The Global Carbon Cycle: a Viewpoint on the Missing Sink

Abstract

Atmospheric carbon budgets that ignore the possibility of terrestrial ecosystem responses to global atmospheric change do not balance; there is a 'missing sink' of about 0.4 - 4 Gt C yr^-1. This paper argues a case that mechanistically it is well within the bounds of possibility that increasing carbon storage in vegetation and soils in response to the globally increasing CO₂ concentration, temperature and nitrogen deposition can account for the missing C sink. Global warming conditions considered alone would be unlikely to cause most ecosystems to emit CO₂, because the N mineralised by any enhanced soil organic matter decomposition would be largely taken up by plants and reconverted into organic matter having a much higher C:N ratio than that in the soil. Models of the global terrestrial C cycle indicate that an extra 0.5 - 4 Gt C yr^-1 could well be being stored in soils and vegetation today in response to the CO₂ fertilising effect, having regard for the interactions with growth restricting water, light and nitrogen levels. To obtain direct proof as to whether that this is happening or not is a major challenge.