Abstract
AbstractThe Earth's climate sensitivity to radiative forcing remains a key source of uncertainty in future warming projections. There is a growing realization in recent literature that research must go beyond an equilibrium and CO2‐only viewpoint, toward considering how climate sensitivity will evolve over time in response to anthropogenic and natural radiative forcing from multiple sources. Here the transient behavior of climate sensitivity is explored using a modified energy balance model, in which multiple climate feedbacks evolve independently over time to multiple sources of radiative forcing, combined with constraints from observations and from the Climate Model Intercomparison Project phase 5 (CMIP5). First, a large initial ensemble of 107 simulations is generated, with a distribution of climate feedback strengths from subannual to 102‐year timescales constrained by the CMIP5 ensemble, including the Planck feedback, the combined water vapor lapse rate feedback, snow and sea ice albedo feedback, fast cloud feedbacks, and the cloud response to sea surface temperature adjustment feedback. These 107 simulations are then tested against observational metrics representing decadal trends in warming, heat and carbon uptake, leaving only 4.6 × 103 history‐matched simulations consistent with both the CMIP5 ensemble and historical observations. The results reveal an annual timescale climate sensitivity of 2.1 °C (ranging from 1.6 to 2.8 °C at 95% uncertainty), rising to 2.9 °C (from 1.9 to 4.6 °C) on century timescales. These findings provide a link between lower estimates of climate sensitivity, based on the current transient state of the climate system, and higher estimates based on long‐term behavior of complex models and palaeoclimate evidence.
Highlights
There is currently significant uncertainty in the sensitivity of Earth’s climate to radiative forcing, with the IPCC Assessment Report 5 (IPCC, 2013) estimating that the EquilibriumClimate Sensitivity (ECS, measuring the surface temperature response in °C to a sustained doubling of CO2) ranges from 1.5 °C to 4.5 °C (Fig. 1a, black)
We notice from (5) that the total radiative forcing divided by the overall effective climate feedback parameter at time t, Rtotal(t)/λ(t), represents the overall warming that would be achieved from all sources of radiative forcing if the global climate system were in energy balance, N(t) = 0, via
This is achieved in Warming Acidification and Sea level Projector (WASP) by using two time-stepping equations: one equation adjusting the climate feedbacks to the existing radiative forcing to the ith source at the previous time-step, and a second equation adjusting the climate feedback to the additional radiative forcing from the ith source since the previous time-step, to account for the feedback to any additional radiative forcing being the Planck feedback initially
Summary
There is currently significant uncertainty in the sensitivity of Earth’s climate to radiative forcing, with the IPCC Assessment Report 5 (IPCC, 2013) estimating that the Equilibrium. Instead of applying efficacy weightings (2), this study explores an alternative approach: Here, the energy balance equation (1) is extended to explicitly include time-varying climate feedbacks from multiple processes, that each respond independently to multiple radiative forcing agents This extended energy balance equation is used to constrain the climate sensitivity over multiple timescales, and used to show that this may explain the discrepancy between climate sensitivity estimates from the current transient energy balance and century timescale approaches (Fig. 1).
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