AFF's Doublethink

As always, my role is the amateur plunging into a pool of expertise. On Thursday, George Will quoted Mark Steyn to the effect,

If you’re 29, there has been no global warming for your entire adult life. If you’re graduating high school, there has been no global warming since you entered first grade. 

Kevin Drum ismissed Steyn’s comment as a denialist talking point:

Global temps have been trending up for over a century, but in any particular year they can spike up and down quite a bit. In 1998 they spiked up far above the trend line and last year they spiked below the trend line. So 2008 was cooler than 1998. 

Kevin provides the relevant graph to make his point.  

I found the graph interesting for several reasons. First of all, it shows that global temperatures were falling from around 1940 through 1975.  Then there were big jumps in the late 70s, late 80s and late 90s.  Whatever’s happening to our planet’s temperatures, they certainly aren’t holding steady.

Anyhow, Kevin’s post (along with those of Ezra Klein and Ryan Avent) led Jim Manzi to defend Will and Steyn on the grounds that they are, at a minimum, technically correct, since average temperatures haven’t risen over the past ten years, regardless of what’s happened over the past thirty-five. Jim also raises some very interesting questions about how much data we actually need to know whether the globe is warming or not. He admits that ten years of stability doesn’t mean much. But how many years of data do we actually need to come up with good answers about the existence/extent of global warming?

Finally, there’s one last response from Kevin in which he takes Jim to task for saying that temperatures have been stable over the past decade, since they actually have risen (by one-fifth of a degree (Celsius) to be precise). Kevin’s baseline year is 1999. Compared to 1998 (a hot year), it seems temperatures have actually fallen slightly. I think Kevin’s argument would’ve been much more robust if he said that the average temperature for the past ten years was significantly higher than the average temperature for the decade before that.

So all I need to figure out is what the average temperature will be a decade from now.