January 11, 2008

Raining on the Drought Parade

Filed under: Climate History

One of the many pillars of fear regarding global warming is the claim that droughts will become more severe in the future, particularly in continental interiors. The story is very simple and is told over and over – temperatures rise, evaporation rates increase, and even with no change in rainfall, soil moisture levels decrease and droughts last longer and are more severe. Then, crops will fail, ecosystems will collapse, major cities will run out of water, diseases will spread – you know the story. There is always some drought occurring some place on the planet, so supporting evidence is easy to find.

We have written on this subject many times, and like everything else, there is a lot more complexity to the story. Changes in wind and/or clouds could impact future evaporation rates, global dimming could cause a decease in evaporation, plants could become more water use efficient thanks to higher levels of atmospheric carbon dioxide and therefore extract less water from the soil, and on and on. One of the problems is that long term soil moisture data are rare to non-existent, but an article in a recent issue of the International Journal of Climatology brings us a story about soil moisture extending back 1,426 years!

The article is by a team of Chinese scientists from the University of San Diego and various institutions in China. The research was funded by NASA, the Chinese Academy of Sciences, the K.C. Wang Education Foundation of Hong Kong, and University of San Diego. They begin the article noting “Anthropogenic climate changes since the Industrial Revolution have attracted much attention in recent years. One often debated question is whether the magnitude of the climate change has exceeded the range of natural variability of the climate system” (sounds like they read World Climate Report). They specifically turn their attention to soil moisture conditions in northwestern China (see very nice map below) and then reveal the plan to reconstruct soil moisture levels for over 1,400 years.


Figure 1. Study area and sampling sites (from Yin et al., 2008)

Yin et al. remind us that tree growth in that part of the world is sensitive to changes in soil moisture, and they identified Qilian junipers in their study area that are over 1,400 years old. They extracted 1050 increment cores from 493 trees (the trees are not harmed), and they carefully measured characteristics of each ring width (there is one ring per year). The tree ring widths are highly statistically significantly related to the soil moisture levels, and just like magic, they could reconstruct soil moisture levels a long way back in time. They further note “We developed tree-ring chronologies over 1400 years long, which included several important climatic events, such as the medieval warming, Little Ice Age, and the post-industrial period warming.”

One of the tricks in dendroclimatology is to link ring widths to actual climate data in the modern period, establish statistical response functions, and then allow the widths to tell us about climate variations hundreds and even thousands of years ago. Yin et al. developed water balance variables for their study area for the period 1955 – 2002 (AE = actual evaporation; DEF = soil moisture deficit; RSM = relative soil moisture; FC = field capacity), and the figure below shows us something very interesting. In their own words regarding the 1955-2002 analyses, we learn that “There was an overall trend to a wetter condition during the study period as indicated by the increasing trends of AE and RSM.” OK – where is this drought trend that we are expecting in this continental interior? Once again, the darn data are not consistent with the predictions we hear from the greenhouse crusade!


Figure2. Interannual variation patterns of the water balance variables during 1955–2002 (from Yin et al., 2008).

Now for the big findings – the figure below shows the reconstructed water balance data back to 566 AD. They note “prominent dry periods during 700–800 AD, 1100–1200 AD, 1425–1525 AD, and 1650–1750 AD, wet periods around 1225 AD, 1350 AD, and 1525–1650 AD, and a general trend toward a wetter condition during the most recent 300 years.” They pour more salt on the greenhouse wound stating “The recent trend to a wetter condition conformed to the ice accumulation record during 1600–1980 based on the ice core taken from the Dunde Glacier (38°06’N, 96°24’E, 5325 m) northwest of the study region. The wetter trend was also corroborated by a recent study in northern Pakistan, in which a reconstructed precipitation record based on tree-ring data indicated that the 20th Century was the wettest period during the past millennium.” Did you notice that little comment about ice accumulation at a nearby glacier – we are sure that is an interesting story as well? Yin et al. definitely are not making any friends with the greenhouse advocates and their findings definitely will not be featured in any self-respecting climate-change-hyping newspapers anytime soon.


Figure 3. Reconstructed water balance data for actual evaporation, accumulated soil moisture deficit, and relative soil moisture (from Yin et al., 2008)

Yin et al. rain all over the drought parade and reveal that predicted drought trends are quite at odds with what is happening (at least locally) in the real world. What a familiar theme at World Climate Report.

Reference:

Yin, Z.-Y., X. Shao, N. Qin, and E. Liang. 2008. Reconstruction of a 1436-year soil moisture and vegetation water use history based on tree-ring widths from Qilian junipers in northeastern Qaidam Basin, northwestern China. International Journal of Climatology, 28, 37-53.




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