Constraints on global temperature target overshoot

The climate science and policy communities are beginning to assess the feasibility and potential benefits of limiting global warming to 1.5°C or 2°C. Understanding the dependence of the magnitude and duration of possible temporary exceedance (i.e., "overshoot") of these targets on sustainable energy decarbonization futures and carbon dioxide (CO2) removal rates will be an important contribution of the scientific community to this policy discussion. Drawing upon results from the mitigation literature and the IPCC Working Group 3 (WG3) scenario database, we examine the global mean temperature implications of differing independent pathways for the decarbonization of global energy supply and the implementation of negative emissions technologies. We find that within the range of decarbonization and negative emissions futures considered by WG3, the most ambitious rates of both decarbonization and deployment of negative emissions technologies are required to avoid overshoot of 1.5°C. The magnitude of temperature overshoot is more sensitive to the rate of decarbonization, but limiting the duration of overshoot to less than two centuries will require ambitious deployment of both decarbonization and negative emissions technology. The dependencies of temperature overshoots properties upon currently untested negative emissions technologies suggests that it will be important to assess how climate impacts depend on the magnitude and duration of overshoot, not just long term residual warming. As a new round of research proceeds with a 1.5°C threshold in mind, it will be important to understand the drivers behind various global temperature linked impacts and how these are influenced by both the duration and magnitude of a temporary overshoot of the target. This understanding will allow policy makers to better link climate policy goals to specific technological needs. Figure: Magnitude and duration of 1.5°C temperature target overshoot for "likely" range of climate response. Contours show the maximum magnitude (in °C) and duration (in years) of the period of overshoot beyond 1.5°C as a function of decarbonization and negative emissions implementation. White areas show scenario spaces with no overshoot, and stippled areas scenario spaces where the quantity is still undefined in 2300.