Background
Climate alarmists typically claim that CO2-induced global warming will lead to the occurrence of more extreme meteorological phenomena, such as droughts, floods and storms.  Whenever some severe weather event or climatic anomaly occurs, therefore, they attribute its apparent extremeness to the ever-increasing CO2 content of the atmosphere, which they say is a result of the burning of fossil fuels.  To have some rational basis for assessing the validity of these claims and either accepting or rejecting them, one must have some idea of how the magnitudes of these extreme phenomena compare with those of similar phenomena that occurred in centuries past, when the air's CO2 concentration was essentially invariant at a much lower value than it is currently.  Woodhouse's paper provides that kind of background for droughts of the western United States.

What was done
Woodhouse reports what is known about natural hydroclimatic variability throughout the western U.S. via descriptions of several major droughts that occurred there over the past three millennia, all but the last century of which had atmospheric CO2 concentrations that never varied by more than about 10 ppm from a mean value of 280 ppm.

What was learned
For comparative purposes, Woodhouse begins by noting that "the most extensive U.S. droughts in the 20th century were the 1930s Dust Bowl and the 1950s droughts."  The first of these droughts lasted "most of the decade of the 1930s" and "occurred in several waves," while the latter "also occurred in several waves over the years 1951-1956 (Diaz, 1983; Karl and Heim, 1990)."

Far more significant than either of these two 20th-century droughts was what has come to be known as the 16th-Century Megadrought, which lasted from 1580 to 1600 and included northwestern Mexico in addition to the southwestern United States and the western Great Plains.  Then there was what is called simply The Great Drought, which spanned the last quarter of the 13th century and was actually the last in a series of three 13th-century droughts, the first of which may have been even more severe than the last.  In addition, Woodhouse notes there was a period of remarkably sustained drought in the second half of the 12th century.

What it means
In the words of Woodhouse, "the 20th century climate record contains only a subset of the range of natural climate variability in centuries-long and longer paleoclimatic records."  She also notes that these earlier droughts "are likely related to slowly varying modes of climate," and that this "long-term natural climate variability will likely continue."  This being the "likely" case, it would take a drought several times more extreme than the most extreme droughts of the 20th century to take the southwestern U.S. and northwestern Mexico into truly uncharted waters (no pun intended) with respect to extreme drought conditions.  We can also conclude from these observations that climate-alarmist claims that we are already feeling the effects of anthropogenic CO2 emissions in various types of extreme weather and climate phenomena are totally without merit, i.e., unproven.

References
Diaz, H.F.  1983.  Drought in the United States: some aspects of major dry and wet periods in the contiguous United States, 1895-1981.  Journal of Climate and Applied Meteorology 22: 3-16.

Karl, T.K. and Heim Jr., R.R.  1990.  Are droughts becoming more frequent or severe in the United States?  Geophysical Research Letters 17: 1921-1924.