At the risk of turning this column into “The Undercover Environmentalist”, I need to return to that vexed question of carbon dioxide emissions. In my first column of the year, I vowed to reduce my carbon footprint from air travel – easy enough, given that it was 50 tonnes of CO2 last year. A kind reader wrote to reassure me that I needn't lose any sleep, because the planes were making the journey anyway. Glib, I know: I've often said it myself to wind up environmentalists.

The answer reminded me of a brain-teaser that's been entertaining me for the past couple of months. Since buses often run almost empty, two people sharing a car emit less CO2 per person than do bus passengers. Shouldn't we then be travelling by car?

The BBC's in-house environmental activist, Justin Rowlatt, aka “Ethical Man”, recently pondered this question and concluded that, no, he'd still be taking the bus. Why? Because the buses are making the journey anyway.