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Canadian Centre for Climate Modelling and Analysis

Equivalent CO2 concentrations used in CCCma coupled global climate model simulations

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:

  • The IS92a scenario has effective CO2 concentration increasing at 1% per year after 1990. In the model, the concentrations are specified by linear interpolation between specified values at 2000, 2025, 2050 and 2100.
  • The equivalent CO2 concentrations for the provisional IPCC SRES A2 and B2 scenarios (which span the period 1990-2100) were provided by Ron Stouffer of GFDL. The forcing changes implied by the concentrations were used to scale the equivalent CO2 values to be consistent with the 1990 value in our IS92a simulation.
IPCC forcing

Additional notes on scenarios:

  1. The IS92a scenario.

    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.

  2. SRES scenarios.

    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.

  3. Description of A2 and B2 scenarios.

    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.

Equivalent CO2 Concentration (ppmv).
Year IS92a A2 B2
1850 330 330 330
1860 343 343 343
1870 345 345 345
1880 347 347 347
1890 349 349 349
1900 351 351 351
1910 357 357 357
1920 363 363 363
1930 370 370 370
1940 376 376 376
1950 382 382 382
1960 388 388 388
1970 408 408 408
1980 437 437 437
1990 476 476 476
2000 526 504 504
2010 540 539
2020 585 578
2025 674 612 599
2030 642 619
2040 707 659
2050 865 779 700
2060 860 740
2070 950 781
2080 1050 823
2090 1180 867
2100 1422 1320 915