気候変動：気候オーバーシュートのリスク評価

Concerns about the growing use and acceptance of overshoot scenarios, in which warming is allowed to temporarily increase past the Paris 1.5 °C limit, are raised in a paper published in Nature. The study indicates that reversing global warming following a climate overshoot may be difficult within the time frames envisioned in the present day. Instead, limiting rises in global temperatures are likely to be the most effective strategy to mitigate climate changes.

The Paris Agreement has established 1.5 °C warming beyond pre-industrial temperatures as an optimistic limit for global temperature increases. It has been proposed that temporarily exceeding this target, termed an overshoot, and then reversing subsequent warming to below 1.5 °C may be possible by reducing carbon dioxide emissions (although this may rely on the development of carbon capture technologies). However, the impacts of overshoots on climate change are unclear.

Carl-Friedrich Schleussner, Joeri Rogelj and colleagues explored modelling simulations of overshoot pathways and long-term climate stabilization. They find that the risk for global and regional climate changes differs between scenarios where overshoots do or do not happen. The authors suggest that temperature decline after an overshoot may not be achievable, owing to possible amplification of warming from strong Earth-system feedbacks, which may result in long-term warming.

Schleussner and colleagues found that following an overshoot, many aspects of the Earth system — such as species abundance, carbon stocks and biodiversity on land — may not return to pre-overshoot levels. They also highlight that sea levels will continue to rise, even if temperatures decline following an overshoot. The authors suggest that efforts to reduce global temperatures will be more likely to limit climate risks than attempting to stabilize global temperature rises resulting from overshoots.

Overshoot scenarios rely on the possibility that future carbon-removal costs will be reduced relative to current costs. The authors indicate that any preventive carbon capture system that could be developed would need to remove in the region of several hundred gigatonnes of carbon following an overshoot to protect against high-risk outcomes. However, current technical, economic and sustainability considerations may prevent a system operating at these levels.

The findings suggest that overconfidence in the controllability of changes following any overshoot should be avoided. They conclude that only rapid declines in emissions are effective in limiting climate changes.