The answer is of course the wind. The graceful sailing ships that sent the likes of Christopher Columbus and Vasco da Gama to the Americas and India are long gone, though, replaced by vast iron vessels loaded with crude oil, minerals and neat stacks of shipping containers to feed the voracious global economy.
These massive vessels, which can reach as long as three soccer fields put together, consume fuel oil that pumps out tons of planet-warming gases such carbon dioxide (CO2) and nitrous oxide as well as other pollutants that cause acid rain.
Shipping is responsible for ferrying over 90 percent of global trade and produces about three percent of mankind's carbon emissions, or more than the CO2 produced by the German economy.
Yet reducing these emissions means tackling idiosyncratic maritime laws, the vested interests of countries such as China that rely on shipping to fuel rapid economic growth, and convincing conservative ship owners to embrace green technology.
Ironically, one of the most promising technologies to reduce ship fuel consumption is the same one used thousands of years ago when ancient mariners first ventured across seas on rickety boats -- sails.
German entrepreneur Stephan Wrage is among those re-inventing the sail and his 21st century version is very different from the sails that graced the masts of Columbus' Santa Maria.
As a teenager, Wrage figured he could speed up his sailing boat if he attached a sports kite overhead. Two decades later, he has translated that idea into a kite system for large vessels such as bulk carriers.
"Wind is unbeatable because it's free and you don't need to transport it with you so you just use it as you need," said Wrage, whose company SkySails GmbH & Co. KG makes giant kites that look similar to paraglider canopies for ships.
Kites propel ships faster than traditional sails, Wrage said, because they are dynamic, not fixed, and can move around to capture winds coming from different directions.
The system, he said, can save between 10 to 30 percent of fuel on a single voyage, a significant cost saving given that 60 to 80 percent of a vessel's operation costs derive from fuel.
Free as the Wind
Convincing shipping companies to buy costly green technologies, however, is easier said than done as the industry recovers from the economic slowdown and faces volatile fuel prices that make it difficult to calculate returns on investment.
A carbon emissions scheme, in which ships would be penalised for inefficient fuel use and rewarded for conserving fuel, would spur green investments in a conservative industry that has been slow to embrace new technologies.
"The most important business driver would be if there was a carbon emissions scheme in place," said Wrage.
"The most important thing is that we get a price on carbon so the industry has security and knows what is coming and knows where to invest," he added.
However, developing a carbon scheme for shipping is politically charged and highly complex as the vast majority of vessels sail under flags of convenience of other countries to avoid tighter regulations and higher taxes and labour costs.
Ships from countries such as Denmark, Britain and Germany are largely registered in developing countries such as Panama, Liberia and even land-locked Mongolia which, under the U.N.'s Kyoto Protocol, have no duty to cut carbon emissions.
"If the whole shipping industry was viewed as a country, it would be the sixth or seventh biggest emitter of greenhouse gases," said Philip Roche, a maritime and trade lawyer at British firm Norton Rose.
Those emissions are set to keep rising as world trade grows. The International Maritime Organisation (IMO) has estimated that greenhouse gas emissions from shipping could grow between 150 and 250 percent by 2050.
"So we're talking about a lot of emissions but at the moment the emissions don't belon
