Overshoot scenarios for Paris Agreement temperature targets: an interdisciplinary perspective

Organizers: Katsumasa Tanaka (NIES), Johannes Bednar (IIASA), Oliver Geden (German Institute for International and Security Affairs), Daniel Johansson (Chalmers University of Technology, Sweden), Michael Obersteiner (IIASA), Kiyoshi Takahashi (NIES)

research SESSION

The Paris Agreement calls for “holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels.” A large body of literature suggests low global decarbonization rates in the first half of this century followed by enormous amounts of negative emissions in the second half to achieve such targets. Less widely recognized, many such scenarios generate an overshoot, during which the temperature temporarily exceeds the target level (especially, the 1.5°C target) from the mid- towards the end of this century.

Our session aims to create an opportunity for exchanging views on different aspects of overshoot and associated consequences (e.g. extensive Bio-energy with carbon capture and storage (BECCS)), with the broader goal of informing the scientific and political discourse around the Paris agreement in an interdisciplinary manner. The session shall act as a platform for various communities (especially CMIP6) to form synergies and collaborations and is open for a wide range of issues, including:

i)        Climate-carbon cycle feedbacks of uncertain magnitude potentially cause problems for termination of an overshoot trajectory.
ii)        The magnitude and duration of overshoot can play a crucial role in determining the timing of achieving net zero emissions and the levels of net negative emissions required to achieve the temperature targets.
iii)        Climate change damages that occurred after the target overshoot may not be reversible during the recovery phase of overshoot.
iv)        How would the signatories of the Paris Agreement deal with temperature overshoot politically? There are grand challenges for communicating overshoot to policymakers, especially with respect to burden shifting to future generations.
v)        Large scale overshoot levels create the necessity for and reliance on a limited set of negative emission technologies that haven’t passed the pilot phase to date. After achieving the climate target in 2100 deployed assets become stranded in typical scenarios.
vi)        BECCS as the negative emission option of choice in overshoot scenarios comes at high external social costs (natural land loss, interferences with the food system and important ecosystems functions), which are currently not benchmarked in AR5 mitigation scenarios.
vii)        Early but moderate ramp-up of BECCS combined with nature based options such as afforestation and reforestation and optional deployment of Direct Air Capture (DAC) have great potential to alleviate the profound weaknesses of late BECCS deployment.

The session will have two invited talks and three regular talks from various backgrounds (both natural and social scientists), contributing to the diversity expected at the conference. We call for abstracts addressing points related to above or overshoot scenarios in general.