October 10, 2007

Drought Update

Filed under: Droughts, Precipitation

Have you been hearing a lot about drought recently? Or maybe you’ve been hearing a lot about flooding recently? Probably depends on where you live. The map below (Figure 1) shows the current state of drought (and extreme moisture) across the United States as of a few days ago, and the map shows “Extreme Drought” in much of the American West (a pattern that extends well into Canada), “Extreme Drought” in the Southeast, but “Extreme Moist” conditions in Texas and parts of the middle of the country. Add in some record-breaking high temperatures in October in the Northeast, and the global warming crowd is once primed to blame everything you see on the dreaded build-up of greenhouse gases. This week, we even learned that the fashion industry is gearing up for a world with milder winters, orangutans are feeling the heat, presidential hopefuls are pledging to do something about the changing climate, and who knows what else?


Figure 1. Moisture status across the United States.

With elevated interest in drought, we went to the recent scientific literature and found two interesting articles on the subject. The first article appears in Climate Research and deals with growing season precipitation in the Canadian prairie. The three scientists from Canada’s Atmospheric Environmental Service examined the precipitation records from 37 stations located across the Canadian prairie (see Figure 2 if your Canadian geography is rusty) over the period 1905 through 1996. Understand that their study focused on linking precipitation to circulation patterns, but within the article, an interesting graphic is presented showing a time series of growing season precipitation through much of the 20th century.


Figure 2. Canadian Prairie study area and precipitation stations. The shaded region in the box to the left displays the geographical location of the Canadian Prairies with respect to North America (from Bonsal et al., 2007)

For most months and for the entire growing season, the first half of the century was much drier that the second half of the century (Figure 3). Bonsal et al. note that “The time series of total growing season precipitation indicates 3 distinct periods: above normal precipitation from 1905–16, anomalously low amounts during 1917–49, and generally higher precipitation from 1950–96.” We are not sure why the ended their study in 1996, and we have not seen the drought of today in central Canada put into historical perspective. But if we simply look at the data presented by these scientists, there is no trend whatsoever to reduced precipitation as the greenhouse gas concentration increased through the 20th century – indeed there is evidence that precipitation was enhanced through the century.


Figure3. Standardized precipitation anomalies over the Canadian Prairies, 1905-1996. (from Bonsal et al., 2007)

A second study is a recent tree ring reconstruction of hydrological drought in the upper Colorado Basin over the period A.D. 762 through 2005 by scientists at the University of Arizona and the University of Colorado. Meko et al. cored living trees from throughout the upper Colorado region, they measured the ring widths from each growth year, and the used some advanced statistical wizardry adjust for growth characteristics of trees over long periods of time. The “Streamflow data consisted of water-year-total natural flow of the Colorado River at Lee Ferry, Arizona, 1906–2004. These data, obtained from the U.S. Bureau of Reclamation, represent the best available estimate of what the flow at Lee Ferry would have been without reservoir regulation and other anthropogenic influences.”

They used a sequence of statistical procedures to link the tree ring widths to the flow of the Colorado River over the period 1906 to 2004. Once they established the statistical transfer functions over the period with both streamflow and tree ring data, the tree ring widths were used to estimate flow of the Colorado River for each year back to A.D. 762.

As seen in Figure 4 and as noted by the authors, “The most extreme low-frequency feature of the new reconstruction, covering A.D. 762-2005, is a hydrologic drought in the mid-1100s. The drought is characterized by a decrease of more than 15% in mean annual flow averaged over 25 years, and by the absence of high annual flows over a longer period of about six decades.” Here we have an annual record of hydrological drought going back more than 1,000 years, and despite the time we’ve spent looking at it, we cannot see anything unusual developing with respect to drought in the area. We see no discernible human impact whatsoever – in fact, we notice that the mean flow for the period of historical records (1906-2005) is generally higher than the flow estimates from 760 to 1905.


Figure 4. Time series plot of 25-year running mean of reconstructed flows. Flows are plotted as percentage of the 1906–2004 mean of observed natural flows (18.53 billion cubic meters, or 15.03 million acre-ft). Confidence interval derived from 0.10 and 0.90 probability points of ensemble of 1000 noise-added reconstructions. Horizontal dashed line is lowest 25-year running mean of observed flows (1953–1977) (from Meko et al., 2007).

Anyone suggesting that recent droughts are somehow the result of emissions of greenhouse gases is overlooking tremendous amounts of evidence suggesting otherwise. Droughts are a natural part of the climate of North America – they have been around a long time and they are not going away anytime soon. Droughts impacted the region in warm periods of the past, cold periods of the past, and will return whether the future is warmer or colder.

References:

Bonsal, B.R., X. Zhang, W.D. Hogg, 2007. Canadian Prairie growing season precipitation variability and associated atmospheric circulation. Climate Research, 11, 191-208.

Meko, D., C.A. Woodhouse, C.A. Baisan, T. Knight, J.J. Lukas, M.K. Hughes, and M.W. Salzer, 2007. Medieval drought in the upper Colorado River Basin. Geophysical Research Letters, 34, L10705, doi:10.1029/2007GL029988.




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