Which causes the most global warming, cheap flights to Marbella, driving a 4x4, belching livestock or burning down the rainforest?
Not a lot of people know this, but first is burning down the rainforest, which puts more carbon into the atmosphere than all forms of transport each year.
As Sir Nicholas Stern points out in his report on the economics of climate change last year, curbing carbon emissions would be a very cost-effective way of tackling global warming.
Aviation currently accounts for around three per cent of the EU's greenhouse gas emissions. Sir Nicholas says global emissions stack up like this: power stations 24 per cent; transport, including aviation, 14 per cent; agriculture 14 per cent; industry 14 per cent; land use, principally deforestation, 18 per cent; fuel used in buildings eight per cent; other sources five per cent.
So aviation bosses, such as Michael O'Leary, of Ryanair, have a slim justification for feeling that they are being picked on in the environmental debate.
On the other hand, aviation is the fastest rising source of carbon emissions, having doubled in 10 years and looking to rise by 142 per cent or so by 2020.
It is human nature to rankle at unfairness. However much we may all hypocritically enjoy the benefits of cheap flights, we must face the fact that aeroplanes are free riders in a world of carbon rationing.
Ken Livingstone proposes to charge drivers of 4x4s £25 every time they drive into central London, yet the plane overhead is travelling on untaxed fuel and its emissions are not even included in the inventories compiled for the Kyoto treaty.
And a flight from New York will have produced as much carbon as it takes to drive 10,000 miles or heat a house for a year. Also, emissions in the stratosphere cause two to four times the pollution at ground level. So one can have some sympathy with Ian Pearson's off-message howl of frustration at Mr O'Leary's arrogance when the airline boss suggests boycotting the EU carbon-trading scheme.
You just wonder whether Mr Pearson's target ought not to have been Tony Blair who has so far got away with talking-up climate change while backing the aviation industry's market strategy of predict-and-provide.