In a new paper in Geophysical Research Letters, NASA scientists estimate that doubling atmospheric carbon dioxide will result in 1.64 degrees Celsius of warming over the next 200 years. Estimates from the Intergovernmental Panel on Climate Change (IPCC) range from 3-to 5 degrees Celsius.
The problems with IPCC climate models, NASA says, is that they “did not allow the vegetation to increase its leaf density as a response to the physiological effects of increased CO2 and consequent changes in climate. Other assessments included these interactions but did not account for the vegetation down regulation to reduce plant’s photosynthetic activity and as such resulted in a weak vegetation negative response. When we combine these interactions in climate simulations with 2 × CO2, the associated increase in precipitation contributes primarily to increase evapotranspiration rather than surface runoff, consistent with observations, and results in an additional cooling effect not fully accounted for in previous simulations with elevated CO2.”
Reference: Bounoua, L., F. G. Hall, P. J. Sellers, A. Kumar, G. J. Collatz, C. J. Tucker, and M. L. Imhoff (2010), Quantifying the negative feedback of vegetation to greenhouse warming: A modeling approach, Geophys. Res. Lett., 37, L23701, doi:10.1029/2010GL045338.
It seems that as climate models get more sophisticated, the carbon dioxide effect gets closer to zero, which would be consistent with the geologic record.
There are many modeling estimates of the warming effect of carbon dioxide, but there is no physical evidence that human carbon dioxide emissions have a significant effect on global temperature.