Attribution science basics: climate change and extreme weather

Extreme weather events such as life-threatening heat waves and record-breaking downpours are part of the natural climate system, but by definition they are relatively infrequent—even rare. In recent decades, however, some kinds of extreme weather events have become more common. Science has linked some of these general increases to climate change, but it was challenging for researchers to clarify the influence of climate change on specific extreme weather events. Advances in computer processing power over the last decade and improved methods for sorting out the many factors that contribute to weather are allowing scientists not only to determine the extent to which climate change contributed to some extreme weather events but also to say with confidence that certain extreme weather events would not or could not have occurred but for climate change. This is the science of extreme event attribution.

• Weather is what’s happening in the atmosphere over a short period of time—hours, days, or weeks. Climate is a description of atmospheric behavior over longer periods, typically years, decades, or longer.
• In the words of Dr. J. Marshall Shepherd, past president of the American Meteorological Society, “Weather is your mood and climate is your personality.”

• “Extreme weather” is a relative term meaning especially intense or very severe weather compared to what normally occurs over a baseline period of time, such as the average for the 20th century.
• Because there is not a widely accepted technical definition, every analysis must define “extreme”.

• Countless scientific papers, as well as assessments of those papers by the U.S. National Academies of Sciences, Engineering, and Medicine (NASEM), the U.S. National Climate Assessment, the Intergovernmental Panel on Climate Change (IPCC), and others have concluded that the Earth’s climate system is warming and that most or all of the warming in recent decades has been caused by human activities—primarily the burning of fossil fuels.
• Knowing whether and how this climate change is contributing to extreme weather (a field sometimes called “extreme weather attribution”) can help decision-makers:
  • predict future prevalence and severity of extreme weather events;
  • determine how to balance efforts to limit climate change (“mitigation”) versus efforts on preparedness and infrastructure resilience (adaptation);
  • identify whether certain efforts to address climate change could save more money than they cost.

• For some kinds of weather events, in some places, there is a clear connection between climate change and weather extremes. When documenting these connections, however, it is important to define terms precisely.
  • For example, an “increase” in a kind of extreme weather can mean “happens more frequently” or “is more extreme when it happens” (in intensity or duration) or both. Careful wording can prevent confusion.
  • It’s also important to distinguish between the influence of climate change on certain classes of extreme weather, and climate change’s contribution to a specific extreme weather event.

• Climate scientists are confident that climate change is already causing an increase in the frequency and/or intensity of the following types of extreme weather or weather-related events (listed here from highest to lowest confidence):
  • multi-day temperature extremes, primarily heat waves
  • drought and heavy rains
  • tropical storms, including hurricanes and typhoons
  • wildfires and perhaps extreme convective storms (e.g., afternoon thunderstorms).
• Wildfires are considered “weather-related,” since they are not meteorological events themselves but are linked to weather extremes such as drought and lightning storms.
• Also not meteorological events per se but among the most certain weather-related impacts of climate change: coastal flooding due to climate-change-linked sea level rise.