Emissions consistent with halting global warming

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 6^th 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).