Drought is a common feature of climate; but every so often when a longer-lasting or somewhat severe drought occurs, it is not long before someone, somewhere, makes the claim that that drought was either caused or made worse by CO₂-induced global warming. A simple test of this thesis can be conducted by examining the historic record of drought for the location in question. If it can be shown that similar (or greater) frequencies or magnitudes of drought have occurred in the past, prior to the modern increase in CO₂, then it cannot be definitively concluded that the current drought is the product of anything other than natural climate variability.

Unfortunately, long-term historical drought records covering more than a few decades of time are lacking for most locations across the planet. As a result, scientists have sought to augment these short-term instrumental drought histories with much longer proxy records, records that will sometimes extend back in time several centuries to millennia. Such is the case in the recent study of Vance et al. (2015), who derived a 1,003-year proxy of historical drought in eastern Australia.

In recent years, concerns of a CO₂-induced influence on drought in eastern Australia were magnified with the 1997–2009 occurrence of what has been called the “Big Dry” – the most persistent drought to envelop the region since the start of the 20^th century. Noting that there is a scarcity of long-term drought records in the region and that “no high-resolution studies cover this era of Australian prehistory,” Vance et al. set out to produce “the first millennial-length Australian drought record.” In doing so, they utilized climate records from the Law Dome ice core in East Antarctica to reconstruct a 1,000 year record of the Interdecadal Pacific Oscillation that they then combined with an eastern Australian rainfall proxy (also derived from the Law Dome site) from which they were able to identify historic megadroughts (defined as more than 5 years of below average rainfall). The resultant record is presented in the figure below.

Top panel: Independent reconstructions of the Interdecadal Pacific Oscillation (blue = decision tree and red = piecewise linear derivation), with positive phases (>0.5 for both reconstructions) highlighted in blue banding. Bottom panel: Annual Law Dome summer sea-salt time series (grey), with 13 year Gaussian smooth (thick black) and drought periods (> 5 year duration, >0.5 for both IPO reconstructions) identified (orange banding). Source: Vance et al. (2015).

As indicated by the orange shading in the figure, eight megadroughts are noted in the proxy record, the longest of which has “no modern analog.” Lasting 39 years (1174–1212 AD), this unparalleled drought was the exclamation point on a uniquely dry period in which 80 out of 111 concurrent years (over the period 1102–1212 AD) persisted in drought. The modern, or so-called “Big Dry,” by comparison, was judged by Vance et al. to be “far from an exceptional eastern Australian drought in the context of the past millennium.” And, as a result of this observation, the researchers conclude that water management in eastern Australia “needs to account for decadal-scale droughts being a normal feature of the hydrological cycle.” Indeed it should; and climate alarmists should take equal notice that there is no evidence to support the claim that CO₂-induced global warming caused or enhanced the occurrence of the Big Dry.

Reference

Vance, T.R., Roberts, J.L., Plummer, C.T., Keim, A.S. and van Ommen, T.D. 2015. Interdecadal Pacific variability and eastern Australian megadroughts over the last millennium. Geophysical Research Letters 42: 129–137.