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Insensitivity of global warming potentials to carbon dioxide emission scenarios

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Abstract

GLOBAL warming potentials for radiatively active trace gases (such as methane and chlorofluorocarbons) have generally been expressed1-2 relative to the time-integrated climate forcing per unit emission of carbon dioxide. Previous attempts to estimate the integrated climate forcing per unit CO2 emitted have focused on perturbations to steady-state conditions in carbon-cycle models. But for non-steady-state conditions, the integrated climate forcing from a CO2 perturbation depends both on the initial conditions and on future atmospheric CO2 concentrations. As atmospheric CO2concentrations increase, the radiative forcing per unit CO2 emitted will become smaller because the strongest absorption bands will already be saturated. At the same time, higher concentrations of dissolved carbon in the surface ocean will reduce the ocean's ability to absorb excess CO2from the atmosphere. Each of these effects taken alone would affect the climate forcing from a pulse of emitted CO2 by a factor of three or more; but here we show that, taken together, they compensate for each other. The net result is that the global warming potential of CO2 relative to other radiatively active trace gases is nearly independent of the CO2emission scenario. Thus, the concept of the global warming potential remains useful, despite the nonlinearities in the climate system and uncertainties in future emissions.
... Such functions have been used to predict atmospheric CO 2 from emissions (Bolin et al., 1981;Siegenthaler and Oeschger, 1978;Oeschger and Heimann, 1983;Maier-Reimer and Hasselmann, 1987;Enting, 1990;, land carbon storage from NPP (net primary production) (Bolin et al., 1981;Thompson and Randerson, 1999) and GPP (gross primary production) (Emanuel et al., 1981), and to disentangle the historical development of the land and ocean carbon sinks from ice core reconstructions of atmospheric 12 CO 2 and 13 CO 2 (Joos and Bruno, 1998). Linear response functions have also been employed to study the dependence of global warming on possible future CO 2 emissions and the associated GWP (global warming potential) (Caldeira and Kasting, 1993;Joos et al., 2013;Caldeira and Myhrvold, 2013;Ricke and Caldeira, 2014;Gasser et al., 2017). ...
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The response function identification method introduced in the first part of this study is applied here to investigate the land carbon cycle in the Max Planck Institute for Meteorology Earth System Model. We identify from standard C4MIP 1 % experiments the linear response functions that generalize the land carbon sensitivities β and γ. The identification of these generalized sensitivities is shown to be robust by demonstrating their predictive power when applied to experiments not used for their identification. The linear regime for which the generalized framework is valid is estimated, and approaches to improve the quality of the results are proposed. For the generalized γ sensitivity, the response is found to be linear for temperature perturbations until at least 6 K. When this sensitivity is identified from a 2×CO2 experiment instead of the 1 % experiment, its predictive power improves, indicating an enhancement in the quality of the identification. For the generalized β sensitivity, the linear regime is found to extend up to CO2 perturbations of 100 ppm. We find that nonlinearities in the β response arise mainly from the nonlinear relationship between net primary production and CO2. By taking as forcing the resulting net primary production instead of CO2, the response is approximately linear until CO2 perturbations of about 850 ppm. Taking net primary production as forcing also substantially improves the spectral resolution of the generalized β sensitivity. For the best recovery of this sensitivity, we find a spectrum of internal timescales with two peaks, at 4 and 100 years. Robustness of this result is demonstrated by two independent tests. We find that the two-peak spectrum can be explained by the different characteristic timescales of functionally different elements of the land carbon cycle. The peak at 4 years results from the collective response of carbon pools whose dynamics is governed by fast processes, namely pools representing living vegetation tissues (leaves, fine roots, sugars, and starches) and associated litter. The peak at 100 years results from the collective response of pools whose dynamics is determined by slow processes, namely the pools that represent the wood in stem and coarse roots, the associated litter, and the soil carbon (humus). Analysis of the response functions that characterize these two groups of pools shows that the pools with fast dynamics dominate the land carbon response only for times below 2 years. For times above 25 years the response is completely determined by the pools with slow dynamics. From 100 years onwards only the humus pool contributes to the land carbon response.
... However, his approach still does not take into account non-linearities in the carbon cycle. This is important since larger fractions of CO 2 emissions pulse stays in the atmosphere, the higher the CO 2 concentration is (Archer et al., 2009;Caldeira & Kasting, 1993;Maier-Reimer & Hasselmann, 1987). In DICE 2016R2, the carbon cycle appears to have been linearized around a relatively high concentration of CO 2 . ...
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... For the well-mixed greenhouse gases CO 2 , CH 4 , and N 2 O, the radiative efficiency (RE) is reduced with increasing atmospheric background concentrations. Previous literature suggests that the sensitivity to emission scenario is small, and the relationship between emissions and temperature response more linear, for CO 2 (Caldeira and Kasting, 1993). However, the same has not been shown for methane (and N 2 O, which is not considered here). ...
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... The way that feedbacks between the climate system and the carbon cycle produce an almost linear relationship between CO 2 and temperature was shown already in the early 1990s (Caldeira and Kasting, 1993). In 2008, H Damon Matthews and Ken Caldeira built on this older study to show that CO 2 emissions effectively would have to cease completely if temperature was to be stabilized at any level (Matthews and Caldeira, 2008). ...
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