October 18, 2004

A Tale of Two Records

Research on the long-term drought history of the western United States doesn’t jibe with research on the long-term temperature reconstructions of the same region. When two records disagree, something is amiss.

On October 7, 2004, SciencExpress (the rush-to-release version of Science magazine) published a report stating that a trend toward warmer temperatures in the future could lead to “a serious long-term increase in aridity over western North America.” The authors, a team of paleoclimate researchers led by Edward Cook, analyzed a 1,200-year reconstruction of moisture conditions in the American West using data a large collection of tree-ring data from the region.

Figure 1 shows Cook’s reconstruction of the percentage of the western United States that experienced drought conditions from the present back to 800 A.D. (the data are smoothed to emphasize decadal and longer-scale variations). Also included on Figure 1 are the central dates of periods that are associated with unusually wet or dry conditions. Notice that during the period from about 900 A.D. to 1300 A.D. a far greater percentage of the West experienced drought conditions than the period since then. Also notice that the drought at the end of the record (the current period) pales in comparison (at least in areal coverage) to droughts in earlier eras.

Drought Reconstruction

Figure 1. Reconstruction of the percentage of the western United States experiencing drought conditions, 800 A.D. to present. Central dates of periods with unusually wet or dry conditions are indicated. (Source: Cook et al., 2004).

Cook and colleagues would have us believe that there are two take-home messages from this chart, 1) the dry conditions experienced from 900 A.D to 1300 A.D. occurred during the Medieval Warm Period (MWP) and that “the coincidence between our megadrought epoch [900 A.D. to 1300 A.D.] 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” and 2) a trend toward warmer temperatures in the future may bring about a return to the drier conditions of the past.

While this may seem well and good, you may wonder about Cook’s reference to the existence of the MWP, especially if you believe in the work of Michael Mann and colleagues. Their temperature reconstructions downplay the magnitude of such an event, especially in the western United States. Consider Mann’s recent article in the American Geophysical Society’s EOS. Figure 2 depicts Mann’s version of the temperature history of the western United States. It is very difficult in this reconstruction to find an extended period that appears to be unusually warm (that is, except for the data at the very end of the record). There is a slight indication that from about 1075 A.D. to 1425 A.D., it was a bit warmer than the period from about 1425 A.D. to 1900 A.D., but that difference certainly does not appear to be remarkable and thus hardly seems as if it could have caused the widespread droughts that Cook found in his record. Perhaps we need look a little more closely.

Temperature Reconstruction

Figure 2. Reconstructed temperature history of the western United States, 200 A.D. to 1980 A.D. Where is the Medieval Warm Period? (Source: Mann et al., 2003)

In Figure 3, we truncate the full 1,800-year Mann temperature reconstruction to a 1,200- year record and then align it with the Cook drought reconstruction. We then look to see how well the two reconstructions correspond. Interestingly enough, the scant correspondence that we did find was opposite to the sense that Cook postulated. The warm periods in the Mann period during his ersatz MWP corresponded to relatively wet periods in the Cook reconstruction, while the cool periods in the Mann record corresponded to the periods of widespread drought as reconstructed by Cook. Or at least that was the case from about 1000 A.D. to about 1500 A.D. Since 1500 A.D. there was little if any similarity between the two reconstructions. About the worse correspondence was found during the last 200 years, which apparently were cold and dry in the early 1800s, then turned warmer and wetter by the turn of the 20th century, and ended the century still warmer but dry. During the past two centuries it seems that absolutely no information about the temperature could be gained from the moisture conditions or vice versa.

Drought Reconstruction

Figure 3. A direct comparison of Cook’s reconstruction of drought conditions and Mann’s reconstruction of temperatures shows that during the time loosely associated with the Medieval Warm Period, extensive drought conditions were associated with cooler temperatures (dashed brown arrows) and periods of less drought were associated with warmer conditions (solid green arrows). This runs counter to Cook’s assertion that warm periods are associated with enhanced aridity in the western United States.

Something is obviously amiss. If both reconstructions are correct, then Cook’s supposition that warm periods are associated with enhanced aridity in the western United States is incorrect. If Cook’s supposition is correct, then Mann’s temperature reconstruction is incorrect. Or, perhaps they’re both wrong. It’s a fact that many theories that almost all scientists espouse are eventually proven wrong by history.

Who’s to say? The one thing that is for sure is that all this uncertainty provides little useful information for planners or policymakers.

References:

Cook E.R., et al., 2004. Long-Term Aridity Changes in the Western United States. Sciencexpress, October 7, 2004.

Mann, M.E., 2003. On Past Temperature and Anomalous Late-20th Century Warmth. EOS, 27, 256-258.




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