Evidence Check: Which Extreme Weather Events Are More Linked with Climate Change – Heat Waves or Hurricanes?

, senior climate scientist | July 16, 2012, 10:38 am EST
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The heat gripping the United States this month has been relentless. And if that weren’t enough, hurricane season is upon us. How does the scientific evidence stack up over the past decades regarding how these extreme events are changing? And how much influence does human-caused climate change have on these events? We created an infographic to serve as a quick reference of the current state of scientific understanding.

UPDATE July 18th (see at bottom of post)

It is the question so many have asked climate scientists over the years, usually right after devastating loss of life or property from an extreme weather event: Are there any connections with climate change?  This infographic hopefully can help answer these questions.

Infographic: Extreme Weather and Climate Change

The size of the circles relate to the strength of the evidence for the connections to climate change of observed extreme events since 1950. Assessment based on the Intergovernmental Panel on Climate Change SREX report (2012). Figure source: Union of Concerned Scientists.

The infographic was based on careful evaluation of the latest authoritative assessment by the Intergovernmental Panel on Climate Change (IPCC) known as the Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX). I worked with UCS communications staff Matt Heid, Aaron Huertas, and Colleen MacDonald to distill into a graphic what the SREX report stated about historical observations of extreme events and their relationship to human-caused  climate change over an entire report.

The SREX report uses different ways of conveying scientific evidence, agreement and confidence regarding different weather phenomena, including how those phenomena have been changing over the past 50 years and attributing if human-caused climate change has played a nonexistent, minor, or major role in driving those changes.

There were traditional likelihood terms defined by the IPCC such as “likely” (66 to 100 percent probability) or “very likely” (90 to 100 percent probability) to describe these relationships as well as expressions of scientists’ overall confidence in their findings, which ranged from low to medium to high.

We decided to depict six phenomena for which we get the most questions or have the strongest evidence. For example, we chose to depict the circle size for the IPCC’s assessment of overall effect on tropical cyclones (we call them hurricanes to use a term most audiences are more familiar with) rather than its separate assessments for tropical cyclone wind speed or tropical cyclone frequency in different ocean basins.

For example, here are the terms used in the SREX report for the following extremes:

Extreme Observed Changes
since 1950
Attribution of Observed Changes
to Human-Caused
Climate Change
Heat very likely likely
Coastal high water likely likely
Precipitation, some regions likely medium confidence
Droughts, some regions medium confidence medium confidence
Tropical Cyclone wind speed likely low confidence
Tropical Cyclone overall low confidence low confidence
Tornadoes low confidence Not Given


In order to make the circles in the infographic relate to the list of combined terms we created a “score” based on the lower range of IPCC likelihood findings. So likely connections were given a score of 66 and very likely connections a score of 90. The report does not give any such guidance for the confidence terms, so we created another scoring system also based on a 100 point scale to at least get us in the ballpark. We assigned medium confidence a score of 50 out of 100 and low confidence a score of 10. If there was no term assessed (such as with tornadoes attribution) that was assigned a score of zero. Hence tornadoes have the smallest size circle on the infographic while heat waves have the largest.

Note this is the evidence to date regarding extreme events since 1950. In many cases we are hampered by changes in historical data gathering techniques and other factors. For future projections of heat waves, coastal high water, drought and other phenomena, the SREX report typically uses much higher confidence language, in large part because scientists are very confident that temperatures are continuing to increase and the underlying physics point to growing incidences of weather extremes under climate change.

Reducing heat-trapping emissions, of course, would decrease the amount and rate of warming we will experience and subsequently lessen the consequences climate change could have for various weather extremes compared to an unabated emissions trajectory.

UPDATE July 18th:

We updated our infographic to clarify that human-caused climate change is increasing heat waves and coastal flooding, not the sole source driving them. Heat waves and coastal flooding would continue, of course, even in the absence of global warming. But in a warming world, both are increasingly frequent and severe.

For example, the heat (and drought) that Texas experienced in the summer 2011 was made more likely by changes in water temperature in the tropical Pacific associated with La Niña.  Recent research suggested that climate change made extreme heat in Texas some twenty times more likely than in equivalent La Niña years in the mid-20th century.  Thanks to Peter Frumhoff (UCS) and Andrew Revkin (New York Times) for suggesting the language change.


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  • rusureuwant2know

    Oh, you forgot colder winters – or did you realize how foolish that claim was? You do realize, I hope, that the “global average temperature” is not a real temperature and won’t melt ice where it’s below freezing – especially in Antarctica where it’s -50 to -100F right now. While we have summer, they have winter, so the heat waves we are experiencing here are not having a global impact in the way you keep putting forth.

    • http://www.ucsusa.org/news/experts/brenda-ekwurzel.html Brenda Ekwurzel

      Thank you for your comment on the northern hemisphere and southern hemisphere seasonal difference. The long-term trend in the Antarctic Peninsula shows rapid warming since records began over 50 years ago (http://bit.ly/QyRtRl).

      • http://www.ucsusa.org/news/experts/brenda-ekwurzel.html Brenda Ekwurzel

        Rick, as you point out there are more extreme weather events than what are depicted on the infographic. One of the quick comprehension recommendations was to limit the number of circles we depicted to no more than six. So we weighted it toward the strongest evidence language used in the report as well as to two categories that we often get asked about – tornadoes and hurricanes. You’re your question of location is important. When the SREX report mentions extreme events they make global assessments as well as regional assessments. For example, the statement for historical temperature observations is as follows: “It is very likely that there has been an overall decrease in the number of cold days and nights, and an overall increase in the number of warm days and nights, at the global scale, that is, for most land areas with sufficient data. It is likely that these changes have also occurred at the continental scale in North America, Europe, and Australia.”

    • Rick Clayton

      You do realize that “extreme weather events” include hotter, drier summers and colder wetter winters (as well as floods and droughts). The likelihood of extreme events occuring is continually increasing due to climate change.

      Also, where is “here”? Do you mean the heat waves/droughts in North America, South America, Europe, Asia, Australia, or the Artic? … That sounds rather global to me.

  • http://www.jilljh.blogspot.com Jill


    I am a huge solar community share advocate, and as I am currently enrolled in college in Buffalo, NY, and have lived in places on the west coast and in Manhattan, I wonder what weather studies are necessary in understanding whether a city, a community, or a college may be a candidate for a solar initiative grant?


    • http://www.ucsusa.org/news/experts/brenda-ekwurzel.html Brenda Ekwurzel

      Jill, you bring up a good point about weather’s influence on solar energy as it is one of the six basic factors (http://1.usa.gov/OmwcEz) that determine how much of the sun’s energy reaches a particular spot on earth. During an intense rainstorm with thick clouds overhead the sunbeam could be reduced nearly entirely (http://1.usa.gov/OmwcEz). The others are geographic location, time of day, season, and local landscape. The influence of geographic location in the US (http://1.usa.gov/PBewtw) means the southwest has the high solar energy potential. Some of the more proactive incentives for solar installations in communities (http://bit.ly/MDzhlw) can currently be found in CA.