Extreme Weather and Climate Change

Nothing makes me cringe more than being asked if an extreme weather event is caused by climate change.  The answer is not simple and straightforward and in most situations the linkages, if they exist, are complex.

Currently, my go-to guide on the subject is a report prepared the Committee on Extreme Weather Events and Climate Change Attribution for the National Academies of Sciences, Engineering, and Medicine and published by National Academies Press in 2016 (available by clicking here).  It is freely available as a pdf.

I have long been concerned about the conflation of weather and climate by the media and special interest groups, and the report expresses my concerns very well.  For example, they summarize my "cringiness" quite well:

"Although climate scientists are frequently asked 'Was a given observed weather event caused by climate change?' we believe this is a poorly formed (or ill-posed) question that rarely has a satisfactory answer." 

They highlight the reasons for this.  For example, not all extreme events are purely meteorological (e.g., droughts, floods, and wildfires).  In other cases, trends may be difficult to detect given the large size of natural variability and/or the quality of the data.  Snow provides a good example.  It is easier to detect trends in some snow measures, such as the fraction of precipitation that falls as snow, than other snow measures, such as water equivalent in the snowpack on 1 April (see Western Snow Trends and Global Warming: Part I).

The authors of the report describe these issues, concluding that:

"Confidence in attribution findings of anthropogenic influence is greatest for those extreme events that are related to an aspect of temperature, such as the observed long-term warming of the regional or global climate, where there is little doubt that human activities have caused an observed change" 

They provide a chart illustrating the state of attribution science for specific event types.  The effect of climate change on event type and confidence in our ability to attribute specific events to human-caused climate change is high for events at upper right and low for events at lower left.  Events at lower left aren't necessarily affected by climate change, but it is very hard to draw conclusions. 

Severe convective storms are the farthest to the left.  Marshall Shepherd, one of the author of the report, recently posted an article on climate change and tornadoes for Forbes.  If you want to know more, the article is available here. 

Ultimately, weather is not climate.  Instead, climate is the statistics of weather.  Although we can't really say that climate change caused an extreme event, we can work to estimate the how much climate change increased or decreased and events magnitude or the probability of an event occurrence.  This is an area of active research where significant progress is being made, although we don't yet have all the answers. 

Let's not take our eye off the ball.  The threat of climate change is very real.  Let us work to create a a society more resilient to the extreme weather of today and the extreme weather of tomorrow.  This doesn't require the conflation weather in climate in a way that is often happening today. 

Postscript: I'm pretty certain I've written this post or something like it a few times before!