In the discussion paper of Weber et al. (2015) a simple model of the anthropogenic carbon cycle is presented. The authors describe a simple linear model, consisting of one ordinary differential equation which describes the changes in CO2 content of the atmosphere over time. The two free parameters of the equation are derived by fitting the results of the model (carbon content of the atmosphere) to the observations or reconstructions covering the last 150 years. The model is then applied to calculate the response of the global carbon cycle to future anthropogenic emissions and some conclusions on the fate of the anthropogenic carbon emissions until the year 2150 are then drawn. The findings show a rather fast reduction in atmospheric CO2 and the conclusions are in contrast to the results of virtually all other global carbon cycle models, (e.g. Meinshausen et al., 2011; Stocker et al., 2013; Friedlingstein et al., 2014), which — in contrast to the simple model presented here — include the current state of understanding of the processes involved in the global carbon cycle. We challenge the overall conclusions of the paper for the following reasons: The simple model (although not perfect) performs well for the anthropogenic period up to today, since the values of the two free parameters in the ordinary differential equation are based on observations (or to be more correct on model-based interpretation of observations). The agreement of the model to the historic atmospheric CO2 record is therefore hardly surprising. Besides the balance of some carbon fluxes in and out of the atmosphere no further theoretical (process-based) understanding is implemented in the simple model. This is a valid approach for simulating the most recent (anthropogenic driven) past, but does not prove that the model contains prognostic value, which justifies its application on future emissions. The model can only be applied to future anthropogenic perturbations on the surmise that the carbon cycle is not fundamentally altered. However, this is clearly not the case for nearly all future emission scenarios, most importantly because the carbon uptake capacity of the ocean depends on the carbonate chemistry (Revelle factor), which is changing at unprecedented speed.