Leading article: All these flights abroad could end up costing us the earth

Just ponder this as you flick through those holiday brochures and even the travel pages of this newspaper to brighten up the grey January days. The number of trips abroad taken by Britons is set to rise again this year - from 65 million visits last year to an expected figure of 77.3 million by 2010. And a decade after that it is projected to top the 100 million mark. Fuelled by cheap flights, the British wanderlust seems far from sated.

Fuelled is the operative word here. For the cost of cheap flights may be low in pounds sterling but the cost to the environment is high. For it is not just we Brits. The affluent world made a staggering total of 433 million overseas visits to foreign countries in 2005 - and the total is set almost to double to more than 838 million a year by 2020.

The ecological impact of all these air miles rarely enters into the public consciousness. True, there are a few far-sighted souls who have pledged themselves to the notion of "eco-tourism" - to take into account the environmental and social impact of holidaymakers on the places they visit.

The idea of "responsible" travel which conserves the environment and is beneficial for local people has not taken off. There were just five million eco-tourists worldwide last year. But many of them meant by that wilderness adventures or travel to destinations where flora, fauna and untamed landscape are the primary attractions. All too often, however, what this amounts to, in countries such as Kenya, Ecuador, Nepal, Costa Rica and Madagascar, is a stay in a hotel plonked in the middle of splendid landscape, to the detriment of the ecosystem. And even where this is not the case some of the destinations visited by eco-tourists, such as Antarctica, are so environmentally sensitive they can be damaged even by careful travellers.

But the key problem, as with the rest of the tourist industry, is that it is the flight to foreign destinations - whether exotic or just to another indistinguishable four-star Mediterranean hotel serviced by the ubiquitous shopping mall - which does the real damage. A 10,000km return journey consumes about 700 litres of fuel per person. A flight to Ontario, for example, produces as many greenhouses gases per passenger as the average car creates in an entire year.

There are moves afoot for a tax on aviation fuel - which, extraordinarily, is still duty free. The French proposed this at the G8 summit at Gleneagles with the revenue to be set aside for additional aid to the Third World. At the otherwise disappointing UN 60th anniversary summit in New York in September, some 66 countries backed environmentally friendly levies on international air travel.

The idea needs to be embraced more widely - most particularly by the United States, Germany and Japan. For though French citizens are forecast to more than double their international travel by 2020, it is Germans who make the most trips. They overtook the US in 2003, with 86 million foreign visits last year. And the Japanese are set to almost quadruple their travel from 36 million trips a year in 2005 to 139 million by 2020. By then China will not be far behind.

All this will have a significant impact on the ever-increasing production of greenhouse gases. The levels of tax on air travel being discussed will go little way to address the scale of this problem. Either they must rise substantially or we must curb our air travel voluntarily. Most of us have not yet reached the level of consciousness to make sacrificial decisions for the good of the planet. But there may come a time when such matters are not a matter of pricking the conscience of the affluent so much as a necessity for the whole world.