Reconciled climate response estimates from climate models and the energy budget of Earth

Abstract

Correcing for these biases and accouning for wider uncertainies in radiaive forcing based on recent evidence, we infer an observaion-based best esimate for TCR of 1.66 °C with a 5-95 % range of 1.0-3.3 °C, consistent with the climate models considered in the IPCC 5 th Assessment Report.TCR for the Climate Model Intercomparison Project, phase 5 (CMIP5) models is deined using simulaions in which atmospheric CO 2 increases at 1 % per year and the muli-model mean is 1.8 °C (1.2-2.4 °C, henceforth bracketed values refer to 5-95 % ranges). 6-8 TCR has also been esimated from Earth's energy budget using: eicacy of ocean heat uptake, 20-22 structural uncertainies in energy-budget calculaions or lower real-world TCR.We focus on potenial biases in temperature series due to geographical incompleteness of the data ('masking') and the combinaion of air and water measurements ('blending') by applying energy-budget TCR calculaions to CMIP5 simulaions and observaions. We calculate energy-budget TCR with the Oto et al. (2013) method, henceforth 'Oto', which uses diferences between an early baseline period and a recent reference period: Where Δ Q is the system heat uptake which, being posiive during warming, means that ECS is larger than TCR. We do not calculate ECS here to avoid uncertainies associated with Δ Q , and to avoid the assumpion of linear climate response which is less accurate over the longer ime periods required for equilibrium. 17 However, as Δ T is in the numerators of Equaions (1) and (3), any Δ T bias afects each calculaion equally in percentage terms.Formally, TCR refers to global near-surface air temperature ('tas' in CMIP5 nomenclature) for Δ T while observaional temperature records have incomplete and varying geographical coverage and combine air temperatures over land and sea ice with near-surface water temperatures over oceans.These diferences introduce biases as warming is not spaially uniform, sea ice coverage changes and Model temperatures are reconstructed in three ways: by using global air temperature ('tas-only'), by blending air temperature over land and sea ice with ocean temperatures over water ('blended') and by blending temperatures and using the historical geographical coverage of observaions in HadCRUT4 ('blended-masked'). We assume that the modelled near-surface water temperature over oceans ('tos' in CMIP5 nomenclature) is equivalent to measured sea surface temperatures. Results are similar between models with diferent ocean layering: for example with 2.5 metre top-layer depth instead of 10 metres, suggesing tos is a robust measure of modelled sea surface temperature (see Supplementary Informaion).The 'tas-only' reconstrucions are used in standard model assessments of TCR, the 'blended' reconstrucions represent the same reconstrucion techniques as HadCRUT4 but with perfect data coverage and the 'blended-masked' reconstrucions represent HadCRUT4. Table 8 shows that the masking bias is largely due to undersampling of rapidly warming polar regions. The blending and masking efects were not acc...

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