The Challenge of a 4°C World by 2100

As evidenced by the Fifth Assessment Report of the IPCC, our understanding of the Earth System and the climate change impacts expected in the coming decades is developing at a rapid pace. Contributing to this progress, the first ever Inter-Sectoral Impact Model Intercomparison (ISI-MIP) has helped to paint a clearer picture of potential impacts at different levels of global mean warming. However, along with such advances the limitations of our understanding become more apparent. A number of processes are scarcely or not at all reflected in current assessments of the risks associated with significant levels of warming. These include critical thresholds in the Earth system which, once breached, can give rise to non-linear impacts. Recent insights from West Antarctica indicate that we have already ‘tipped’ several large glacier systems there, suggesting that the risk of crossing such thresholds might be much greater than previously thought. Also excluded from a sectoral perspective are the intricate interdependencies between systems and the potential for an initial impact to cascade into a chain of impacts, or for impacts to occur simultaneously and interact in complex ways. Finally, we need to take into account the different degrees of vulnerability not only across but also within nation states. The ramifications of non-linear impacts and their uneven distribution are likely to be deleterious to the stability and wellbeing of our societies and will, we hope, never be realized. However, if we wish to understand the challenges associated with a 4°C world2, such a world needs to be imagined.