Based on projections of Solar cycles. So the IPCC made their predictions of runaway warming due to CO2 being a forcing (ie positive feedback) for climate. Here these folks make their case on the traditional view of CO2 as a GHG sans positive feedback and the role of the Solar cycles controlling the temps here on Earth. Archibald: Solar Based Climate Forecast to 2050 | Watts Up With That? I especially like this chart that shows the effect of CO2 as a GHG absent a positive feedback from CO2 impacting water vapor (which contributes >95% of the planetary GH effect). The vertical (Y) axis is the Greenhouse contribution of CO2 in degrees C, the horizontal (X) axis is the ppm of CO2 in the atmosphere. Note that the effect is logarithmic, IOW CO2 has contributed about everything it can and even doubling CO2 will have a minor effect on temps, also that reducing CO2 will also have a very minor effect on temps.