The old man told me that, as a child, he watched in amazement as the first plane lifted off the ground. Today, he laments, two thousand million airline passengers barely even notice when their plane takes off.

Some old people were alive when the modern air travel industry started with the Wright Brothers in 1903.

Thre were a lot less humans back then, about 1.6 billion (1,600,000,000) compared with today's 6.7 billion.

But air travel has expanded rapidly and now over 2 billion passengers fly every year with at least 200,000 people flying any one time.

These numbers are growing fast and are expected to double over the next 50 years.

Early attempts by humans to fly, from Icarus and Deadalus, to Leonardo Di Vinci, resulted in gravitational failure of one sort or another. Hot air balloons, with the famous first human flight undertaken by Joseph and Etienne Montgolfier in 1783, proved more successful. The first heavier-than-air aircraft was flown by Orville and Wright in 1903, taking one man a distance of about 40 metres. Early airlines carried mail and freight. However, as aviation technology developed, it became possible to offer services for ‘self loading cargo', an industry term for human passengers. It was the helium filled dirigible airship, that would carry the first commercial passengers and form the first passenger airline in 1911.

Commercial airlines using fixed wing planes commenced in 1914 in the United States. Following the Second World War, many airlines in Allied countries were released from military contracts and changed their business models to accommodate increased demand for civil air transport, both passenger and cargo. New airline designs including the Boeing Stratocruiser, Lockheed Constellation, and Douglas DC-6 became available after benefiting from developments to high altitude bombers such as the B-29, which had spearheaded research into new technologies such as pressurization. The development of the jet engine in the Second World War soon made passenger flight much faster, and new routes around the world opened up.

Pressurization allowed airliners to travel higher in the atmosphere where oxygen was scarce but the conditions were more suited to passenger air travel. This new capacity to fly in the lower stratosphere permitted smoother air travel, above the turbulence of local weather systems which are mostly found lower in the atmosphere. As airline technology developed, planes were able to fly faster, for longer distances and carry more passengers and freight. Most airliners fly at subsonic speeds, below the speed of sound (i.e. less than 1,200 kmh). However a few airlines operate at supersonic speeds. The most famous of these is Concorde which commenced flights in 1976 and ended spectacularly in 2003.

According to the International Air Travel Association (IATA), air travel moves about 2.1 billion passengers a year (2006 figures). The livelihoods of 32 million people are tied to aviation, accounting for US$3.5 trillion in economic activity. The most commonly used passenger plane is the Boeing 737, 5,000 of which have been built since 1967. In total, it is believed that at any one time, some 2 million human beings are moving through the stratosphere in airliners. Increased air travel is promoted by low cost airlines, the growth in the global economy creating the financial resources for more and more people to fly, larger planes, plus larger and more numerous airports.

Many countries have nationally owned airlines operated by the government. Other airlines are privately owned. Airlines generally receive high levels of government subsidy, particularly during times of economic stress such as following the September 11 attacks. Following this incident, airlines were grounded across the United States, causing billions of dollars in losses. Aviation is exempt from the Kyoto Protocol, the international treaty to manage global greenhouse emissions. This leaves groups such as the International Civil Aviation Organization to develop global management of the environmental cost of air travel. These groups also play leading roles in managing the many concerns of air passengers, including terrorism, aviation-spread disease like swine flu, and spiralling oil prices.

Technological solutions to reduce the environmental impact of air travel have been proposed. Advanced, fuel efficient jet liners are not anticipated to be available for many years. An alternative solution is to develop jet fuel made from plant oils (biofuels). By using plant-based oils, the greenhouse emissions from air travel could be lowered, although not completely removed. Perhaps combining energy efficiency with biofuels will make a difference to the environmental cost of air travel. However, even then, the increasing number of air passengers will likely outstrip any benefits these solutions provide. Ultimately, a continually growing global economy and more passengers are putting severe pressure on the biophysical earth systems such as the climate. As such, the long term future of passenger air travel, and the biosphere itself, is uncertain.