For the hundreds of climate-change activists who've camped out by Heathrow Airport this week, there is just one way to reduce aircrafts' carbon footprint: stop flying.
"Aviation is a luxury we can live without," says a protester named Merrick. Air travel, he says, is booming, multiplying greenhouse gases just as the climate-change imperative starts to bite. "It has to be scaled right back."
As protesters plan an unspecified action this Sunday, aircraft engineers, scientists, and climate experts around the world are urgently assessing if technology, taxation, and rationing – or a combination of all three – is required to stop aircraft from overbalancing the climate-change equation.
The statistics look ominous. Aviation currently contributes about 3 percent of global carbon emissions, but air travel is growing at some 5 percent a year, meaning numbers of air passenger kilometers will triple by 2030. Boeing estimates that aircraft numbers will double to more than 30,000 in little more than a decade.
Added to this is the complication that aircraft do not just give off carbon dioxide but nitrous oxide, thought to have at least double the impact of CO2, and condensation trails, which also may contribute to global warming.
For some, including President Bush's senior environmental adviser James Connaughton, the answer lies chiefly with technology. Aircraft manufacturers are constantly improving design of bodywork and engines, deriving greater fuel efficiency that reduces carbon emissions.
A British study group dubbed Omega (Opportunities for Meeting the Environmental Challenge of Growth in Aviation) is looking at a range of technological and other factors, including aircraft design, sustainable fuels, and open rotor-propelled aircraft that reduce fuel burn, to assess how they could mitigate aircraft pollution.
Boeing last month unveiled the 787 Dreamliner, which it says will use 20 percent less fuel than similar-sized aircraft. The UN International Panel on Climate Change says perennial improvements have made planes 70 percent more efficient than they were 40 years ago. Another 40-50 percent improvement can be expected over the next 30 years.
The problem, climate experts say, is that current projections indicate that air travel is set to grow 400 percent in the same time period.
"Efficiency is only set to improve at 1 or 2 percent per year at best, while the number of passenger kilometers is growing at 5 or 6 percent," says Peter Lockley, head of policy development at the Aviation Environment Federation, a British think tank. "So emissions are going up steadily in the gap between the two."
Then there are alternative fuels. Radical concepts like hydrogen-powered aircraft are still considered to be decades away. But serious work is being done on biofuels as an alternative to kerosene in aircraft. Last year, British entrepreneur Richard Branson promised to plough all profits from his air and rail companies into a new business, Virgin Fuels, that would fund development of biofuels.
Scientists are skeptical, though, of the potential for running jets on biofuels. Then there is the area of land required to produce fuel in sufficient volume. Already, environmentalists are concerned at the way rainforest is being destroyed to make way for palm oil, a biofuel crop.
Lockley says that one study concluded that supplying the US commercial fleet with a 15-percent mix of biofuel would require planting an area the size of Florida with soya beans.
Given the limited prospects for a technological solution, a growing body of opinion is arguing for efforts to manage demand for air travel. "What matters is the next 10 to 15 years, and technology can do very little in that time frame," says Kevin Anderson of Britain's Tyndall Centre for Climate Change Research. "The principal issue is to reduce the rate of growth of air travel."
Experts point to several options. Europe is planning to include aviation in its emissions-trading plan starting in 2011. The hope is to set an example to the rest of the world, chiefly China and India, where aviation growth is surging, that concerted efforts can make a difference.
Airlines will get a limited number of CO2 permits that can be traded; top polluters will have to buy additional permits, hurting their bottom line. The idea is to give airlines incentive to operate cleaner aircraft; higher ticket prices may result as well, reining in demand.
But experts note that caps will be set fairly high, weakening the imperative; ticket prices are expected to rise by only a couple of euros, if that. Consumer behavior may thus be little affected.
An alternative is direct taxation. John Stewart, chairman of AirportWatch, a British movement opposed to aviation growth, says air travel in Britain, at least, is "artificially cheap" because there is no tax on aviation fuel and the industry "really doesn't pay the cost of the pollution and the noise that it generates."
He says without a radical price change, it will be impossible to change the mind-set of a generation that thinks little of hopping $20 flights for weekend pursuits. Some have lobbied for cigarette-style health warnings on ads for air travel and long-distance holidays, but Mr. Stewart argues that the only way to change behavior is to hit the pocketbook.
"The first step is for governments to get together across Europe and come to an arrangement whereby aviation is taxed, to cut demand in particular for short-haul flights," he says. Stewart notes that 45 percent of all flights in Europe are less than 500 kilometers (310 miles) in distance. "The French and Germans are showing that if you invest in good railways, you can persuade people to travel by rail and not by air."
But it's not just about leisure travel. Business travel makes up, by some estimates, about 40-50 percent of all air travel. One element of the British Omega project is a study that looks at how business can reduce its aviation carbon footprint.
Keith Mason, who is spearheading the study, say it involves persuading businesses to measure the carbon they consume, choose flights that are not just the lowest cost but are least environmentally damaging, use rail where possible, and make greater use of videoconferencing and webcast solutions.
"We are aiming to come up with a range of practical tools that will help companies start managing their carbon consumption," says Professor Mason. One company, PriceWaterhouseCoopers, has, he notes, introduced an internal 'carbon budget' whereby its 1,000 top travelers must reduce their CO2 footprint by 20 percent.
Indeed, some experts believe that personal carbon budgets – rationing – may be the only solution.
"It's too late for voluntary mechanisms," says Dr. Anderson. "Carbon allowances are the only fair way to deal with this."