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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 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 Sheets – Carbon cycle System in timescales longer than thousands of years. The model is successful in reproducing the glacial-interglacial cycles of the last 800 kyr, in good agreement with the timing and amplitude of paleorecord fluctuations, with the best correlation between modelled and paleo global ice volume of 0.86. Using different model realisations, we produce a probabilistic forecast of the evolution of the Earth system over 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 present and 120 kyr after present, and between 400 kyr 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 PgC) are capable of affecting climate evolution for up to half million years, indicating that the beginning of the next glaciation is highly unlikely in the next 120 kyr. High cumulative anthropogenic CO2 emissions (3000 PgC or higher), which could potentially be achieved in the next two to three 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 million years, postponing the natural occurrence of the next glacial inception to 600 kyr after present or later.