Abstract
Projections of future climate made by model-ensembles have credibility because the historic simulations by these models are consistent with, or near-consistent with, historic observations. However, it is not known how small inconsistencies between the ranges of observed and simulated historic climate change affects the future projections made by a model ensemble. Here, the impact of historical simulation–observation inconsistencies on future warming projections is quantified in a 4-million member Monte Carlo ensemble from a new efficient Earth System Model (ESM). Of the 4-million ensemble members, a subset of 182,500 are consistent with historic ranges of warming, heat uptake and carbon uptake simulated by the Climate Model Intercomparison Project 5 (CMIP5) ensemble. This simulation–consistent subset projects similar future warming ranges to the CMIP5 ensemble for all four RCP scenarios, indicating the new ESM represents an efficient tool to explore parameter space for future warming projections based on historic performance. A second subset of 14,500 ensemble members are consistent with historic observations for warming, heat uptake and carbon uptake. This observation–consistent subset projects a narrower range for future warming, with the lower bounds of projected warming still similar to CMIP5, but the upper warming bounds reduced by 20–35 %. These findings suggest that part of the upper range of twenty-first century CMIP5 warming projections may reflect historical simulation–observation inconsistencies. However, the agreement of lower bounds for projected warming implies that the likelihood of warming exceeding dangerous levels over the twenty-first century is unaffected by small discrepancies between CMIP5 models and observations.
Highlights
Two parameters are altered between ensemble-members to encapsulate current uncertainty in the magnitude of anthropogenic radiative forcing over time (Myhre et al 2013): 1. The coefficient relating radiative forcing to the log change in atmospheric CO2, aCO2 (Wm−2), is relatively well-constrained (Myhre et al 2013, 1998), and so is varied with a random-normal distribution with mean 5.35 Wm−2 and standard deviation 0.27 Wm−2 (Fig. 3a, black), to reflect the mean and uncertainty range used in Myhre et al (2013)
Equations (1) and (2) are applied to the Representative CO2 Concentration Pathway (RCP) scenarios (Meinshausen et al 2011) to prescribe total radiative forcing over time, with aCO2 and funcert are varied between ensemble members with random-normal distributions to reflect uncertainty in the magnitude of anthropogenic radiative forcing in 2011 (Fig. 3, black)
A new Earth System Model was presented for efficient projection of the global mean impacts of carbon emissions: the Warming Acidification and Sea-level Projector (WASP: Fig. 2; “Appendix” section)
Summary
Earth System Model (ESM) ensembles forced with prescribed Representative CO2 Concentration Pathway (RCP) scenarios (Meinshausen et al 2011) show significant spread in twenty-first century projections of warming and compatible carbon emissions (e.g. Collins et al 2013; Gillet et al 2013; Matthews et al 2009; Zickfield et al 2009; 2012) (Fig. 1), even though each ensemble-member is consistent, or close to consistent, with observations of historic and present-day climate change (e.g. Hartmann et al 2013; Rhein et al 2013; Flato et al 2013). Collins et al 2013; Gillet et al 2013; Matthews et al 2009; Zickfield et al 2009; 2012) (Fig. 1), even though each ensemble-member is consistent, or close to consistent, with observations of historic and present-day climate change (e.g. Hartmann et al 2013; Rhein et al 2013; Flato et al 2013) This future spread leads to significant uncertainty in the sensitivity of future warming to carbon emissions, termed the Transient Climate Response to Emission (Gillet et al 2013), or TCRE [K (1000 PgC)−1].
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