London, April 17 -- The number of days each year that the world's oceans experience extreme surface heat has tripled over the past 80 years due to global warming, a new study has found. Researchers found that, on average, the global sea surface saw about 15 days of extreme heat annually in the 1940s. Today that figure has soared to nearly 50 days per year, according to a new study published this week in the journal Proceedings of the National Academy of Sciences. Global warming is responsible for almost half of the occurrence of marine heatwaves -- periods when sea surface temperatures rise well above normal for an extended time. The study, produced by a team of scientists from the Mediterranean Institute for Advanced Studies, the Universit...