For UNFCCC-compliance & assessing the risks of failure, we should measure Contraction:Concentrations in the same unit [Gt C].
Doing this shows that for public policy development, shrinking the 'Carbon-Emission-Budget' is more risk-averse than letting 'Atmospheric-Carbon-Concentrations' [ACC] rise out of control due to positive-feedback.
An assessment of 'Contraction & Concentrations' and 'Contraction & Convergence' and the C&C targets and modelling behind rates of 'sink-efficiency' in the UK Government's 'Climate Act' .
The '50:50' odds the UK Government gave for avoiding a temperature rise globally of more than two degrees with their emissions scenario are examined in this context. They are linked the Government's wholly unsubstantiated claim that atmosphere concentrations will fall after 2050 even though we are projected as only halfway through a 100-year emissions 'contraction-event'.
A letter 8th June 2011 from eminent persons sent to the UK Secretary of State for Energy and Climate Change about these matters ishere
C&C in the context of COP-15 Copenhagen [12/2009] with a view on what went wrong and what it takes to get it right. Presentation/Animation - Also for 'download and save' as an swffile for internet browsers or a self-executing [virus-free]Flash file for PCs.