A new paper, pointed out by Timo Hämeranta, has appeared that further documents the vulnerability of the western United States to drought, even in the absence of human climate forcings.
Stone, Jeffery R., and Sherilyn C. Fritz, 2006. Multidecadal drought and
Holocene climate instability in the Rocky Mountains. Geology Vol. 34, No
5, pp. 409-412, May 2006
The Abstract reads,
“Time series analysis of a diatom-inferred drought record suggests that Holocene hydroclimate of the northern Rocky Mountains has been characterized by oscillation between two mean climate states. The dominant climate state was initiated at the onset of the Holocene (ca. 11 ka); under this climate state, drought was strongly cyclic, recurring at frequencies that are similar to twentieth century multidecadal phase changes of the Pacific Decadal Oscillation. This pattern remained consistent throughout much of the mid-Holocene, continuing until ca. 4.5 ka. After this time the mean climate state changed, and drought recurrence became unstable; periods of cyclic drought alternated with periods of less predictable drought. The timing of this shift in climate was coincident with widespread severe drought in the mid-continent of North America. Overall, the strongest periodicity in severe drought occurred during the mid-Holocene, when temperatures in the northern Rocky Mountains were warmer than today.”
This paper further illustrates why a focus on vulnerability, as an inclusive framework to assess risk is so useful. Relying on multi-decadal global climate model projections to plan for what could happen in the future is very risky, as the climate clearly can respond even to natural climate forcings and feedbacks in complex ways. A reoccurrence of prehistoric droughts today would result in enormous social and environmental consequences.
As we have shown in our paper
Pielke Sr., R.A., N. Doesken, O. Bliss, T. Green, C. Chaffin, J.D. Salas, C. Woodhouse, J.L. Lukas, and K. Wolter, 2005: Drought 2002 in Colorado – An unprecedented drought or a routine drought? Pure Appl. Geophys., Special Issue in honor of Prof. Singh, 162, 1455-1479, doi:10.1007/200024-005-2679-6.
the state of Colorado was affected by the 2002 drought out of proportion to its intensity. This event illustrated that we are more vulnerable to drought today, than with the same dry conditions in the past. Regardless of the extent of human intervention in the climate system, this sobering assessment indicates that we need to prepare for long term drought in the West, regardless of the global model projections.