Overshoot scenarios and their climate response (A3)

Updated on 25/12/2021

This report reviews the current literature regarding “overshoot” scenarios where some aspect of the climate such as the temperature or GHG concentrations temporarily exceed a given target. The aim is to characterise such scenarios from Integrated Assessment Models (IAM) and where possible the climate to these scenarios, however most simulations with comprehensive climate models consider only highly idealised extreme scenario of future climate change.

The assessment of IAM literature includes some additional analysis of the IAM scenario database compiled by the IPCC for the 5th Assessment Report from more than 30 different models and several major international model intercomparison projects.

The natural uptake of CO2 by the carbon cycle means that a small degree of atmospheric CO2 concentration overshoot would be recoverable from by natural means. However a predominant feature of ambitious mitigation scenarios is the presence of slightly larger overshoot (in duration or magnitude) which could be facilitated by enhanced Carbon Dioxide Removal (CDR) technologies. These are almost exclusively assumed by IAM to be produced through the combinations of BioEnergy with Carbon Capture and Storage (BECCS) which is the subject of further scrutiny within AVOID reports D2a and D1a. The limits of CCS scalability or land and water availability for bio-fuels are likely to place additional constraints on the feasibility of the majority of the overshoot scenarios described in the IPCC 5th Assessment Report.

We show that uncertainty regarding BECCS availability is the leading order sensitivity for the possible magnitude and duration of a recoverable climate overshoot. Without BECCS the IPCC WG3 database indicates an upper bound to temperature overshoot of 0.1 °C for 40-50 years when still meeting a 2 °C target in 2100. If BECCS is available scenarios in the IPCC WG3 AR5 database suggest that the duration of the overshoot could be halved (25 years) or allowed to become 3 times the size (0.3 °C) with optimistic assumptions about CDR. It should be noted that the limits to the use of CDR in these IPCC WG3 scenarios are a consequence of minimising mitigation costs in these scenarios under a range of different scenario assumptions. Non- cost optimum scenarios incorporating more CDR would also be possible and allow larger overshoots, however availability of BECCS may turnout to be smaller than assumed in the IAM and lead to smaller possible overshoots while still meeting a 2 °C target in 2100.

Climate response to an overshoot of atmospheric greenhouse gas concentrations has been examined in a number of studies of idealised emissions and concentration scenarios in Earth System Models (ESM). There is consensus that stabilization of CO2 concentrations, or even global temperatures, does not imply stabilization of all aspects of the climate at the same time. Many parts of the climate system have inherently long time-scale responses (e.g. ice sheets, carbon cycle, ocean heat content) which may take many hundreds to thousands of years to reach a new equilibrium.

An aspect of the physical response for which there has been insufficient study in the available scientific literature to draw firm conclusions about is the temporary resilience of large-scale components of the climate system, such as polar ice sheets or the Amazon forest, to abrupt or irreversible changes during a temperature overshoot of the scale detailed in the IPCC WG3 AR5 scenario database. Whilst it is broadly accepted that there is potential for such changes, and that the probability of them occurring increases at higher temperature, the precise details of the trigger points are still too uncertain to draw robust conclusions about the relative increase in risk posed by temporarily overshooting a climate target.

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