Correlation and Causality

Today’s example of lazy thinking uses the religious debate of the day: global warming. If you’re a person who would rather not let the facts get in the way of a good opinion, it’s time to hit the eject button. But be warned: ignorance is not bliss.

There is a wonderful scene in Al Gore’s Inconvenient Truth where he shows this graph and he sniggers that clearly, CO2 causes global warming:

Yet there are at least another 3 possible explanations:

Correlation does not automatically equal causality. While a rise in variable A (CO2) could cause a rise in variable B (temp), it could also be the other way around…

... which is exactly what some scientists have been arguing: based on ice core samples, CO2 follows temperature, therefore temperature, they argue, is the cause for rising CO2 levels, not the other way around.

Further, variables A and B could be caused by an external variable, C. In this third case, neither A causes B, nor B causes A. Global warming has such a possibility : the variance in the tilt in the earth’s axis. There is compelling data suggesting that the most likely cause of the ice ages is due to fluctuations in the intensity and the distribution of solar radiation caused by changes in the tilt in the Earth's axis, as first theorised by Milutin Milankovitch, in 1938. (or it could also be an external variable that we have not yet identified – that’s also a possibility)

The fourth (though in this case probably unlikely) reason for the correlation of CO2 and temperature, could be pure coincidence.

In summary, we’ve already witnessed the judgment errors and risk of “slam dunk” conclusions drawn from convenient data subsets. Let's learn from history.

That is not to say that we should not worry about burning fossil fuels or doing more with less, striving for a zero carbon footprint, baking in sustainable practices or pricing in the true cost of production to incentivise change – far from it. Just that we need to avoid “Ready, Fire, Aim” conclusions and apply appropriate thinking to causality.

Update (15 Dec '08): Data points from Mars supporting the Milankovitch theory: http://www.space.com/scienceastronomy/081204-mars-climate-cycles.html