There have always been extreme weather events, so why are scientists talking so much about extreme events now?

Recent climate research synthesized in the 2007 and 2014 Intergovernmental Panel on Climate Change Assessment Reports documents that there have been observable changes in extreme weather events over the past fifty years that are consistent with the expected impacts in a warming climate. The report shows that there has been a significant shift towards fewer cold and more heat extremes across many parts of the globe over the past fifty years. The 2014 National Climate Assessment reaffirms that U.S. average temperature has increased by 1.3 to 1.9 degrees F since 1895, with most of this increase occurring since 1970. In fact, the most recent decade was the world’s hottest on record, and 2012 was the hottest year on record in the continental U.S.  Land area affected by drought has also grown in recent decades due to the combination of decreasing precipitation and increasing evaporation with higher temperatures. Trends towards an increase in heavy precipitation events, with more water vapor available in the atmosphere, have occurred in some high latitude areas, but are not consistently observed across the globe. Over most of North America, extreme precipitation episodes (heavy downpours) have become more frequent and more intense in recent decades. For example, intense precipitation (the heaviest 1% of daily precipitation totals) in the continental U.S. increased by 20% over the past century, according to a synthesis and assessment report released in 2008, by the U.S. Climate Change Science Program. The report notes that, in particular, intense precipitation has increased in the central U.S. during the past century. The report goes on to say that “[h]eavy precipitation events averaged over North America have increased over the past 50 years, consistent with the observed increases in atmospheric water vapor, which have been associated with human-induced increases in greenhouse gases.”