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The CCCma coupled global climate models, CGCM1 and CGCM2, represent the net radiative effect of all greenhouse gases by means of an "equivalent" CO2 concentration. The equivalent CO2 concentration is necessarily higher than observed CO2 concentration since it represents the climate forcing due to CO2 and also the forcing associated with all other greenhouse gases.
In transient climate change simulations, the change in greenhouse gas forcing is represented in the model as a perturbation relative to the 330ppmv equivalent CO2 concentration used in the control simulation. That is, 330ppmv is taken as a reference value, and climate change simulations involve changes relative to this value.
Equivalent CO2 values from 1850 to 1990 are based on historical changes in greenhouse gas forcing, provided by the Hadley Centre.
From 1990 onward we have employed 3 different scenarios:
Additional notes on scenarios:
The IPCC IS92a scenario specifies equivalent greenhouse gas (GHG) concentrations and sulphate aerosol loadings from 1850 to 2100. Climate change simulations based on this scenario have been performed by a number of climate modelling groups who have contributed to the IPCC Third Assessment Report.
The IPCC Special Report on Emission Scenarios (SRES) provides 40 different scenarios which are deemed "equally likely". For the Third Assessment Report, the IPCC facilitated the conversion of two of these emission scenarios (A2 and B2) into concentration scenarios for use in climate simulations.
The A2 scenario envisions population growth to 15 billion by the year 2100 and rather slow economic and technological development. It projects slightly lower GHG emissions than the IS92a scenario, but also slightly lower aersol loadings, such that the warming response differs little from that of the earlier scenario. The B2 scenario envisions slower population growth (10.4 billion by 2100) with a more rapidly evolving economy and more emphasis on environmental protection. It therefore produces lower emissions and less future warming. Climate change results based on the A2 and B2 scenarios are also discussed in the IPCC Third Assessment Report.
Specific values are provided in the table below.