page_title: Climate change in the United States
section_title: Extreme weather events
hierarchical_section_title: Climate change in the United States / Current and potential effects of climate change in the United States / Extreme weather events
caption_attribution_description: English: One measure of heavy precipitation events is a two-day precipitation total that is exceeded on average only once in a 5-year period, also known as the once-in-five-year event. As this extreme precipitation index for 1901-2012 shows, the occurrence of such events has become much more common in recent decades. Changes are compared to the period 1901-1960, and do not include Alaska or Hawai‘i. (Figure source: adapted from Kunkel et al. 2013). The mechanism driving these changes is well understood. Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming. This extra moisture is available to storm systems, resulting in heavier rainfalls. Climate change also alters characteristics of the atmosphere that affect weather patterns and storms.
context_page_description: Climate change in the United States refers to historical changes in the climate of the United States, as well as the regional climactic, economic, and cultural responses to global warming.
  The current effects of global warming in the United States are widespread and varied. In 2012, the United States experienced its warmest year on record. As of 2012, the thirteen warmest years for the entire planet have all occurred since 1998, transcending those from 1880. Different regions experience widely different climatic changes. Changes in climate in the regions of the United States appear significant. For example, drought conditions appear to be worsening in the southwest while improving in the northeast. Some research has warned against possible problems due to American climate changes such as the spread of invasive species and possibilities of floods as well as droughts. Climate change is seen as a national security threat to the United States.
  The United States is among the most significant emitters of greenhouse gasses in the world. In terms of both total and per capita emissions, it is among the largest contributors.
context_section_description: The number and severity of high-cost extreme weather events has increased in the 21st century in the United States. By August 2011 alone, the NOAA had registered nine distinct extreme weather disasters for that year, each totalling $1 billion or more in economic losses. Total losses for 2011 were evaluated as more than $35 billion before Hurricane Irene.
  Though the costs and frequency of cyclones have increased on the east coast, it remains unclear whether these effects have been driven primarily by climate change. When correcting for this, a comprehensive 2006 article in Geophysical Research Letters found "no significant change in global net tropical cyclone activity" during past decades, a period when considerable warming of ocean water temperatures occurred. However, the study found major regional shifts, including  a general rise of activity in the North Atlantic area, including on the U.S. eastern coast.
  From 1898 through 1913, there have been 27 cold waves which totalled 58 days. Between 1970 and 1989, there were about 12 such events. From 1989 until January 6, 2014, there were none. The one on the latter date caused consternation because of decreased frequency of such experiences.
  Looking at the lack of certainty as to the causes of the 1995 to present increase in Atlantic extreme storm activity, a 2007 article in Nature used proxy records of vertical wind shear and sea surface temperature to create a long-term model. The authors found that "the average frequency of major hurricanes decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s." As well, they also found that "hurricane activity since 1995 is not unusual compared to other periods of high hurricane activity in the record and thus appears to represent a recovery to normal hurricane activity, rather than a direct response to increasing sea surface temperature."  The researches stated that future evaluations of climate change effects should focus on the magnitude of vertical wind shear for answers.
  The frequency of tornadoes in the U.S. have increased, and some of this trend takes place due to climatological changes though other factors such as better detection technologies also play large roles. According to a 2003 study in Climate Research, the total tornado hazards resulting in injury, death, or economic loss "shows a steady decline since the 1980s". As well, the authors reported that tornado "deaths and injuries decreased over the past fifty years". They state that addition research must look into regional and temporal variability in the future.
caption: Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming. This extra moisture is available to storm systems, resulting in heavier rainfalls.
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