Abstract
Abstract. We propose a reduced-complexity process-based model for the long-term evolution of the global ice volume, atmospheric CO2 concentration, and global mean temperature. The model's only external forcings are the orbital forcing and anthropogenic CO2 cumulative emissions. The model consists of a system of three coupled non-linear differential equations representing physical mechanisms relevant for the evolution of the climate–ice sheet–carbon cycle system on timescales longer than thousands of years. Model parameters are calibrated using paleoclimate reconstructions and the results of two Earth system models of intermediate complexity. For a range of parameters values, the model is successful in reproducing the glacial–interglacial cycles of the last 800 kyr, with the best correlation between modelled and global paleo-ice volume of 0.86. Using different model realisations, we produce an assessment of possible trajectories for the next 1 million years under natural and several fossil-fuel CO2 release scenarios. In the natural scenario, the model assigns high probability of occurrence of long interglacials in the periods between the present and 120 kyr after present and between 400 and 500 kyr after present. The next glacial inception is most likely to occur ∼50 kyr after present with full glacial conditions developing ∼90 kyr after present. The model shows that even already achieved cumulative CO2 anthropogenic emissions (500 Pg C) are capable of affecting the climate evolution for up to half a million years, indicating that the beginning of the next glaciation is highly unlikely in the next 120 kyr. High cumulative anthropogenic CO2 emissions (3000 Pg C or higher), which could potentially be achieved in the next 2 to 3 centuries if humanity does not curb the usage of fossil fuels, will most likely provoke Northern Hemisphere landmass ice-free conditions throughout the next half a million years, postponing the natural occurrence of the next glacial inception to 600 kyr after present or later.
Highlights
The long-term evolution of the Earth system is of theoretical interest but a matter of utter practical relevance at present when its assessment is required in the decisionmaking process for guaranteeing the safe permanent storage of radioactive waste
We use paleoclimate reconstructions of the late Quaternary. This period was selected for two reasons: (1) so far this is the only period of time for which accurate reconstructions of the atmospheric CO2 concentration are available, and (2) this period is dominated by long glacial cycles which are expected to continue in the future, at least for some time in the absence of significant anthropogenic influence
The last equation in our model describes the global mean surface air temperature anomaly as a linear combination of two terms: first, the direct effect of global ice volume and, second, a term representing the radiative forcing of CO2, which is proportional to the logarithm of CO2 concentration: T = d1v + d2 log where d1 and d2 are adjustable model parameters
Summary
The long-term evolution of the Earth system is of theoretical interest but a matter of utter practical relevance at present when its assessment is required in the decisionmaking process for guaranteeing the safe permanent storage of radioactive waste. To provide a set of possible scenarios of the future coupled climate–ice sheet–carbon cycle evolution we develop a reduced-complexity model based on paleoclimate data and results of physically based Earth system models This approach constitutes a significant further development of the method proposed in Archer and Ganopolski (2005) and Lord et al (2019). In the former, Paillard’s conceptual model of glacial cycles was combined with the results of future CO2 simulations performed with a simple marine carbon cycle.
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