With record rainfall in Louisiana and other parts of the south this week, inevitably someone is going to ask, "Was it caused by climate change?" I cringe when I hear this question because it is so abused by ALL sides of the climate discussion:
Person X: "This event is clearly caused by climate change.....blah blah blah"
Person Y: "See they say every extreme event is caused by climate change, but the climate changes naturally and there were always extreme events.....blah blah blah"
This is the cliche dance that plays out on Twitter, the media and elsewhere.
But Dr. Shepherd my _________(fill in the blank with: uncle, favorite blog, news network, favorite twitterologist) says it is a hoax, unproven and so-on. Once you filter through the noise, the signal is clear. The majority of peer review literature notes that there is a anthropogenic "steroid" on top of the naturally-varying climate. For example, home run frequency and length varied naturally in Major League Baseball, but after the Steroid era the influence was seen in the home run statistics. But was a certain baseball players 300th homerun caused by steroids? That's an ill-posed question. The National Academy of Sciences released a report, Attribution of Extreme Events in the Context of Climate Change. Within its key findings, we find an expert plea to make the "Was it Caused By Climate Change" question extinct. Some key findings from the report are summarized, but the full 144-page report can be downloaded here. I was honored to be one of the experts that authored the report.
Asking the right questions. "Was that event caused by climate change?" just cannot be answered because natural variability almost always plays some role. When a team plays basketball, there is a natural variability in the outcome of each game, but as team practices and attains better players, there is a bit more "signal" on the potential outcome of the game. More appropriate questions are:
Are events of this severity becoming more or less likely because of climate change?
To what extent was the storm more or less intense because of climate change?
Some events are more attributable than others. Confidence is greatest for extreme events related to aspects of temperature (e.g. extreme heat and lack of extreme cold events). Attribution science is relatively young but has advanced rapidly. The National Academy panel noted that attribution is most reliable when there are sound physical principles, consistent observational evidence, and the ability for numerical models to replicate the event. These "three legs of the stool" were used as benchmarks to rate the confidence. Attribution of extreme rainfall and hydrological drought are found in the moderate confidence category. Why? There is good physical understanding of how the warming aspects might lead to greater evaporation rates (to further amplify a drought or make more water vapor available in a precipitating storm). There was only moderate confidence in the length of record and ability of model to simulate other aspects. There is little to no confidence in attribution of tornadic storms or Nor'easter-Mid-latitude cyclone type storms. There is emerging literature that may increase our confidence going forward, but the body of literature does not support a higher confidence rating at this time.
Danger Will Robinson, Danger. These ratings DO NOT mean that every single event is linked to climate change. It simply means that the science is now able to provide attribution with moderate to high confidence for a subset of events studied. The report also argues that future attribution studies need a set of standards to guide how cases are selected for study. There is a degree of randomness and opportunity in selection that hinders the broader representation of attribution studies.
Framing is also important. The Russian heatwave of 2010 was discussed in the report. In the 1960s, it was a "once in a lifetime event" (every 100 years or so). It now has become, as Panel Chair Admiral David Titley says, a "once in a mortgage event" (every 30 years) according to published studies. However, if you frame it in terms of mean summer temperature, the increase over that period is a few degrees Fahrenheit. A few degrees is still significant to the climate system but to some it may not be perceived the same way as the framing with the return period. Oh by the way, the DC area is running a few degrees above normal right now and it is quite noticeable in early March (and would be in August for sure).
The report notes that attribution studies can eventually benefit from being linked to an integrated weather-to-climate forecasting effort on a range of time scales (0 to several years). The ultimate goal would be to provide predictive (probabilistic) forecasts of future extreme events at lead times of days, seasons, or longer. With this goal in mind, it is simply ridiculous to frame "weather research" vs"climate research." They need to be evaluated as a continuum.
Now for the last time.....
I know the climate changes naturally. I promise you I didn't miss that in graduate school.
Dr. J. Marshall Shepherd, a leading international expert in weather and climate, was the 2013 President of American Meteorological Society (AMS) and is Director of the University of Georgia’s (UGA) Atmospheric Sciences Program. Dr. Shepherd is the Georgia Athletic Associatio...MORE
Dr. Marshall Shepherd, Dir., Atmospheric Sciences Program/GA Athletic Assoc. Distinguished Professor (Univ of Georgia), Host, Weather Channel's Popular Podcast, Weather Geeks, 2013 AMS President