Agence France-Presse: Melting permafrost could eventually disgorge a billion tonnes a year of greenhouse gas into the atmosphere, accelerating the threat from climate change, scientists said Wednesday. Their probe sought to shed light on a fiercely-debated but poorly-understood concern: the future of organic matter that today is locked up in the frozen soil of Alaska, Canada, northern Europe and Siberia. The fear is that, as the land thaws, this material will be converted by microbes into carbon …