ALBERT

Description: This comment deals only with the atmospheric CO2 data plotted in the discussion paper, an issue which - as far as I was able to follow - has not yet been brought up by the reviews published until today.

Description: The paper of Paillard investigates the Plio-Pleistocene carbon cycle by setting up a conceptual model, consisting of differential equation for the carbon content of the atmosphere-ocean-biosphere C, the alkalinity of the ocean, A, and the stable carbon isotope values of C, d13C. I find the conceptual idea how to understand the observed long-term changes in the carbon cycle very interesting. However, I have some fundamental comments to Equation 3 describing the evolution of the the carbon isotope of the system.

Description: It is still an open question how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – depends on background climate. We here present palaeodata-based evidence on the state dependency of S, by using CO2 proxy data together with a 3-D ice-sheet-model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. This nonlinearity has not, so far, been accounted for in similar approaches due to previously more simplistic approximations, in which land ice albedo radiative forcing was a linear function of sea level change. The latitudinal dependency of icesheet area changes is important for the non-linearity between land ice albedo and sea level. In our set-up, in which the radiative forcing of CO2 and of the land ice albedo (LI) is combined, we find a state dependence in the calculated specific equilibrium climate sensitivity, STCO2,LIU, for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods, STCO2,LIU is on average � 45% larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6–5 MyrBP) the CO2 data uncertainties prevent a well-supported calculation for STCO2,LIU, but our analysis suggests that during times without a large land ice area in the Northern Hemisphere (e.g. before 2.82 MyrBP), the specific equilibrium climate sensitivity, STCO2,LIU, was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state change in the climate system with the widespread appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land ice albedo radiative forcing, which is important for similar palaeodata-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO2 doubling are not yet entirely clear since the details of necessary corrections for other slow feedbacks are not fully known and the uncertainties that exist in the ice-sheet simulations and global temperature reconstructions are large.

Description: Continuous records of the atmospheric greenhouse gases (GHGs) CO2, CH4, and N2O are necessary input data for transient climate simulations, and their associated radiative forcing represents important components in analyses of climate sensitivity and feedbacks. Since the available data from ice cores are discontinuous and partly ambiguous, a well-documented decision process during data compilation followed by some interpolating post-processing is necessary to obtain those desired time series. Here, we document our best possible data compilation of published ice core records and recent measurements on firn air and atmospheric samples spanning the interval from the penultimate glacial maximum ( ∼  156 kyr BP) to the beginning of the year 2016 CE. We use the most recent age scales for the ice core data and apply a smoothing spline method to translate the discrete and irregularly spaced data points into continuous time series. These splines are then used to compute the radiative forcing for each GHG using well-established, simple formulations. We compile only a Southern Hemisphere record of CH4 and discuss how much larger a Northern Hemisphere or global CH4 record might have been due to its interpolar difference. The uncertainties of the individual data points are considered in the spline procedure. Based on the given data resolution, time-dependent cutoff periods of the spline, defining the degree of smoothing, are prescribed, ranging from 5000 years for the less resolved older parts of the records to 4 years for the densely sampled recent years. The computed splines seamlessly describe the GHG evolution on orbital and millennial timescales for glacial and glacial–interglacial variations and on centennial and decadal timescales for anthropogenic times. Data connected with this paper, including raw data and final splines, are available at doi:10.1594/PANGAEA.871273.

Description: A still open question is how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – is depending on background climate. We here present paleo-data based evidence on the state-dependency of S, by using CO2 proxy data together with 3-D ice-sheet model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land-ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. This non-linearity was in similar approaches not accounted for due to previously more simplistic approximations of land-ice albedo radiative forcing being a linear function of sea level change. Important for the non-linearity between land-ice albedo and sea level is a latitudinal dependency in ice sheet area changes.In our setup, in which the radiative forcing of CO2 and of the land-ice albedo (LI) is combined, we find a state-dependency in the calculated specific equilibrium climate sensitivity S[CO2 ,LI] for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods S_[CO2,LI] is on average ∼45% larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6–5 Myr BP) the CO2 data uncertainties prevents a well-supported calculation for S_[CO2,LI], but our analysis suggests that during times without a large land-ice area in the Northern Hemisphere (e.g. before 2.82MyrBP) the specific equilibrium climate sensitivity S_[CO2,LI] was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state-change in the climate system with the wide appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land-ice albedo radiative forcing, which is important for similar paleo data-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO2 doubling are not yet entirely clear since the necessary corrections for other slow feedbacks are in detail unknown and the still existing uncertainties in the ice sheet simulations and global temperature reconstructions are large.

Description: 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.