Tracts of ice in Greenland and Antarctica melted when temperatures were around or slightly higher than today in ancient thaws in the past three million years, a US-led international team wrote in the journal Science. And the world may be headed for a repeat even if governments cut greenhouse gas emissions to limit global warming to a United Nations goal of 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times. "Present temperature targets may commit Earth to at least six meters sea level rise," the authors at the Past Global Changes project wrote. Some greenhouse gases can linger for centuries in the atmosphere. Such a thaw would threaten cities from Beijing to London, and swamp low-lying tropical island states. Lead author Andrea Dutton, of the University of Florida, said it could take many centuries for a six-meter rise, despite some ancient evidence that more rapid shifts were possible. "This is a long-term projection. It's not going to happen the day after tomorrow," she told Reuters. The United Nations' panel of climate scientists said in 2013 that global warming could push up world sea levels by 26 to 82 cm (10 to 32 inches) by the late twenty-first century, on top of a 19 cm gain since 1900. Thursday's study, based on studies of everything from ancient ice to fossil corals, said sea levels rose by between 20 and 30 feet (6-9 metres)  in a warm period about 125,000 years ago when temperatures were similar to those of today. Ocean levels gained between 20 and 42 feet (6-13 metres) 400,000 years ago when temperatures were up to about 1C warmer than present. And in a warm period three million years ago, sea levels were also at least 20 feet (6 metres) higher than now. The ancient shifts were probably linked to natural variations in the Earth's orbit around the sun. Last year, some scientific studies indicated that parts of West Antarctica's ice sheet had already passed a "tipping point", and were locked in an unstoppable long-term thaw. "Tipping is not just a theoretical possibility, it is a reality," Ricarda Winkelmann of the Potsdam Institute for Climate Impact Research told a science conference in Paris.