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What will the upper limit in the IPCC's ‘likely’ range of its climate sensitivity estimate be, in its 6th Assessment Report?

Climate sensitivity is arguably the most important number in climate change. It is the global-mean surface temperature change associated with a doubling of atmospheric CO2 concentration over its preindustrial value (Vial et al. 2013). The standard metric for climate sensitivity is the equilibrium climate sensitivity (ECS) given as the change in temperature at equilibrium for a doubling of CO2, neglecting long-term feedbacks associated with vegetation changes, carbon feedbacks and ice sheet dynamics (Skeie et al. 2017).

The first attempt to calculate climate sensitivity was in 1896, by the Swedish scientist, Svante Arrhenius. He found that CO2 doubling should increase the Earth's mean temperature by 5–6°C (Lapenis, 2006). Since then, hundreds of estimates have been made based on climate modelling and/or estimates based on physical evidence, such as climate changes in the distant past (Knutti et al., 2017).

Some have argued that from the 1980s onward, the uncertainty in climate projections has not narrowed appreciably, despite tremendous increases in computing power, in observations, and in the number of scientists studying the problem. For instance, In 2013, the IPCC widened the range of its estimate of climate sensitivity once again, opting for the "likely" range of 1.5°C to 4.5°C (66% confidence interval) (Freeman et al., 2015). The resolution of this dilemma has important implications for climate research and policy.

The ‘likely’ range (i.e. its 66% confidence interval) of ECS as stated in the IPCC's 5th annual report (IPCC, 2014) is 1.5–4.5 degrees Celsius. This is the same range that was givien in the IPCC's first report (IPCC, 1990).

A 2018 article aims to revise the estimate of climate sensitivity, and reduces the range of possible end-of-century outcomes by more than half. It finds a central estimate of 2.8 degrees Celsius with 66 per cent confidence limits (equivalent to the IPCC ‘likely’ range) of 2.2–3.4 degrees Celsius. The IPCC's sixth report is currently scheduled for publication in 2022, will it revise its upper bound of 4.5 degrees Celsius of its ‘likely’ range?

What will the upper limit in the IPCC's assessed 'likely' range of equilibrium climate sensitivity, in its sixth Assessment Report?


Resolution

This question resolves as the upper of the 66% confidence limits, i.e. the upper number of its 'likely' range, of the IPCC's estimate for climate sensitivity in its Sixth Assessment Report. The question resolves ambiguous if the sixth Assessment Report is not published before the end of 2024, or if it does not issue the estimate in its report.

In case of format changes to the IPCC's representation of their estimate for equilibrium climate sensitivity (i.e. different confidence interval is presented), an admin shall examine the relevant technical publication(s) produced by the IPCC, or those that principally inform the IPCC estimates for climate sensitivity, to work out the associated 66% confidence interval from their cumulative probability function (CDF). In case different confidence intervals are provided and the relevant CDF is not disclosed, we shall assume that the CDF is equivalent to the analytic form found in Roe and Baker (2007), so that the 66% confidence interval can be worked out by an admin.

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