Background
The authors write that "the central premise explored in this study is whether extreme forcing is necessary for extreme climate events, and thus whether an event would have either failed to occur or failed to achieve extreme magnitude if such forcing were lacking," among which forcings they mention "changes in atmospheric chemical composition resulting from volcanic aerosols, or human-induced greenhouse gas changes," the latter of which are often promoted by climate alarmists as causing more extreme types of both climate and weather.

What was done
Kumar et al. addressed this subject within the context of the severe U.S. Great Plains drought of May-July 2012, which had "a rapid onset, and little indications or early warnings for its sudden emergence." This they did via the use of "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
As a result of their findings, the four U.S. researchers felt confident in stating that "based on the diagnosis of a spectrum of possible outcomes for precipitation over the Great Plains from this system, it is 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
Among a number of other things, Kumar et al. write that "processes inherent to the atmosphere alone, with no intervention of external forcing, may cause extreme events." And they say that "the evolution of these atmospheric processes is highly sensitive to initial conditions such that two analyzed states that are only slightly different rapidly diverge," which they say "is typically viewed as a consequence of the so-called 'butterfly effect'," citing Hilborn (2004). And so it was that they ultimately concluded that the mechanism responsible for the occurrence of extreme events in such nonlinear systems may well be simply "the (unpredictable) sensitivity to small differences in initial conditions."

Reference
Hilborn, R.C. 2004. Sea gulls, butterflies, and grasshoppers: A brief history of the butterfly effect in nonlinear dynamics. American Journal of Physics 72: 425-427.