An observation-based scaling model for climate sensitivity estimates and global projections to 2100
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An observation‑based scaling model for climate sensitivity estimates and global projections to 2100 Raphaël Hébert1 · Shaun Lovejoy2 · Bruno Tremblay3 Received: 30 May 2017 / Accepted: 3 November 2020 © The Author(s) 2020
Abstract We directly exploit the stochasticity of the internal variability, and the linearity of the forced response to make global temperature projections based on historical data and a Green’s function, or Climate Response Function (CRF). To make the problem tractable, we take advantage of the temporal scaling symmetry to define a scaling CRF characterized by the scaling exponent H, which controls the long-range memory of the climate, i.e. how fast the system tends toward a steady-state, and an inner scale 𝜏 ≈ 2 years below which the higher-frequency response is smoothed out. An aerosol scaling factor and a non-linear volcanic damping exponent were introduced to account for the large uncertainty in these forcings. We estimate the model and forcing parameters by Bayesian inference which allows us to analytically calculate the transient climate response and the equilibrium climate sensitivity as: 1.7+0.3 K and 2.4+1.3 K respectively (likely range). Projections to 2100 accord−0.2 −0.6 ing to the RCP 2.6, 4.5 and 8.5 scenarios yield warmings with respect to 1880–1910 of: 1.5+0.4 K and 4.2+1.3 K. K , 2.3+0.7 −0.9 −0.2 −0.5 These projection estimates are lower than the ones based on a Coupled Model Intercomparison Project phase 5 multi-model ensemble; more importantly, their uncertainties are smaller and only depend on historical temperature and forcing series. The key uncertainty is due to aerosol forcings; we find a modern (2005) forcing value of [−1.0, −0.3] Wm−2 (90 % confidence interval) with median at −0.7 Wm−2 . Projecting to 2100, we find that to keep the warming below 1.5 K, future emissions must undergo cuts similar to RCP 2.6 for which the probability to remain under 1.5 K is 48 %. RCP 4.5 and RCP 8.5-like futures overshoot with very high probability. Keywords Global mean temperature · Projections · Climate sensitivity · RCP scenarios · Global warming · Scaling
1 Introduction
* Raphaël Hébert [email protected] Shaun Lovejoy [email protected] Bruno Tremblay [email protected] 1
Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Telegrafenberg A45, Potsdam 14473, Germany
2
Department of Physics, McGill University, 3600 rue University, Montréal, Québec H3A 2T8, Canada
3
Department of Atmospheric and Oceanic Sciences, McGill University, 845 rue Sherbrooke Ouest, Montréal, Québec H3A 0G4, Canada
The atmosphere is a complex system involving turbulent processes operating over a wide range of scales starting from millimeters at the Kolmogorov dissipation scale up to the size of the Earth, spanning over 10 orders of magnitudes in space. The dynamics are sensitive to initial conditions and there are deterministic predictability limits that are roughly equal to the eddy turn-over time (lifetime) of structures. For planetary scale
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