Transient and Quasi‐Equilibrium Climate States at 1.5°C and 2°C Global Warming
Andrew D. KingAlexander R BorowiakJosephine R. BrownDavid J. FrameLuke J. HarringtonSeung‐Ki MinAngeline G. PendergrassMaria RugensteinJ. M. Kale SnidermanDáithí A. Stone
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Abstract Recent climate change is characterized by rapid global warming, but the goal of the Paris Agreement is to achieve a stable climate where global temperatures remain well below 2°C above pre‐industrial levels. Inferences about conditions at or below 2°C are usually made based on transient climate projections. To better understand climate change impacts on natural and human systems under the Paris Agreement, we must understand how a stable climate may differ from transient conditions at the same warming level. Here we examine differences between transient and quasi‐equilibrium climates using a statistical framework applied to greenhouse gas‐only model simulations. This allows us to infer climate change patterns at 1.5°C and 2°C global warming in both transient and quasi‐equilibrium climate states. We find substantial local differences between seasonal‐average temperatures dependent on the rate of global warming, with mid‐latitude land regions in boreal summer considerably warmer in a transient climate than a quasi‐equilibrium state at both 1.5°C and 2°C global warming. In a rapidly warming world, such locations may experience a temporary emergence of a local climate change signal that weakens if the global climate stabilizes and the Paris Agreement goals are met. Our research demonstrates that the rate of global warming must be considered in regional projections.Keywords:
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Abstract. In the ongoing political debate on climate change, global mean temperature change (ΔTglob) has become the yardstick by which mitigation costs, impacts from unavoided climate change, and adaptation requirements are discussed. For a scientifically informed discourse along these lines, systematic assessments of climate change impacts as a function of ΔTglob are required. The current availability of climate change scenarios constrains this type of assessment to a narrow range of temperature change and/or a reduced ensemble of climate models. Here, a newly composed dataset of climate change scenarios is presented that addresses the specific requirements for global assessments of climate change impacts as a function of ΔTglob. A pattern-scaling approach is applied to extract generalised patterns of spatially explicit change in temperature, precipitation and cloudiness from 19 Atmosphere–Ocean General Circulation Models (AOGCMs). The patterns are combined with scenarios of global mean temperature increase obtained from the reduced-complexity climate model MAGICC6 to create climate scenarios covering warming levels from 1.5 to 5 degrees above pre-industrial levels around the year 2100. The patterns are shown to sufficiently maintain the original AOGCMs' climate change properties, even though they, necessarily, utilise a simplified relationships between ΔTglob and changes in local climate properties. The dataset (made available online upon final publication of this paper) facilitates systematic analyses of climate change impacts as it covers a wider and finer-spaced range of climate change scenarios than the original AOGCM simulations.
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This paper is an attempt to show that global warming and climate change are real, and are greatly affecting the biosphere. Evidences suggesting that the increasing concentration of atmospheric greenhouse gases due to human activities is mainly responsible for global warming and climate change are presented. Uncertainties about the future course of global warming and climate change centred on the role of two important mechanisms, forcings and feedback processes, and their influence on earth’s surface temperature are also discussed. Suitable and innovative geo-engineering measures that could make effective and efficient use of the scarce resources and maximize returns from the resources invested to limit the emissions of greenhouse gases are evaluated. Preferred actions to control and stabilize global warming and climate change which can be widely applied to reorient economic developmental policies in developing countries are examined.
Key words: Global warming, climate change, greenhouse gases, uncertainties, feedback mechanisms, measures.
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Abstract As the greenhouse gas concentrations increase, a warmer climate is expected. However, numerous internal climate processes can modulate the primary radiative warming response of the climate system to rising greenhouse gas forcing. Here the particular internal climate process that we focus on is the Atlantic meridional overturning circulation (AMOC), an important global-scale feature of ocean circulation that serves to transport heat and other scalars, and we address the question of how the mean strength of AMOC can modulate the transient climate response. While the Community Earth System Model version 2 (CESM2) and the Energy Exascale Earth System Model version 1 (E3SM1) have very similar equilibrium/effective climate sensitivity, our analysis suggests that a weaker AMOC contributes in part to the higher transient climate response to a rising greenhouse gas forcing seen in E3SM1 by permitting a faster warming of the upper ocean and a concomitant slower warming of the subsurface ocean. Likewise the stronger AMOC in CESM2 by permitting a slower warming of the upper ocean leads in part to a smaller transient climate response. Thus, while the mean strength of AMOC does not affect the equilibrium/effective climate sensitivity, it is likely to play an important role in determining the transient climate response on the centennial time scale.
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There are a variety of uncertainties of understanding on climate change,although much important progress has been achieved so far.In this paper,some hot issues related with disagreements are explored on both sides,which could be significant to climate change research.These issues can be listed as follows:1)the past climate change-whether the Medieval Warm Period and the Little Ice Age existed in the past 2000 years or not,whether the global mean temperature of the 20th century was the highest in the latest 1000 years or not,and whether the global warming trend in the last decade has disappeared or not;2)greenhouse effects-different viewpoints on its mechanism,greenhouse effects on climate change,and effects of water vapor in the atmosphere on global warming.3)simulation of climate system-differences of the global mean temperature by observed and simulated climate models as well as their shortcomings.4)2℃ threshold of the global warming-the physical significance of 2℃ threshold for the earth,and disputation on itself.Finally,it is proposed that five topics on climate change research need to be studied further in the near future.
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Climate sensitivity is an important index that measures the relationship between the increase in greenhouse gases and the magnitude of global warming. Uncertainties in climate change projection and climate modeling are mostly related to the climate sensitivity. The climate sensitivities of coupled climate models determine the magnitudes of the projected global warming. In this paper, the authors thoroughly review the literature on climate sensitivity, and discuss issues related to climate feedback processes and the methods used in estimating the equilibrium climate sensitivity and transient climate response (TCR), including the TCR to cumulative CO2 emissions. After presenting a summary of the sources that affect the uncertainty of climate sensitivity, the impact of climate sensitivity on climate change projection is discussed by addressing the uncertainties in 2°C warming. Challenges that call for further investigation in the research community, in particular the Chinese community, are discussed.
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