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Western U.S. Drought-Climate Connection: A Testament to the Magnitude of the Medieval Warm Period?
Volume 7, Number 47: 24 November 2004

In our Editorial of 17 Nov 2004, we reviewed the work of Pierce et al. (2004), who concluded from their study of fire-related sediment deposits in alluvial fans in central Idaho, USA, that century-scale warm periods such as the Medieval Warm Period tend to experience more frequent and severe large-scale stand-replacing fires than do century-scale cool periods such as the Little Ice Age. In this Editorial, we revisit the work of Cook et al. (2004), wherein they analogously conclude from a study of centuries-long annually resolved tree-ring records that century-scale warm periods such as the Medieval Warm Period tend to experience more severe and longer-lasting large-scale droughts than do century-scale cool periods such as the Little Ice Age.

The parameter used by Cook et al. to represent drought was the summer-season Palmer Drought Severity Index (PDSI). For the entire western half of the United States plus adjacent strips of Canada and Mexico - hereafter simply called the West - they constructed a 103-point 2.5 by 2.5 grid that covers the time interval AD 1380 to 1978, where 68 of the grid points possess PDSI reconstructions stretching all the way back to AD 800. Then, using instrumental data, they extended the PDSI histories of each grid point from 1979 to 2003, which allowed them to compare the severe multiyear drought that still holds sway in the West - and, in their words, "is unprecedented in some hydroclimatic records" - with droughts of the Medieval Warm Period (MWP) that occurred within the same region.

As serious as the current drought is, Cook et al. find that it "pales in comparison to an earlier period of elevated aridity and epic drought in AD 900 to 1300, an interval broadly consistent with the Medieval Warm Period." Within that period of exceptional warmth, they identified four megadroughts, centered on AD 936, 1034, 1150 and 1253. Their data also revealed "an abrupt change to persistently less arid conditions after AD 1300 that lasted for ~600 years," essentially concurrent with the duration of the Little Ice Age, after which they found that "overall aridity in the West has increased in an irregular manner," broadly coincident with 20th-century global warming.

Commenting on the obvious relationship between drought and temperature that is evident in their data, Cook et al. state that "the overall coincidence between our megadrought epoch and the MWP suggests that anomalously warm climate conditions during that time may have contributed to the development of more frequent and persistent droughts in the West." Then, after recounting several possible reasons for such a relationship, they say that "large-scale warming, such as what plausibly occurred during the MWP, is again [i.e., as a result of these other studies] suggested as a contributor to the AD 900 to 1300 epoch of elevated aridity and epic drought in the West." Hence, they suggest that the severe drought currently afflicting the West may be a consequence of 20th-century global warming, further stating that "any trend toward warmer temperatures in the future could lead to a serious long-term increase in aridity over western North America," and noting that "future droughts in the 'West' of similar duration to those seen prior to AD 1300 would be disastrous."

The upshot of these many observations linking drought with warming is that the Medieval Warm Period must have experienced much warmer temperatures than those yet experienced in the Modern Warm Period. This implication is also evident in still other of Cook et al.'s comparisons of the droughts of each period: (1) "Compared to the earlier 'megadroughts' ... the current drought does not stand out as an extreme event, because it has not yet lasted nearly as long," (2) the MWP droughts "dwarf the comparatively short-duration current drought in the 'West'," (3) "more intense droughts of longer duration have occurred in the past and could occur in the future," and (4) "the epoch of unprecedented aridity revealed in [the MWP] might truly be a harbinger of things to come in the West."

All of these observations suggest that the droughts of the past century have been nowhere near as significant as those of the Medieval Warm Period; and since all of the evidence discussed by Cook et al. tends to attribute drought in the western half of the United States and adjacent parts of Canada and Mexico to warming, the body of evidence they describe suggests that the warmth of the past century has also been nowhere near as significant as that of the Medieval Warm Period. Interestingly, this is also the take-home message of the study of Pierce et al., who related the occurrence of large-scale stand-replacing fires in central Idaho, USA, to global warming.

All of these data-based revelations, which continue to accumulate, week after week, make one wonder how much longer the world's climate alarmists can continue to claim, in contradiction of a mountain of evidence, that 20th-century global warming was unprecedented over the past two millennia. Although they clearly promote a falsehood in making this claim, they simply refuse to back down, for if the Medieval Warm Period was significantly warmer than it is today, when there was approximately 100 ppm less CO2 in the air than there is now, there is absolutely no reason in the world to attribute the warming of the past century to the concomitant increase in the air's CO2 content; and, hence, there is absolutely no reason in the world to reduce anthropogenic CO2 emissions, which are a boon to the biosphere. In truth, it is likely a repeat performance of whatever was responsible for establishing and maintaining the warmth of the Medieval Warm Period that is responsible for bringing us the warmth of the Modern Warm Period, which has a long way yet to go before it can begin to compare with the magnitude of that earlier period of significantly elevated temperature.

Sherwood, Keith and Craig Idso

Cook, E.R., Woodhouse, C.A., Eakin, C.M., Meko, D.M. and Stahle, D.W. 2004. Long-term aridity changes in the Western United States. Science 306: 1015-1018.

Pierce, J.L., Meyer, G.A. and Jull, A.J.T. 2004. Fire-induced erosion and millennial-scale climate change in northern ponderosa pine forests. Nature 432: 87-90.