Natural variability-focused assessment of climate overshoot timing

The Paris Agreement legally commits the international community to keep anthropogenic global warming well below 2.0°C, while major efforts shall be made to hold the 1.5°C-line. This is supported by ample scientific evidence indicating that climate change becomes difficult to manage beyond those lines due to highly nonlinear impacts (like tipping processes). The time range when the Paris guardrails will be transgressed under business as usual is highly relevant for precautionary adaptation measures and for justifying the rapid transformation towards a post-fossil economy. Fully-fledged Earth System Models (ESMs) are usually employed for the pertinent projections, yet they are not only computationally expensive but also lack explicit accounting for natural climate system variability. The latter may significantly distort (and invalidate) the ESM overshoot-timing projections. As an alternative, we present here a purely data-driven stochastic approach based on the persistence properties of the observed global temperatures. We are able to quantify, in a probabilistic way, the natural variability that must be superimposed on the anthropogenic trends in order to retrieve the observed warming behavior. When assuming that the anthropogenic warming continues at the current rate, we actually arrive at comparable overshoot timing estimates as the ESMs and provide an explanation for this finding.