Tony Blair would need to plant 7,200 trees – a veritable forest – to offset the amount of carbon dioxide that he is burning up as he embraces the Earth to say goodbye, The Times has calculated.

For Mr Blair, who sees himself as a champion of the crusade against climate change, is leaving one almighty carbon footprint as he strides the world stage in the twilight of his premiership.

He has flown to Washington to bid farewell to President Bush, to Iraq to address stony-faced British troops; and he plans an odyssey across Africa, revisiting the scene of past triumphs.

He was quick to shake the hand of Nicolas Sarkozy, before he became President of France. International summits in Germany and Brussels beckon. In eight separate trips between announcing his departure and leaving No 10 next month for the last time he will have clocked up 34,300 miles.

Worse, his carbon footprint is not like those of ordinary citizens. As Prime Minister, he generally flies on commercially chartered aircraft, usually a British Airways Boeing 777.

A leading carbon offsetting company calculated that 34,300 miles in such an aircraft, with about 20 take-offs and landings, would burn 600 tonnes of CO2 best practice, this should be doubled to take account of the impact of flying at high altitude, equating to 1,200 tonnes of CO2 (equivalent). Another carbon offsetting specialist suggested a rule of thumb that one tree would, over a lifespan of 99 years, soak up three quarters of a tonne of CO2 footprint amount to 900 more mature trees.

Here, however, things get tricky. Allowing for death, disease, felling and other mishaps, experts suggest you need to plant 5, 8 or even 12 trees to produce a single mature specimen. Taking a conservative figure of 8, Mr Blair would need to plant 7,200 saplings to ensure enough survived to old age. Blair-wood Forest, perhaps?

Alas, the final legacy of Mr Blair’s swansong is unlikely to be as tangible. The Prime Minister is an enthusiastic, if some would say late, convert to “green” travel. But in the fast-moving world of carbon offsetting, tree planting is so 1990s.

Like many such schemes, the Government’s own offsetting fund, through which Mr Blair is offsetting all his official travel since April 2005, focuses on so-called clean development: small renewable energy projects in developing countries.

Rather than a forest in his name, Mr Blair may help to endow a digester that turns pig effluent into methane gas to generate electricity in Mexico, or hydroelectricity generators in Fiji. Both are supported by EEA fund management, the Government’s recently appointed carbon offsetting broker.

His carbon credits may even end up financing wind turbines on the tin roofs of mud huts in Indian villages, a neat contrast with David Cameron’s own contribution to saving the planet in Notting Hill.

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