Even so, the increased probability of extremes with changing average conditions is physically and statistically sound. (niskanencenter.org)
Research during the past decade has shown that attribution to climate change can be analyzed by examining the risk or probability of extreme events (Stone et al. 2009). (climateprediction.net)
So an «increase in extremes» can be described as an increase in probability of an extreme with a given value (say temperature), or an increase in this value for a given probability of occurrence. (realclimate.org)