In a recent article in the New York Review of Books, Yale University economist William Nordhaus presents a graph showing changes in global mean temperatures from 1880 to 2011. Take a look. The data appear to be annual and he is graphing changes in temperature, not absolute level. So at first I thought he must mean that each year, the temperature changed by the amount on the graph compared to the last year. So, for example, that would mean that in 1945 (I'm eyeballing it here), where there is a huge spike, the temperature changed by almost 0.5 degrees C in one year.
But a little reflection shows that that can't be the case. (I'm not assuming that Nordhaus thinks it is the case. I'm simply clarifying for people who initially have my problem with understanding the graph.) Here's why. Look at the changes from about 1950 to 2011. They average at least 0.4 degrees C. So that would mean that the world has warmed by 60 times 0.4 degrees C or 24 degrees C! Clearly that hasn't happened. QED.
So what does all this mean? It must mean that Nordhaus is not graphing changes in temperature from the year before, but, rather, changes from some baseline temperature. It's not clear what baseline he has in mind. One might think 1880, but it's not clear. Read this way, what his graph says is that in 2011, the global mean temperature was higher by 0.8 degree C than it was in 1880 or some year earlier than 1880. Notice that that interpretation is consistent with two other well-agreed-on claims: (1) global temperatures are about 0.8 degrees C higher than they were 80 years ago, and (2) global temperatures are slightly lower than they were 10 years ago.