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DOI	10.1007/s00382-020-05242-1
	Improved methods for estimating equilibrium climate sensitivity from transient warming simulations
	Dai A.; Huang D.; Rose B.E.J.; Zhu J.; Tian X.
发表日期	2020
ISSN	0930-7575
起始页码	4515
结束页码	4543
卷号	54
英文摘要	Equilibrium climate sensitivity (ECS) refers to the total global warming caused by an instantaneous doubling of atmospheric CO2 from the pre-industrial level in a climate system. ECS is commonly used to measure how sensitive a climate system is to CO2 forcing; but it is difficult to estimate for the real world and for fully coupled climate models because of the long response time in such a system. Earlier studies used a slab ocean coupled to an atmospheric general circulation model to estimate ECS, but such a setup is not the same as the fully coupled system. More recent studies used a linear fit between changes in global-mean surface air temperature (ΔT) and top-of-atmosphere net radiation (ΔN) to estimate ECS from relatively short simulations. Here we analyze 1000 years of simulation with abrupt quadrupling (4 × CO2) and another 500-year simulation with doubling (2 × CO2) of pre-industrial CO2 using the CESM1 model, and three other multi-millennium (~5000 year) abrupt 4 × CO2 simulations to show that the linear-fit method considerably underestimates ECS due to the flattening of the −dN/dT slope, as noticed previously. We develop and evaluate three other methods, and propose a new method that makes use of the realized warming near the end of the simulations and applies the −dN/dT slope calculated from a best fit of the ΔT and ΔN data series to a simple two-layer model to estimate the unrealized warming. Using synthetic data and the long model simulations, we show that the new method consistently outperforms the linear-fit method with small biases in the estimated ECS using 4 × CO2 simulations with at least 180 years of simulation. The new method was applied to 4 × CO2 experiments from 20 CMIP5 and 19 CMIP6 models, and the resulting ECS estimates are about 10% higher on average and up to 25% higher for models with medium–high ECS (> 3 K) than those reported in the IPCC AR5. Our new estimates suggest an ECS range of about 1.78–5.45 K with a mean of 3.61 K among the CMIP5 models and about 1.85–6.25 K with a mean of 3.60 K for the CMIP6 models. Furthermore, stable ECS estimates require at least 240 (180) years of simulation for using 2 × CO2 (4 × CO2) experiments, and using shorter simulations may underestimate the ECS substantially. Our results also suggest that it is the forced −dN/dT slope after year 40, not the internally-generated −dN/dT slope, that is crucial for an accurate estimate of the ECS, and this forced slope may be fairly stable. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
英文关键词	Climate feedback; Climate models; Climate sensitivity; CMIP5; Equilibrium response; Global warming
语种	英语
scopus关键词	carbon dioxide; climate forcing; climate modeling; climate variation; equilibrium; estimation method; global warming; warming
来源期刊	Climate Dynamics
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/145455
作者单位	Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, NY 12222, United States; School of Atmospheric Sciences, Nanjing University, Nanjing, China; College of Hydrology and Water Resources, Hohai University, Nanjing, China; ICCES, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
推荐引用方式 GB/T 7714	Dai A.,Huang D.,Rose B.E.J.,et al. Improved methods for estimating equilibrium climate sensitivity from transient warming simulations[J],2020,54.
APA	Dai A.,Huang D.,Rose B.E.J.,Zhu J.,&Tian X..(2020).Improved methods for estimating equilibrium climate sensitivity from transient warming simulations.Climate Dynamics,54.
MLA	Dai A.,et al."Improved methods for estimating equilibrium climate sensitivity from transient warming simulations".Climate Dynamics 54(2020).