Serendipity

I gave my talk on SE for the Planet again this afternoon, to the local audience. We recorded it, and will get the whole thing up on the web soon.

I mentioned during the talk that the global greenhouse emissions growth curve and the world population growth curve are almost identical, and speculated that this means effectively that emissions per capita has not changed over the last century or so. After the talk, Jonathan pointed out to me that it means no such thing. While globally the average emissions per capita might have remained roughly constant, the averages probably hide several very different trends. For example, in the industrialized world, emissions per capita appears to have grown dramatically, while population growth has slowed. In contrast, in the undeveloped world the opposite has happened: huge population growth, with very little emissions growth. When you average both trends you get an apparent static per capita emissions rate.

Anyway, this observation prompted me to go back and look at the data. I’d originally found this graph, which appears to show the growth curves are almost identical:

Greenhouse gas emissions versus population growth

The problem is, I didn’t check the credibility of the source. The graph comes from the site World Climate Report, which turns out to be a denialist site, full of all sorts of misinformation. In this case, they appear to have cooked the graph (note the low resolution and wide aspect ratio) to make the curves look like they fit much better than they really do. To demonstrate this, I reconstructed them myself.

I got amazingly detailed population data by year from digital survivors. They’ve done a wonderful job of collating data from many different sources, although their averaging technique does lead to the occasional anomaly (e.g. in 1950, there’s a change in availability of source datasets, and it shows up as a tiny glitch on my graph). I got the CO2 emissions data from the US government’s Carbon Dioxide Information Analysis Center (CDIAC).

Here’s the graph from 1850 to 2006 (click it for a higher resolution version):

Notice that emissions grew much more sharply than population from 1950 onwards, with the only exceptions being during the economic recessions of the early 1980’s, early 1990’s, and around 2000. Since 2000, emissions have been growing at more than double the population growth rate. So, I think that effectively explodes the myth that population growth alone explains emissions growth. It also demonstrates the importance of checking your sources before quoting them…