This picture is figure 9.5 in IPCC AR4. These two graphs are IPCC’s case CO2 is responsible for global warming. The top graph shows when their models are run backwards including the effects of CO2 the models reproduce observed temperatures. The bottom graph shows the models run backwards without the effect of CO2. Although the models reproduce temperatures for most of the last century they fail to match the rise from 1980 to 2000. The IPCC say this means CO2 must be responsible – even though their models leave out other possible factors such as change in Sun’s output and change in cloud cover.