"Virtually Impossible Without Human-Caused Climate Change"

Here's the press release from World Weather Attribution about their study just out showing that the recent Pacific Northwest heat wave "was virtually impossible without human-caused climate change." 

There's a link to the study on that page as well.

Here's the NY Times article about it. 

They estimate this was about a 1 in 1,000 year event, but say it was so rare it's difficult to say for sure. 

This is where it gets interesting:

There are two possible sources of this extreme jump in peak temperatures. The first is that this is a very low probability event, even in the current climate which already includes about 1.2°C of global warming — the statistical equivalent of really bad luck, albeit aggravated by climate change. The second option is that nonlinear interactions in the climate have substantially increased the probability of such extreme heat, much beyond the gradual increase in heat extremes that has been observed up to now. We need to investigate the second possibility further, although we note the climate models do not show it. All numbers below assume that the heatwave was a very low probability event that was not caused by new nonlinearities.

[First, I wish scientists would stop using the word "nonlinear" when communicating with the public or journalists, because I wonder if either of them understands what it means. But that's a subject for another time.] 

The second possibility -- which I take to mean that the climate system may have passed some tipping point -- is most intriguing. Because if so, it means we don't know for sure what might be ahead. More events like the recent heat wave? Deep cold events? I don't know, heavier precipitation events, does anybody know? I think that's what's so worrisome, that this Pac NW event blew the records away by such large jumps it simply looks too strange and suspicious. 

Also

Also, this heatwave was about 2°C hotter than it would have been if it had occurred at the beginning of the industrial revolution (when global mean temperatures were 1.2°C cooler than today).

That's 3.6°F, meaning 117°F would have been 113.4°F. Still pretty bad and still a record.

Looking into the future, in a world with 2°C of global warming (0.8°C warmer than today which at current emission levels would be reached as early as the 2040s), this event would have been another degree hotter. An event like this – currently estimated to occur only once every 1000 years, would occur roughly every 5 to 10 years in that future world with 2°C of global warming.

So 117°F would be 118.8°F. In the Pac NW. Every 5 to 10 years. Does this include the possibility such a heat wave might happen in Arizona or Saudi Arabia or India? What would be the maximum temperature there. Because that sounds scarier.