A few notes from the AR6 Technical Summary on Equilbrium Climate Sensitivity
Magnitude of climate system response: In this Report, it has been possible to reduce the long-standing uncertainty ranges for metrics that quantify the response of the climate system to radiative forcing, such as the equilibrium climate sensitivity (ECS) and the transient climate response (TCR), due to substantial advances (e.g., a 50% reduction in the uncertainty range of cloud feedbacks) and improved integration of multiple lines of evidence, including paleoclimate information. Improved quantification of ERF, the climate system radiative response, and the observed energy increase in the Earth system over the past five decades demonstrate improved consistency between independent estimates of climate drivers, the combined climate feedbacks, and the observed energy increase relative to AR5. (Section TS.3.2)
Equilibrium climate sensitivity. Paleoclimate data provide evidence to estimate equilibrium climate sensitivity (ECS15) (Section TS.3.2.1). In AR6, refinements in paleo data for paleoclimate reference periods indicate that ECS is very likely greater than 1.5°C and likely less than 4.5°C, which is largely consistent with other lines of evidence and helps narrow the uncertainty range of the overall assessment of ECS. Some of the CMIP6 climate models that have either high (>5°C) or low (<2°C) ECS also simulate past global surface temperature changes outside the range of proxy-based reconstructions for the coldest and warmest reference periods. Since AR5, independent lines of evidence, including proxy records from past warm periods and glacial–interglacial cycles, indicate that sensitivity to forcing increases as temperature increases (Section TS.3.2.2). {7.4.3.2, 7.5.3, 7.5.6, Table 7.11}
Constraints on equilibrium climate sensitivity (ECS) and transient climate response (TCR) (see Glossary) are based on four main lines of evidence: feedback process understanding, climate change and variability seen within the instrumental record, paleoclimate evidence, and so-called ‘emergent constraints’, whereby a relationship between an observable quantity and either ECS or TCR established within an ensemble of models is combined with observations to derive a constraint on ECS or TCR. In reports up to and including the IPCC Third Assessment Report, ECS and TCR derived directly from ESMs were the primary line of evidence. However, since AR4, historical warming and paleoclimates provided useful additional evidence (Figure TS.16a). This Report differs from previous reports in not directly using climate model estimates of ECS and TCR in the assessed ranges of climate sensitivity. {1.5, 7.5}
A few notes from the AR6 Technical Summary on Effective Radiative Forcing (ERF) and Feedbacks
Anthropogenic emissions of greenhouse gases and their precursors contribute an ERF of 3.84 [3.46 to 4.22] W m–2 over the industrial era (1750–2019). Most of this total ERF, 3.32 [3.03 to 3.61] W m–2, comes from the well-mixed greenhouse gases, with changes in ozone and stratospheric water vapour (from methane oxidation) contributing the remainder. The ERF of greenhouse gases is composed of 2.16 [1.90 to 2.41] W m–2 from carbon dioxide, 0.54 [0.43 to 0.65] W m–2 from methane, 0.41 [0.33 to 0.49] W m–2 from halogenated species, and 0.21 [0.18 to 0.24] W m–2 from nitrous oxide. The ERF for ozone is 0.47 [0.24 to 0.71] W m–2. The estimate of ERF for ozone has increased since AR5 due to revised estimates of precursor emissions and better accounting for effects of tropospheric ozone precursors in the stratosphere. The estimated ERF for methane has slightly increased due to a combination of increases from improved spectroscopic treatments being somewhat offset by accounting for adjustments (high confidence). {7.3.2, 7.3.5}
Climate Feedbacks and Sensitivity
The net effect of changes in clouds in response to global warming is to amplify human-induced warming, that is, the net cloud feedback is positive (high confidence). Compared to AR5, major advances in the understanding of cloud processes have increased the level of confidence and decreased the uncertainty range in the cloud feedback by about 50%. An assessment of the low-altitude cloud feedback over the subtropical oceans, which was previously the major source of uncertainty in the net cloud feedback, is improved owing to a combined use of climate model simulations, satellite observations, and explicit simulations of clouds, altogether leading to strong evidence that this type of cloud amplifies global warming. The net cloud feedback, obtained by summing the cloud feedbacks assessed for individual regimes, is 0.42 [–0.10 to +0.94] W m–2 °C–1. A net negative cloud feedback is very unlikely (high confidence). {7.4.2, Figure 7.10, Table 7.10}
The combined effect of all known radiative feedbacks (physical, biogeophysical, and non-CO2 biogeochemical) is to amplify the base climate response, also known as the Planck temperature response (virtually certain). Combining these feedbacks with the base climate response, the net feedback parameter based on process understanding is assessed to be –1.16 [–1.81 to –0.51] W m–2 °C–1, which is slightly less negative than that inferred from the overall ECS assessment. The combined water-vapour and lapse-rate feedback makes the largest single contribution to global warming, whereas the cloud feedback remains the largest contribution to overall uncertainty. Due to the state-dependence of feedbacks, as evidenced from paleoclimate observations and from models, the net feedback parameter will increase (become less negative) as global temperature increases. Furthermore, on long time scales the ice-sheet feedback parameter is very likely positive, promoting additional warming on millennial time scales as ice sheets come into equilibrium with the forcing (high confidence). {7.4.2, 7.4.3, 7.5.7}
Radiative feedbacks, particularly from clouds, are expected to become less negative (more amplifying) on multi-decadal time scales as the spatial pattern of surface warming evolves, leading to an ECS that is higher than was inferred in AR5 based on warming over the instrumental record. This new understanding, along with updated estimates of historical temperature change, ERF, and Earth’s energy imbalance, reconciles previously disparate ECS estimates (high confidence). However, there is currently insufficient evidence to quantify a likely range of the magnitude of future changes to current climate feedbacks. Warming over the instrumental record provides robust constraints on the lower end of the ECS range (high confidence), but owing to the possibility of future feedback changes it does not, on its own, constrain the upper end of the range, in contrast to what was reported in AR5. {7.4.4, 7.5.2, 7.5.3}
Based on multiple lines of evidence the best estimate of ECS is 3°C, the likely range is 2.5°C to 4°C, and the very likely range is 2°C to 5°C. It is virtually certain that ECS is larger than 1.5°C. Substantial advances since AR5 have been made in quantifying ECS based on feedback process understanding, the instrumental record, paleoclimates and emergent constraints. There is a high level of agreement among the different lines of evidence. All lines of evidence help rule out ECS values below 1.5°C, but currently it is not possible to rule out ECS values above 5°C. Therefore, the 5°C upper end of the very likely range is assessed to have medium confidence and the other bounds have high confidence. {7.5.5}
Based on process understanding, warming over the instrumental record, and emergent constraints, the best estimate of TCR is 1.8°C, the likely range is 1.4°C to 2.2°C and the very likely range is 1.2°C to 2.4°C (high confidence). {7.5.5}
Emphases mine
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