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, Max Planck Institute for Meteorology), Daniel Johansson (Chalmers University of Technology, Sweden), Michael Obersteiner (IIASA), Kiyoshi Takahashi (NIES)

research SESSION

4:00-4:15 PM
Climatic, environmental and socioeconomic risks and benefits of scenarios with temporary overshooting
Invited Speaker: Nico Bauer, Potsdam Institute for Climate Impact Research

4:15-4:30 PM
Different Implementations of UNFCCC climate targets and its impact on CO2 emission pathways
Daniel Johansson, Physical Resource Theory, Chalmers

4:30-4:45 PM
Path-dependent reductions in CO2 emission budgets caused by permafrost carbon release
Thomas Gasser, International Institute for Applied Systems Analysis

4:45-5:00 PM
Can we bet on overshooting the Paris Agreement temperature targets under carbon cycle feedback uncertainties?
Katsumasa Tanaka, National Institute for Environmental Studies

5:00-5:15 PM
The politics of overshoot scenarios
Invited Speaker: Oliver Geden, German Institute for International and Security Affairs

5:15-5:20 PM
Environmental, social and financial risk implications of late century CDR deployment and alternative pathways
Michael Obersteiner, International Institute for Applied Systems Analysis

5:20-5:45 PM
General discussion
Moderators: Michael Obersteiner and Johannes Bednar

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.