Back in 2014 I made a global warming bet with fellow economists Bryan Caplan and Alex Tabarrok about global temperatures over the following 15 years (2015-2029) compared with the previous 15 years (2000-2014). The bet can be illustrated with this graphic, so I’m calling it our “traffic light” bet:


The short version is that the red line at 0.92°C represents average temperatures during the first 5 years of our 15-year betting period; the yellow line at 0.67°C shows the finish line for the bet: if the red line is above the yellow line after another 10 years then I win the bet, and otherwise they win the bet; and the green line at 0.55°C shows how high average temperatures can be over the next 10 years for Bryan and Alex to win the bet. The good news for Bryan and Alex is that they can still win our global warming bet if average temperatures for the next 10 years are about 0.4°C lower than the average for the past 5 years! (The bad news is that the green line is moving down: last year it was at 0.58°C.)

The long version (mostly copied from last year) starts with a 2014 tweet from Bryan to me:


Bryan and I ended up making a bet about whether the NOAA global land-ocean average for 2015-2029 (the first 4 years of which is represented by the red line) would be more or less than the average for 2000-2014 plus 0.05°C (the yellow line). Bryan asked me for 2 to 1 odds, but I was so confident I gave him 3 to 1 odds. (Keep in mind that IPCC 2007 projected warming of 0.2C per decade for the next two decades, and IPCC 2013 said something similar.) I was also confident enough to suggest an “exploding bet”—where the amount doubles every year unless someone gives in—but he declined. So Bryan and I bet his $333.33 against my $1000, and then for good measure I gave his colleague Alex Tabarrok 5 to 1 odds, his $200 against my $1000, on the same topic:



So far it’s not looking so good for the Bryan and Alex side of the bet. I’ll post another update in a year or three; for now let me confess that I’m not so good at econometrics—I’ll leave that to folks like Steve Levitt, who noted in his 2009 book Superfreakonomics that “a little-discussed fact [is that] over the past several years, the average global temperature… has in fact decreased“—but I will humbly suggest that you take a good look at the following image, which shows the raw data from the “traffic light” graph, before you write me to ask if you can throw your money on the table as well:


This graphic also suggests that Bryan may not be quite right when he says “But we’re only a few years into a 15-year bet, so I won’t start fretting until 2025 or so.” In any case, I’m happy to bet more about this with Bryan, Alex, or anybody else with good references and great credit!