 Thirteen percent of Americans have never heard of global warming even though their country is the world's top source of greenhouse gases, a 46-country survey showed on Monday. 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'. "It has taken extreme and life-threatening weather patterns to finally drive the message home that global warming is happening and is here to stay unless a concerted, global effort is made to reverse it," said Patrick Dodd, the President of ACNielsen Europe. People in Latin America were most worried while US citizens were least concerned with just 42 percent rating global warming 'very serious'. The United States emits about a quarter of all greenhouse gases, the biggest emitter ahead of China, Russia and India. Thirteen percent of US citizens said they had never heard or read anything about global warming, the survey said. Almost all climate scientists say that temperatures are creeping higher because of heat-trapping greenhouse gases released by burning fossil fuels. The study also found that 91 percent of people had heard about global warming and 50 percent reckoned it was caused by human activities. A UN report due on Friday is set to say it is at least 90 percent probable that human activities are the main cause of warming in the past 50 years. People in China and Brazil were most convinced of the link to human activities and Americans least convinced. The survey said that people living in regions vulnerable to natural disasters seemed most concerned -- ranging from Latin Americans worried by damage to coffee or banana crops to people in the Czech Republic whose country was hit by 2002 floods. In Latin America, 96 percent of respondents said they had heard of global warming and 75 percent rated it 'very serious'. Most industrial nations have signed up for the UN's Kyoto Protocol, which imposed caps on emissions of greenhouse gases, mainly from factories, power plants and vehicles. President George W Bush pulled the United States out of Kyoto in 2001, but said last week that climate change was a 'serious challenge'.