Thread by @AndrewDessler

Late night 🧵 on how to think about extreme event attribution and whether the TX cold event was caused by climate change.

I think of extreme event attribution as resting on three pillars. 1/

Pillar 1: If you have observations of the climate over a long enough period, the data can be statistically analyzed to determine the likelihood that an observed extreme event occurring today could have occurred prior to human-induced warming. 2/

But even if the observations are good enough for that sort of analysis, they usually can’t tell you whether an observed trend was caused by global warming or by something else because correlation does prove causality. 

That brings us to ... 3/

Pillar 2: Our understanding of the physics of the phenomenon.  It should be obvious why, in a warmer world, we expect to get more frequent heat waves.  This physical understanding adds to our confidence that climate change is a factor in the occurrence of heat waves.  4/

Pillar 3: Climate models. We can run climate models with and without global warming and if we see that events like the one being studied occur in the warmer world but not in the colder one, then we have another piece of evidence. 5/

If you've got all three pillars (e.g., 2003 European heat wave), we can be very confident that climate change made the event worse.  6/

If you have zero pillars (e.g., tornadoes), you can’t confidently conclude anything — at least for now. The evidence for most types of extreme events is somewhere in between. 7/

For last week's TX cold event, we have maybe one pillar. There is a plausible mechanism concerning the equator-pole temperature gradient.

But we don't have observational evidence that this event is occurring more frequently today than prior to global warming. 8/

I also don't know of any computer simulations showing that these TX cold extremes become more frequent as the climate warms. [if someone's done the analysis, let me know] 9/

Given this, I am skeptical of the connection between the TX cold event and global warming. However, there are people I respect who disagree with me — that's science. As more work on this is done, consensus will emerge on way or the other. 10/10

I wonder what @MCHammer thinks about this. 11/11