This image has become a widely used illustration of C&C . . . . . . also . . . 'mouse-touch' to see C&C in this Carbon Budget Analysis Tool (CBAT)
Contraction refers to the 'full-term event' in which the future global total of greenhouse gas [GHG] emissions from human sources is shrunk over time in a measured way to zero net-emissions within a specified time-frame. The example above shows 90% by 2100. Calculating future emissions contraction, looking at concentrations and sink performance, is a non-random way of responding to the objective of the UNFCCC.
Convergence refers to the full international sharing of the emissions contraction-event, where the 'emissions-entitlements' for all countries result from them converging on the declining global per capita average of emissions arising under the contraction rate chosen. Converging at a rate to be agreed - the example shows 2030 - is a non-random way of responding to the principle of 'equity' in the UNFCCC, whilst still meeting its objective. Negotiating the rate of convergence is 'the main equity lever'.
Contraction and Convergence - "C&C has the virtue of simplicity. Equal per capita emissions is a natural focal point. Contestable computations based on economic variables do not need to enter the allocation formula." Professor Ross Garnaut
Transparently goal-focusing negotiations at the UNFCCC to become teleological this way, recognizes its 'precautionary principle'. It makes compliance with its objective less unlikely than does the randomness of the negotiations there so far - eg at COP-15 and eg at COP-16
Different rates of C&C principle shown here & here & here
First tabled at UN by GCI in 1996 briefing -
Agreed in principle at COP-3 - Described by UNFCCC as "inevitably required" at COP-9 - now widely cited and supported -
information on C&C 'rates' is here - 'incontestability' of C&C is debated here.
C&C was adopted in the UK Climate Act in 2008 but then mishandled at COP-15 also linked here.
The 'climate-gap' is now wider than ever & COP-18 failed to correct this.
GCI's current memo to UNFCCC is here Support is here