EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Emissions consistent with halting global warming

Myles Allen1,2, Stuart Jenkins2, Thomas Froelicher3, and Pierre Friedlingstein4,5
Myles Allen et al.
  • 1School of Geography and the Environment, University of Oxford, United Kingdom (
  • 2Department of Physics, University of Oxford, United Kingdom (
  • 3Climate and Environmental Physics & Oeschger Centre for Climate Change Research, University of Bern (
  • 4College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK (
  • 5Laboratoire de Météorologie Dynamique, Institut Pierre-Simon Laplace, CNRS-ENS-UPMC-X, Département de Géosciences, Ecole Normale Supérieure, Paris, France

The IPCC Special Report on 1.5°C (SR1.5)[i] stated “Reaching and sustaining net-zero global anthropogenic CO2 emissions and declining net non-CO2 radiative forcing would halt anthropogenic global warming on multi-decadal timescales (high confidence)”, implying that net zero CO2 emissions and declining non-CO2 forcing was a sufficient condition for any ongoing global warming to be indistinguishable from natural climate variability on interdecadal timescales. The IPCC 6th Assessment Report (AR6)[ii] went much further: “limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions”, implying that net-zero CO2 emissions was a necessary condition for reducing the ongoing rate of global warming to zero. We discuss interpretations of these statements in the context of a policy environment focussed on the coming century, rather than multi-century timescales. We show that two quantities are important in determining the CO2 emissions and non-CO2 forcing consistent with halting global warming: the Rate of Adjustment to Constant Forcing (RACF), or the fraction rate of global warming over the decades following forcing stabilisation, and the Rate of Adjustment to Zero Emissions (RAZE), or the RACF minus the centennial rate of CO2 forcing decline after CO2 emissions reach net zero. We use results from the Zero Emissions Commitment Model Intercomparison Project to show that the best-estimate value of the RAZE is close to zero, possibly negative at low warming levels, with a range of uncertainty that straddles zero. Hence the evidence currently available suggests only that achieving net zero or net negative CO2 emissions is as likely as not required to halt CO2-induced warming on interdecadal timescales. That said, it is virtually certain that any residual emission consistent with no further warming would be an order of magnitude lower than current emission rates and within the uncertainty of CO2 sources and sinks in the second half of this century.

[i] Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, et al: Global Warming of 1.5°C, IPCC, 2018

[ii] IPCC, 2021: Summary for Policymakers. In: Climate Change 2021, the Physical Science Basis [Masson-Delmotte, V., P. Zhai, et al (eds.)]. Cambridge University Press (2021).

How to cite: Allen, M., Jenkins, S., Froelicher, T., and Friedlingstein, P.: Emissions consistent with halting global warming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4362,, 2022.