I’ve been reading an interesting article from the December edition of the Journal of Climate on droughts in North America over the last 1,000 years. (Cook, B., J. Smerdon, R. Seager, and E. Cook, 2013: Pan-continental droughts in North America over the last millennium. J. Climate. doi:10.1175/JCLI-D-13-00100.1, in press.)
In summary, Cook et al use tree ring data from across the USA and other parts of North American and identify many instances of ‘pan-continental’ droughts, meaning droughts affecting several large regions of North America at the same time. The drought in North America in 2012 being an example. The tree ring record clearly identifies conditions during the Medieval Climate Anomaly (MCA) (aka Medieval Warm Period) and the authors note that pan-continental droughts were more frequent and longer-lasting (megadroughts) during that period than since the “little ice age” and afterwards.
The authors find that Pan-continental drought conditions occur on average at least 12% of the time over the last millennium, although more so during the MCA and less so in more recent times. The occurrence seems to be connected to such natural oscillations such as La Niña, Pacific Decadal Oscillation and Atlantic Multi-decadal Oscillation. As such oscillations will continue, it is likely that so will occasional pan-continental droughts; if the past is a guide to the future, at an average rate of about every 10 to 15 years.
The paper is not offered in support or opposition to the Anthropogenic (man-made) Global Warming hypothesis, but the authors do discuss the likelihood of increased pan-continental drought conditions in North America should the planet warm. This is by way of analogy to the MCA.
An early online release version of this paper is available for download here: https://www.ldeo.columbia.edu/res/div/ocp/drought/PDFS/CookEtAl2013PanContin.pdf
This paper clearly demonstrates that one should not automatically attribute the 2012 drought in North America to any AGW effects, as some have (e.g. World Resources Institute and Climate Central) including it seems a certain POTUS , although most scientists have not actually argued this . I would be interested to know if any similar work has been done on other known drought areas, such as many parts of Australia.
I wonder why the authors thought it necessary to mention AGW in any case. The discussion of possible increase in drought should the planet warm seems a bit speculative. Without addressing the possible cause or causes of MCA one should not assume that any global warming that might arise for any reason would have the same effect, especially if the cause was different. There might be wetter conditions instead, if humidity rose. The meat of the paper does not seem to me to require AGW at all, and it has no effect on any of their conclusions save the speculative ones which are only weakly supported by the rest of the paper. I can only conclude it was either to support their research grants (past, present or future) or to increase the chances that the media would pick it up – as indeed Nature did (£).