Background
The authors write that "over the tropical latitudes, it is certainly true that extremes in climate variability owe their origin to sea surface temperature (SST) variability associated with El Niņo-Southern Oscillation (ENSO) [Hoerling and Kumar, 2002]." But they say that this conclusion "cannot be extended to climate variability in extratropical latitudes where [the] contribution of noise associated with random variability in the climate system dominates," citing Madden (1976) and Kumar and Hoerling (1995).

What was done
In further exploring this subject, Kumar et al. addressed it within the context of the severe Great Plains drought of May-July 2012, which they say had "a rapid onset, and little indications or early warnings for its sudden emergence." This they did by analyzing the origins of the drought "based on a dynamical seasonal climate forecast system where states of the ocean, atmosphere, land, sea ice, and atmospheric trace gases were initialized in late April 2012 and an ensemble of forecasts was made."

What was learned
The four researchers report that "based on the diagnosis of a spectrum of possible outcomes for precipitation over the Great Plains from this system, it [was] concluded that the extreme Great Plains drought did not require extreme external forcings and could plausibly have arisen from atmospheric noise alone."

What it means
"The broader implications of this analysis," in the words of Kumar et al., "were to demonstrate the fundamental role of atmospheric noise in shaping extreme climatic events, particularly in extratropical latitudes." But "this is not to say," as they continue, "that the presence of external forcings cannot also influence the magnitude and probabilities of extreme climate events." Yet their primary finding still poses "a challenge for developing an appropriate early warning system for extreme climate events." And it also poses a challenge for those who proclaim that nearly every extreme climatic event that occurs nowadays is caused by global warming. It just ain't so.

References
Hoerling, M.P. and Kumar, A. 2002. Atmospheric response patterns associated with tropical forcing. Journal of Climate 15: 2184-2203.

Kumar, A. and Hoerling, M.P. 1995. Prospects and limitations of seasonal atmospheric GCM predictions. Bulletin of the American Meteorological Society 76: 335-345.

Madden, R.A. 1976. Estimates of the natural variability of time-averaged sea-level pressure. Monthly Weather Review 104: 942-952.