and neighboring countries
The maps use a common measure, the Palmer Drought Severity Index, which assigns positive numbers when conditions are unusually wet for a particular region, and negative numbers when conditions are unusually dry. A reading of -4 or below is considered extreme drought….
There are also uncertainties in how well the Palmer index captures the range of conditions that future climate may produce. The index could be overestimating drought intensity in the more extreme cases, says Dai. On the other hand, the index may be underestimating the loss of soil moisture should rain and snow fall in shorter, heavier bursts and run off more quickly. Such precipitation trends have already been diagnosed in the United States and several other areas over recent years, says Dai.
The PDSI in the Great Plains during the Dust Bowl apparently spiked very briefly to -6, but otherwise rarely exceeded -3 for the decade (see here). So the numbers projected by NCAR are beyond catastrophic by the 2060s. By the 2090s, the damage would be unimaginable, as NCAR explained:
By the end of the century, many populated areas, including parts of the United States, could face readings in the range of -8 to -10, and much of the Mediterranean could fall to -15 to -20. Such readings would be almost unprecedented.
This is, of course, just one of many devastating impact the nation and the world face on our current path of unrestricted greenhouse gas emissions (see “A stunning year in climate science reveals that human civilization is on the precipice” and “Royal Society special issue details ‘hellish vision’ of 7°F (4°C) world — which we may face in the 2060s!”
- Absolute must read: Australia today offers horrific glimpse of U.S. Southwest, much of planet, post-2040, if we don’t slash emissions soon”
- “Australia faces collapse as climate change kicks in”: Are the Southwest and California next?
- Dust Bowl-ification hits Eastern Australia “” next stop the U.S. Southwest.