There is less evidence that precipitation is changing. The report found no significant trend in North American precipitation since 1951, although there have been substantial changes from year to year and even decade to decade. Moreover, it is unlikely that a fundamental change has occurred in either how often or where severe droughts have occurred over the continental United States during the past half-century. Nevertheless, drought impacts have likely become more severe in recent decades. It is likely that the impacts have been more severe because the recent droughts have lasted a few years, and because warmer temperatures have created stresses in plants, which make them more vulnerable.