"Aviation is a luxury we can live without," said a protester named Merrick. Booming air travel, he said, is multiplying greenhouse gases just as the climate-change imperative starts to bite. "It has to be scaled right back," he said.
The protesters are also targeting the proposed addition of a runway at Heathrow. As they planned an unspecified action for today, aircraft engineers, scientists and climate experts around the world were urgently assessing if technology, taxation and rationing — or a combination of the three — are needed.
The statistics look ominous. Aviation contributes about 3 percent of global carbon emissions, but air travel is growing at about 5 percent a year, meaning numbers of air-passenger miles will more than triple by 2030. Boeing estimates aircraft numbers will double to more than 30,000 within about 10 years.
Added to this is the complication that aircraft do not just give off carbon dioxide but nitrous oxide, believed to have at least double the impact, and condensation trails, which also may contribute to global warming.
Some, including President Bush's senior environmental adviser, James Connaughton, believe the answer lies chiefly with technology. Aircraft manufacturers constantly are improving design of bodywork and engines, deriving greater fuel efficiency that reduces carbon emissions.
A British study group, 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.
In July, Boeing unveiled the 787 Dreamliner, which it says will use 20 percent less fuel than similar-size aircraft.
However, when the International Air Transport Association — the trade group for the world's major airlines — in June called for manufacturers to produce a "zero-emissions jet" within 50 years, senior Boeing executive Mike Cave cautioned that the technology to do that is not in view.
Cave instead recommended a broader approach, with the whole industry — not just airplane makers — working to create more efficient engines and aerodynamic technology, improved air-traffic control systems, and better ground taxiing procedures. To offset any remaining emissions, he supported a global carbon-emissions trading system for aviation.
The U.N. International Panel on Climate Change says perennial improvements have made planes 70 percent more efficient than they were 40 years ago. An additional 40 to 50 percent improvement can be expected in the next three decades, the panel says.
The problem, climate experts say, is that current projections indicate air travel will grow 400 percent in the same period.
"Efficiency is only set to improve at 1 or 2 percent per year at best, while the number of passenger [miles] is growing at 5 or 6 percent," said 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."
There are alternative fuels. Concepts such as hydrogen-powered aircraft are considered to be decades away. But serious work is being done on biofuels as an alternative to kerosene in aircraft. Entrepreneur Richard Branson promised last year to funnel all profits from his air and rail companies into a new business, Virgin Fuels, that would pay for development of biofuels.
In April, when Virgin Atlantic ordered 787s, the airline also announced an environmental partnership with Boeing that includes a joint biofuel project aimed at developing sustainable fuel sources.
Scientists are skeptical, though, of the potential for operating jets on biofuels. And there is the amount of land required to produce fuel in sufficient volume. Environmentalists already are concerned at the way rain forest is being destroyed to make way for palm oil, a biofuel crop.
Lockley said one study concluded that supplying the U.S. commercial fleet with a 15 percent mix of biofuel would require planting an area the size of Florida with soybeans.
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," said 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 plans to include aviation in its emissions-trading plan starting in 2011. The hope is to show the rest of the world, chiefly China and India, where aviation growth is surging, that concerted efforts can make a difference.
Airlines will receive a limited number of carbon-dioxide 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.
But experts note that caps will be set fairly high, weakening the imperative, and ticket prices are expected to rise only slightly, if that. Thus, consumer behavior may be little affected.
An alternative is direct taxation. John Stewart, chairman of AirportWatch, a British movement opposed to aviation growth, said that without a radical price change, it will be impossible to change the mind-set of a generation that thinks little of hopping cheap flights for weekend pursuits. Some people have lobbied for cigarette-style health warnings on ads for air travel, but Stewart argues that the only way to change behavior is to hit the pocketbook.
He noted that 45 percent of all flights in Europe are less than 310 miles. "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 only about leisure travel. Business travel makes up, by some estimates, 40 to 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 leading the study, said it involves persuading businesses to measure the carbon they consume, choose flights that are not just the cheapest but are least environmentally damaging, use rail when possible and make greater use of videoconferencing and Internet solutions.
"We are aiming to come up with a range of practical tools that will help companies start managing their carbon consumption," Mason said. He noted that one company, PricewaterhouseCoopers, has introduced an internal "carbon budget" whereby its 1,000 top travelers must reduce their CO2 footprint by 20 percent.
Some experts think similar personal carbon budgets — rationing — may be the solution.
"It's too late for voluntary mechanisms," Anderson said. "Carbon allowances are the only fair way to deal with this."