Abstract
In both the observational record and atmosphere-ocean general circulation model (AOGCM) simulations of the last $$\sim$$ 150 years, short-lived negative radiative forcing due to volcanic aerosol, following explosive eruptions, causes sudden global-mean cooling of up to $$\sim$$ 0.3 K. This is about five times smaller than expected from the transient climate response parameter (TCRP, K of global-mean surface air temperature change per W m−2 of radiative forcing increase) evaluated under atmospheric CO2 concentration increasing at 1 % yr−1. Using the step model (Good et al. in Geophys Res Lett 38:L01703, 2011. doi: 10.1029/2010GL045208 ), we confirm the previous finding (Held et al. in J Clim 23:2418–2427, 2010. doi: 10.1175/2009JCLI3466.1 ) that the main reason for the discrepancy is the damping of the response to short-lived forcing by the thermal inertia of the upper ocean. Although the step model includes this effect, it still overestimates the volcanic cooling simulated by AOGCMs by about 60 %. We show that this remaining discrepancy can be explained by the magnitude of the volcanic forcing, which may be smaller in AOGCMs (by 30 % for the HadCM3 AOGCM) than in off-line calculations that do not account for rapid cloud adjustment, and the climate sensitivity parameter, which may be smaller than for increasing CO2 (40 % smaller than for 4 × CO2 in HadCM3).
Highlights
The global-mean surface air temperature, expressed as the difference T from an unperturbed steady state, is widely used as an indicator of the magnitude of global climate change, both in observations and in simulations of the past and future
The step model includes this effect, it still overestimates the volcanic cooling simulated by atmosphere–ocean general circulation models (AOGCMs) by about 60 %. We show that this remaining discrepancy can be explained by the magnitude of the volcanic forcing, which may be smaller in AOGCMs than in offline calculations that do not account for rapid cloud adjustment, and the climate sensitivity parameter, which may be smaller than for increasing CO2 (40 % smaller than for 4 × CO2 in HadCM3)
We demonstrate this by comparing the AOGCM ensemblemean historical T with the ensemble mean of estimates derived from the AR5 historical forcing according to the zero-layer model F = ρT (Fig. 2a, black and blue lines) using ρ for each model from its own 1pctCO2 experiment (Table 1)
Summary
The global-mean surface air temperature, expressed as the difference T from an unperturbed steady state, is widely used as an indicator of the magnitude of global climate change, both in observations and in simulations of the past and future. The Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change considered a set of scenarios under which the nominal radiative forcing at 2100 (relative to pre-industrial, regarded as the unperturbed steady state) ranges between 2.6 and 8.5 W m−2 (e.g. Fig. 12.4 of Collins et al 2013). When integrated with historical changes in radiative forcing agents, coupled atmosphere–ocean general circulation models (AOGCMs) show an ensemble-mean historical warming trend due to anthropogenic forcing that is very similar to the observed, as many studies have demonstrated (recently assessed by Bindoff et al 2013). AR5 anthropogenic AR5 anthropogenic + natural AR5 volcanic HadCM3 sstPIhistVol for the historical period from the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (Fig. 1a; Myhre et al 2013) shows a maximum magnitude of −3.6 W m−2 for the Krakatau eruption of 1883. 1.0 Observations CMIP5 AOGCMs Estimate using TCRP (1/ρ) from CMIP5 1pctCO2 Step model with CMIP5 abrupt4xCO2 and AR5 forcing
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