Craig Loehle sends word of a new publication that looks at CO2 trajectories in the context of Hoffman et al. Excerpt posted below. A link to the full paper follows.
THE ESTIMATION OF HISTORICAL CO2 TRAJECTORIES IS INDETERMINATE: COMMENT ON “A NEW LOOK AT ATMOSPHERIC CARBON DIOXIDE”
Craig Loehle, PhD, National Council for Air and Stream Improvement, Inc., Naperville, Illinois
Atmospheric Environment doi:10.1016/j.atmosenv.2010.02.029
A paper by Hofmann et al. (2009, this journal) is critiqued. It is shown that their exponential model for characterizing CO2 trajectories for historical data is not estimated properly. An exponential model is properly estimated and is shown to fit over the entire 51 year period of available data. Further, the entire problem of estimating models for the CO2 historical data is shown to be ill-posed because alternate model forms fit the data equally well. To illustrate this point the past 51 years of CO2 data were analyzed using three different time-dependent models that capture the historical pattern of CO2 increase. All three fit with R2 > 0.98, are visually indistinguishable when overlaid, and match each other during the calibration period with R2 > 0.999. Projecting the models forward to 2100, the exponential model comes quite close to the Intergovernmental Panel on Climate Change (IPCC) best estimate of 836 ppmv. The other two models project values far below the IPCC low estimates. The problem of characterizing historical CO2 levels is thus indeterminate, because multiple models fit the data equally well but forecast very different future trajectories.
Three equally plausible models give very different expectations for future CO2 trajectories under business as usual assumptions. No inference is possible at this time as to which model is “right” because the three models are virtually identical in the CO2 data
period (Fig. 2) and the understanding of the carbon cycle in this context is not precise enough. The factors governing CO2 in the atmosphere may or may not lend themselves to long-term predictability even if they were understood better. It is clear, however, that simply using an exponential model because it fits the data represents an incomplete analysis, as other models fit equally well. The IPCC “best estimate” of 836 ppmv in 2100, which is equivalent to extrapolation of the exponential model, is indeterminateand could just as easily be 569.8 or 672.5 ppmv (or even 747.7 ppmv by Hofmann et al., 2009), as found using equally likely models that fit the same data. These much lower “best estimate” values affect the IPCC “high” estimate, which is derived from the base estimate exponential model by adding a growth term (based on higher economic growth rates and other factors). Because projections of future climate depend on future CO2 (and other greenhouse gas) levels, a future value below the IPCC low estimate would preclude the more extreme climate change forecasts made by the IPCC.
PDF of the entire paper is available at: http://www.ncasi.org/publications/Detail.aspx?id=3282