Two young British men are travelling from pole to pole to highlight how the world is changing through global warming. On Friday (September 14) they crossed the U.S.-Mexico border on bicycles. James Hooper and Rob Gauntlett, aged 19 and 20, want to create awareness of climate change by promoting the use of non-polluting means of transport. So far they've cycled, skied, sailed and walked. Their expedition "180 degrees: Pole to Pole Manpowered" is due to cover 17,000 miles (27,500 kilometres) from the North Pole to the South Pole. They also want to inspire other young people to "reach for the sky and accomplish their goals". Their journey is being recorded on video. Gauntlett says travelling across the Arctic was a real challenge. They had to survive temperatures of minus 40 degrees or even lower. Yet despite that, they had to strip off as they were sweating so much. The expedition took off at the Geomagnetic North Pole on April 8. The youngsters skied across Northern Greenland on a dog sled. At one point, Gauntlett fell through soft ice when he tried to recover a glove and was rescued by Hooper. He hit his head in the fall and remained unconscious for three hours. They arrived in New York in June by yacht from Greenland and then cycled down the east coast of the U.S. and into Mexico. Hooper says they hope to encourage as many people as possible to understand what's happening with global warming, like the fact in the Arctic the sea ice has thinned by 50 percent in the last 20 years. The pair are now halfway through the trip which should end in Sydney, Australia in February next year.