Drought and heat combine to expand fire size and extend the burning season. In Arizona, in the 20th Century, lightning ignited the largest fires in early summer when low humidity and high temperature had prepped the vegetation combustion. Human-caused fires during the more-humid conditions of the rest of the year were smaller. However, as things have warmed up and the drought has intensified, large human-caused fires are beginning to flare up in other seasons. The devastating result is that the fire recurrence interval is becoming too short for regeneration by most native perennial plants. Much of the Sonoran Desert is becoming an annual weedland (http://garryrogers.com/2014/01/14/desert-fire/).