An irreversible climate tipping point could occur within the next 20 years as a result of the release of huge quantities of organic carbon locked away as frozen plant matter in the vast permafrost region of the Arctic,scientists have found. Billions of tons of frozen leaves and roots that have lain undisturbed for thousands of years in the permanently frozen ground of the northern hemisphere are thawing out,with potentially catastrophic implications for climate change,the researchers said. A study into the speed at which the permafrost is melting suggests that the tipping point will occur between 2020 and 2030 and will mark the point at which the Arctic turns from being a net sink for carbon dioxide into an overall source that will accelerate global warming. The study found that by 2200 two-thirds of the Earths permafrost melt,releasing an estimated 190 billion tons of carbon dioxide and methane.Our results show as the Arctic warms up,frozen carbon will thaw out,releasing carbon into the atmosphere, said Kevin Schaefer of the US National Snow and Ice Data Centre.THE INDEPENDENT