"Arctic and sub-Arctic ecosystems (particularly ombrotrophic bog communities, see Glossary) above permafrost were considered likely to be most vulnerable to climatic changes, since impacts may turn Arctic regions from a net carbon sink to a net source. Literature since the TAR suggests that changes in albedo and an increased release of methane from carbon stocks, whose magnitudes were previously substantially underestimated, will lead to positive radiative climate forcing throughout the Arctic region."
https://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch4s4-4...
Citations elided:
"Arctic and sub-Arctic ecosystems (particularly ombrotrophic bog communities, see Glossary) above permafrost were considered likely to be most vulnerable to climatic changes, since impacts may turn Arctic regions from a net carbon sink to a net source. Literature since the TAR suggests that changes in albedo and an increased release of methane from carbon stocks, whose magnitudes were previously substantially underestimated, will lead to positive radiative climate forcing throughout the Arctic region."