SHEFFIELD, UNITED KINGDOM - FEBRUARY 7: UN Peace prize winner Senator George Mitchell talks to the press at the International Climate Change Conference at the Octagon Centre on February 7, 2007 in Sheffield, England. Mitchell was addressing the major climate change challenges that threaten economic sustainability. Getty Images logo Getty Images 33 months ago

SHEFFIELD, UNITED KINGDOM - FEBRUARY 7: UN Peace prize winner Senator George Mitchell talks to the press at the International Climate Change Conference at the Octagon Centre on February 7, 2007 in Sheffield, England. Mitchell was addressing the major climate change challenges that threaten economic sustainability.