What accounts for the relationship between total world GDP and world average temperature? For background, here is some data on GDP, from chapter 5 of Brad DeLong's Macroeconomics textbook (I did the multiplication to get the last column).
world population (billions)
GDP per capita (thousands of dollars)
World GDP (trillions of dollars)
So, at a world level, we have been spewing out GDP at a much higher rate in the past ten years than we did even 25 years ago, and at a way, way higher rate than in the early 20th century.
I looked for data on global temperatures in this century, and I wound up, for better or worse, at this Wikipedia page. What it shows, I think (the scale on the graph is not explained clearly enough for a layman like me), is that temperatures rose somewhat linearly from 1900 to 1940, took a break from 1940 to 1980, and then rose at a comparable linear rate from 1980 to the present.
The reason I bring this up is this: if I look at the world GDP data, and I assume that emissions of greenhouse gases track world GDP, then the accumulation of greenhouse gases in the past 10 years should totally swamp anything up to 1980. So, I would think that we would see either no temperature change prior to around 1990 or else an enormous spike going on right now. I don't see any reason why temperatures should be following a straight line up from 1900 to today.
Another way to put this is that I think that if global warming is caused by humans, then we ain't seen nothing yet. I don't see how we can use the past as any guide to the future. That is not an effort to deny global warming--I just think in designing policies going forward we're taking a crapshoot any way you look at it.
a) The Global Warming alarmists are being way too conservative, perhaps because they have not grasped how fast GDP has gone up. In fact, temperatures are in the process of spiking now and will spike a lot in the next ten years--we'll see something worse than what the so-called worst-case scenarios project.
b) Emissions of greenhouse gases don't really track with world GDP. We're only spewing out a few more tons of greenhouse gases, even though world GDP has shot up. (But see the more link, below.)
c) There is something about the relationship between greenhouse gases and temperature that I am missing. That is, there is a mathematical way to reconcile a low level of emissions in 1900 and a high level of emissions in 2000 with a rate of temperature increase that is approximately the same.
I'm open to any possible answer, but if I had to guess right now, I think I would go with (b). But I would have expected a reasonably close relationship between greenhouse gas emissions and GDP. If that relationship is weak, then I wonder if there are aggregate variables that do track well with greenhouse gas emissions.
So I welcome pointers to papers that speak to this issue.
Of course, William Nordhaus is the go-to guy on this. I think figure 3-3 in this chapter gives me some relevant data. The blue column labeled "W" shows that world CO2 emissions relative to GDP have been declining at an average rate of 1 percent per year since 1975. But that only makes the global emissions rate in 2000 2.1 times the rate in 1975 instead of instead of 2.6.
It still suggests to my layman's way of thinking that a graph of temperature change over time should not even be able to show global warming as a 50-year phenomenon or a 100-year phenomenon. The temperature increases for the latest 10 years should be a huge multiple of any increase that occurred prior to 1950.