Temperature overshoot means more than cleaning up a slightly bigger mess a little later. TL;DR: If we ever return to 1.5 °C, it will likely take centuries. The way overshoot scenarios are often sketched fails to highlight that. iopscience.iop.org
iopscience.iop.org
Long-term warming will soon exceed 1.5 °C (1). To align this reality with the ambition of the Paris Agreement to limit warming to 1.5 °C, there is a rise in so-called “overshoot scenarios”. In an overshoot scenario, global warming temporarily exceeds a temperature limit and later returns to it (2). The rate of global temperature decline is largely determined by the rate of net negative CO2 emissions. Illustrations of “overshoot” routinely show temperature trajectories that are almost symmetric in the increase and decrease in global temperatures. Just to be clear, this would require net negative CO2 emissions on the order of today’s human-caused positive CO2 emissions and an industry to take out CO2 from the atmosphere about the size of the current fossil fuel industry! In a new perspective paper, we argue that the current thinking and communication around global temperature decline after peak warming is far too optimistic. We show that even under extremely optimistic assumptions, the time above 1.5 °C will extend well beyond 2100, and probably much longer. We trace the prevalence of overly optimistic temperature decline in overshoot pathways back to three interconnected problems: 1) The irreversibility of CO2-induced warming is non-intuitive, even to experts. 2) Strong cooling rates depend on overly optimistic assumptions regarding CO2 removal to compensate for emissions that took place in the past. Currently, we are even failing to reduce CO2 emissions globally (3), let alone plan for net negative emissions. 3) The models that generate future emission scenarios are structurally prone to generating overshoot pathways. Temperature decline faces barriers in the real world that are not represented in such models that explore cost-optimal trajectories. Together, these three elements lead to an overly rosy communication that creates the expectation that a return after overshoot will be quick and easy. We and others argue that it is not (4, 5). Are such overshoot scenarios desirable? They are clearly more desirable than warming stabilizing at a higher level without returning to 1.5 °C. Are overshoot scenarios plausible or even likely as often depicted? Our answer: Absolutely not. From a current perspective, long-term exceedance of 1.5 °C seems much more plausible than a fast return to it. In any case, neither our children nor grandchildren are likely to ever experience warming returning to 1.5 °C. A lack of strong near-term mitigation commits many generations to live under an overshoot of 1.5 °C, with impacts that could have been avoided. For scientists: Please stop drawing symmetric overshoot pathways. (1) unep.org (2) iopscience.iop.org (3) essd.copernicus.org (4) nature.com (5) granthaminstitute.com
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