Survey shows 13% of Americans never heard of global warming - "The report, by ACNielsen of more than 25,000 Internet users, showed that 57 percent of people around the world considered global warming a 'very serious problem' and a further 34 percent rated it a 'serious problem.'" [Reuters]

NB: Being from an online (and therefore not randomly sampled) survey, these numbers should be treated with some suspicion.