The Science Connecting Extreme Weather to Climate Change

The document outlines the quantitative scoring system used to translate qualitative likelihood and confidence levels from the Climate Science Special Report (CSSR) and the IPCC Fifth Assessment Report (AR5) into a visual guide for extreme weather attribution in the continental United States.

• The methodology converts qualitative likelihood and confidence statements from the CSSR and AR5 into numerical values ranging from 0 to 99 to enable comparisons. A value of 0 indicates "little or no evidence to support a claim," while 99 represents a likelihood of "virtually certain."
• Specific numerical values were assigned to qualitative terms: 'high confidence' was given a value of 83, 'medium confidence' 50, and 'low confidence' 17. For likelihoods, 'about as likely as not' was assigned 50, while terms 'likely' and higher used lower bounds of their ranges, and 'unlikely' and lower used upper bounds.
• The resulting figure categorizes extreme weather events based on a threshold of 50. Events with values of 50 or higher are presented as having "at minimum strong support for a connection," while those below 50 are characterized as having "weak or growing support."
• The analysis prioritizes statements from the Climate Science Special Report (CSSR) over the IPCC's Fifth Assessment Report (AR5) because the CSSR contains more recent research.

APA

Union of Concerned Scientists (2018). The Science Connecting Extreme Weather to Climate Change. https://www.ucs.org/sites/default/files/attach/2018/06/extreme-weather-Appendix-B3.pdf

Chicago

Union of Concerned Scientists. The Science Connecting Extreme Weather to Climate Change. 2018. https://www.ucs.org/sites/default/files/attach/2018/06/extreme-weather-Appendix-B3.pdf.

Wikipedia

{{cite report |author=Union of Concerned Scientists |title=The Science Connecting Extreme Weather to Climate Change |date=June 2018 |url=https://www.ucs.org/sites/default/files/attach/2018/06/extreme-weather-Appendix-B3.pdf |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{unionofconcernedscientists2018science, author = {{Union of Concerned Scientists}}, title = {{The Science Connecting Extreme Weather to Climate Change}}, institution = {Union of Concerned Scientists}, year = {2018}, month = jun, url = {https://www.ucs.org/sites/default/files/attach/2018/06/extreme-weather-Appendix-B3.pdf}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }