Inter-annual variation in the land carbon sink is a key uncertainty in the feedback between anthropgenic emissions and climate. Analysis of the CMIP5 models demonstrates that the land sink is twice as uncertain as the ocean sink. While we can now readily measure net exchange of carbon on land using eddy covariance techniques – we have little constraint on long term carbon storage in ecosystems. This research outlines the theoretical framework that underpins the use of tree increment cores (tree rings) as a constraint for Earth System Models.