Journal Article

Committed Global Warming Risks Triggering Multiple Climate Tipping Points

Armstrong McKay, David I., Lenton, Timothy M., Abrams, Jesse F., Williamson, Mark S., Sakschewski, Boris, Winkelmann, Ricarda, Zimm, Caroline, Buxton, Joshua E., Huntingford, Chris, Boulton, Chris A., Loriani, Sina

2023

Overview

Many scenarios for limiting global warming to 1.5°C assume planetary‐scale carbon dioxide removal sufficient to exceed anthropogenic emissions, resulting in radiative forcing falling and temperatures stabilizing. However, such removal technology may prove unfeasible for technical, environmental, political, or economic reasons, resulting in continuing greenhouse gas emissions from hard‐to‐mitigate sectors. This may lead to constant concentration scenarios, where net anthropogenic emissions remain non‐zero but small, and are roughly balanced by natural carbon sinks. Such a situation would keep atmospheric radiative forcing roughly constant. Fixed radiative forcing creates an equilibrium “committed” warming, captured in the concept of “equilibrium climate sensitivity.” This scenario is rarely analyzed as a potential extension to transient climate scenarios. Here, we aim to understand the planetary response to such fixed concentration commitments, with an emphasis on assessing the resulting likelihood of exceeding temperature thresholds that trigger climate tipping points. We explore transients followed by respective equilibrium committed warming initiated under low to high emission scenarios. We find that the likelihood of crossing the 1.5°C threshold and the 2.0°C threshold is 83% and 55%, respectively, if today's radiative forcing is maintained until achieving equilibrium global warming. Under the scenario that best matches current national commitments (RCP4.5), we estimate that in the transient stage, two tipping points will be crossed. If radiative forcing is then held fixed after the year 2100, a further six tipping point thresholds are crossed. Achieving a trajectory similar to RCP2.6 requires reaching net‐zero emissions rapidly, which would greatly reduce the likelihood of tipping events. Plain Language Summary The importance of reaching net‐zero greenhouse gas emissions to help avoid dangerous anthropogenic climate change is widely acknowledged. However, current national commitments do not align with this target and instead will lead to about 2.7°C warming by 2100. If the large‐scale carbon dioxide removal needed to reach net‐zero emissions is unfeasible and instead, the remaining hard‐to‐mitigate emissions approximately balance natural sinks, atmospheric greenhouse gas (GHG) concentrations will remain constant. Such fixed GHG levels will result in continued warming until the climate system reaches a state of radiative balance, which we call “committed warming.” We investigate the committed warming associated with the CO2 equivalent (CO2e) for each year for three emission scenarios. Critically, we then examine the probability of breaching tipping point thresholds at different levels of committed warming, finding that under the scenario that best matches current national commitments, we will be committed to crossing the critical temperature threshold for six key climate tipping points by 2100. Maintaining radiative forcing at only slightly elevated levels above present GHG concentrations will substantially alter parts of the Earth System through such “locked‐in” impacts. Society will only be able to avoid breaching tipping point thresholds through rapid and very substantial reduction of human emissions. Key Points We conduct a thought experiment on equilibrium global warming and tipping point likelihood under constant greenhouse gas concentration scenarios Maintaining radiative forcing at or above current levels would commit multiple parts of the climate system to passing tipping points Only a lower emissions scenario, which would require rapidly reaching net‐zero emissions, avoids crossing most climate tipping points

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Anthropogenic factors / Carbon dioxide / Carbon dioxide removal / Carbon sequestration / Carbon sinks / Climate / Climate change / Climate models / Climate sensitivity / Climatic changes / committed warming / Control / Emissions / Emissions (Pollution) / Environmental aspects / Equilibrium / Global warming / Greenhouse effect / greenhouse gas / Greenhouse gas emissions / Greenhouse gases / Heat / irreversibility / Net zero / Oceans / Prevention / Radiative forcing / Temperature / Thresholds / Tipping point