LONDON – MARCH 16: A plane comes into land at Heathrow Airport on March 16, 2007 in London. A US government agency has announced that winter in the Northern Hemisphere this year has been the warmest since records began. Air travel continues to be a major source of debate; attempts to slow climate change with radical policies are being tabled by scientists and politicians. Hybrid buses, which use a combination of diesel and electric power, are said to be central to the Mayor and Transport of London’s plans for a cleaner, greener bus fleet in London, and aim to start cutting the capital’s Carbon Dioxide emissions helping to tackle climate change. (Photo by Matt Cardy/Getty Images)