(CNN) -- The airplane has become, for many, climate change public enemy number one. And for good reason, say environmentalists. The air travel sector now carries the label of "the world's fastest growing source of greenhouse gases" according to Friends of the Earth (FoE), with airplanes pumping out more than 600 million tons of carbon dioxide (CO2) every year. That's nearly as much CO2 as the African continent annually expels.
A plane leaves vapor trails as it flies over London. Concern is growing over aviation's environmental impact.
The aviation industry's official contribution to the world's greenhouse gas emissions tally, however, gives a very different impression. According to the International Air Transport Association (IATA) and the U.N.'s International Panel on Climate Change (IPCC), aviation globally only accounts for around 3 percent of greenhouse gas emissions.
That figure has environmentalist up in arms. They say it fails to take into account a process known as radiative forcing, where the impact of emissions from planes in the upper atmosphere -- according to the UK-based Aviation Environmental Federation (AEF) -- are greater by a factor of 2.7 -- or more. Once radiative forcing has has been factored in, the total contribution of aviation to greenhouse gas emissions looks more like 12 percent to 13 percent.
Many still dispute the science behind radiative forcing. However, in a victory for its proponents earlier this year, the UK's aviation minister said in a parliamentary session that aviation did in fact represent 13 percent of greenhouse gas emissions once this was taken into account. (That being said, her figures were based on flights departing the UK and didn't take into account flights arriving which would push the number up even more.)
Whatever the figure, it's going to keep rising, but by what amount is anyone's guess. Today, there are around 17,700 commercial airplanes in operation, according to the UK's Observer. In the next 20 years, the newspaper reports that another 25,600 could have arrived on the scene -- mainly in India, China and Russia. It could be more. Boeing predicts that in slightly more than a decade, by 2019, another 31,750 planes will have come into operation, according to AEF.
These new planes will be there to serve the emergence of developing countries, such as China and India on the global tourism scene. While the British may currently produce more carbon emissions from air travel per head than any other nationality, (double that of Americans), according to The Guardian, in years to come the picture could look very different. More people will be taking to the skies in the coming few years than at any other time in history and they are more than likely to come from Asia (between 1970 and 1995, aviation in Asia grew by 1,870 percent, according to the UK's Ashden Trust).
Today, around 2.2 billion of us fly each year, according to IATA. In 15 years time, according to FoE, that number will have doubled.
To accommodate what could be more than 4 billion people in our skies, governments around the world are continuing to invest heavily in the aviation industry's growth and development. What that basically means is we are going to see a lot more airports. By AEF estimates, we will have on our hands 1,300 new international airports by 2050, being built at a rate of two a month.
To the environmentalist, that is clearly at odds with the higher aim of combating global warming. To the economist, it's a no-brainer.
There is an overwhelming economic argument for keeping planes in the sky. Tourism, for one, is one of the world's most profitable industries, if not its most profitable (it's certainly the world's No. 1 employer). According to the World Travel and Tourism Council (WTTC), global tourism will generate around $7 trillion this year; keep 230 million people in jobs; and contribute 10 percent of global GDP.
One of the things that keeps tourism in its position as such a great money generator is flying. According to IATA, the aviation industry directly and indirectly employs 28 million people, and generates at least $1.4 trillion. According to The Economist, aviation alone could be contributing as much as 8 percent of global GDP. Stopping people from flying in the hope of cutting emissions simply isn't an option.
The only option -- for governments and the aviation industry at least -- is to make flying greener. IATA says its aim is for the world's fleet to be 25 percent more fuel efficient by 2020, but ultimately it has loftier ambitions. It says it wants zero emissions planes in the next 50 years.
That has people such as Richard Branson talking a lot about biofuels. Branson says he plans to test-fly a Virgin Boeing 747-400 next year flying on nothing but biofuel, which he is designing with GE, according to Treehugger.com. Air New Zealand is another airline also investigating flying, partially at least, with the aide of biofuels.
The problem, however, with biofuels that is of concern to an increasing number of people, is the possibility that we are creating a situation where farmland normally reserved for food crops will give way to land set aside for fuel crops. For those championing the end of hunger in the developing world, this idea isn't exactly appealing.
Another way to achieve carbon neutral flying is by building hydrogen fuel cell-powered planes. The problem here is a current lack of infrastructure to support them, not to mention the costs involved in creating what would in effect be a brand new industry with brand new planes. That being said, according to the Globe Foundation of Canada, "implementation could come within 15-20 years".
And then there is solar. Solar-powered aircraft have to date not taken off on a commercial basis, despite the efforts of organizations such as NASA, for example. In 1999 NASA came up with Helios, a solar-powered by day, fuel cell-powered by night unmanned aircraft that, according to Fortune, cost a mere $15 million to build.
The idea, says NASA, was to build an aircraft which could carry out high altitude environmental science or telecommunications missions for weeks or months at a time. There would be no need for refueling stops, because there was no fuel -- which means no emissions. Unfortunately on one of its earliest test flights, it crashed into the sea, 30 minutes after take off.
(Dick Swanson, whose company SunPower designed the plane's solar cells, gave a telling insight into the solar industry's hopes for sun-powered flight, telling Fortune afterwards: "It was kind of sad, although in retrospect it wasn't a technology that has much practical value.")
One of the problems with solar flight has been speed. Despite a 247-foot wingspan, exceeding that of any jumbo jet, and 62,000 solar cells gathering energy to propel it, the Helios only ever managed to reach speeds of 19-27 miles per hour, with takeoff and landing speeds comparable to that of a bicycle, according to Treehugger.com. That's not exactly promising news for the plane loads of tourists in a hurry to start their holidays.
The other problem is weight. The heavier the load, the more energy you need to generate to carry it, so solar-powered aircraft to date have focused on being as light as possible to maximize all the energy they are getting from the sun. The Solar Impulse project, which is being unveiled to the public this month in Zurich, Switzerland, is essentially powered by lithium batteries which collect and store power from the sun during the day.
But even 24 hours' worth of full charging only gives the plane's motors power that's "equal to the amount available to the Wright brothers in 1903" according to Solarimpulse.com. (That being said, it is expected to reach speeds of 40 miles per hour, according to The Times, which is an improvement at least on NASA's efforts.)
What all this means, is that far from being able to carry "plane loads" of passengers, any solar aircraft around today can only really carry one passenger --one who can fly the plane, that is.
For passengers around the world, perhaps the medium-term plan while we wait for carbon neutral planes is to take the train? E-mail to a friend
(Sources: Friends of the Earth; International Air Transport Association; International Panel on Climate Change (IPCC); Aviation Environmental Federation; The Observer; The Guardian; Ashden Trust; World Travel and Tourism Council; The Economist; Treehugger.com; Globe Foundation of Canada; NASA; Fortune; Solarimpulse.com; The Times)