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Shao-Yi Lee
,
Ho-Jeong Shin
, and
Chien Wang

1. Introduction Field observations in recent decades have revealed an abundance of anthropogenic aerosols, particularly absorbing aerosols, over South Asia and its surrounding ocean during the dry season ( Ramanathan et al. 2001 ). Modeling studies and observation-based analyses have indicated that the radiative effects of these aerosols can interrupt not just localized meteorological features but also large-scale climate features such as monsoons. Using prescribed direct radiative forcing

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S. J. Ghan
,
X. Liu
,
R. C. Easter
,
R. Zaveri
,
P. J. Rasch
,
J.-H. Yoon
, and
B. Eaton

1. Introduction Anthropogenic aerosol is thought to play an important role in driving climate change, but its role is so complex that uncertainty in estimates of radiative forcing of climate change is dominated by uncertainty associated with forcing by anthropogenic aerosol ( Forster et al. 2007 ). This complexity arises because anthropogenic aerosol alters the planetary energy balance through a variety of mechanisms operating across a wide range of spatial scales: direct effects ( Haywood and

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Hong Yin
and
Ying Sun

influence has warmed the atmosphere, ocean, and land” ( IPCC 2021 , p. 4). Hegerl et al. (2019) reviewed the causes of climate change from the nineteenth century, identifying that greenhouse gases have been the main driver of global warming since the beginning of industrialization. The cooling effects from anthropogenic aerosol forcing are likely to have masked some warming since the post-1950s when sulfate aerosols emissions increased. The combined effects of solar and volcanic forcings also

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G. Chagnaud
,
G. Panthou
,
T. Vischel
, and
T. Lebel

hand, the partial recovery of the last decades seems to be a combination of an increase in greenhouse gases (GHG) concentrations ( Dong and Sutton 2015 ; Park et al. 2016 ) and the decline of anthropogenic aerosols concentrations ( Giannini and Kaplan 2019 ; Marvel et al. 2020 ), with the reversal of the AMO from a negative to a positive phase also involved ( Mohino et al. 2011 ). Marvel et al. (2020) underline, though, that unraveling the respective roles of aerosols forcing and oceanic

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Bjorn Stevens
and
Stephanie Fiedler

Stevens (2015 , hereinafter S15) used three lines of reasoning to argue that present-day effective aerosol radiative forcing is very likely less negative than −1 W m −2 . The most quantitative bound arose from the logic that if one wishes to maintain that some component of the warming in the first half of the twentieth century was anthropogenic in origin, then this bounds to have a smaller magnitude than other positive forcings over the same time period. By adopting a simple model that

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Nikolaos Christidis
and
Peter A Stott

Knutson and Zeng (2018) demonstrated that CMIP5 models yield realistic regional precipitation trends and reported consistency with the observations in most areas. It has also been claimed that spatial characteristics of the response to climatic forcings may in fact render certain regional changes more detectable ( Sarojini et al. 2016 ). Attribution studies set out to establish whether the effect of external, and, in particular, anthropogenic forcings can be detected in observed climatic changes

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Rei Chemke
and
Guy Dagan

. As mentioned in section 1 , aerosols may modulate the temperature field by both changing its spatial structure and its global-mean value. A global change in temperature (even if driven by radiative forcing that is homogenously distributed) would affect the global circulation (e.g., Held and Soden 2006 ; Vallis et al. 2015 ). To focus only on the effects of the spatial distribution of anthropogenic aerosols on the circulation, the ARF is normalized such that its global average value is 0. This

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Isla R. Simpson
,
Nan Rosenbloom
,
Gokhan Danabasoglu
,
Clara Deser
,
Stephen G. Yeager
,
Christina S. McCluskey
,
Ryohei Yamaguchi
,
Jean-Francois Lamarque
,
Simone Tilmes
,
Michael J. Mills
, and
Keith B. Rodgers

1. Introduction Historically, Earth’s climate system has evolved under a mixture of natural and anthropogenic forcings, and it will continue to do so moving forward. A common approach that is used to disentangle and understand the relative contributions of such forcings to the evolution of the climate system is to perform Earth system model (ESM) experiments in which only some forcings are evolving in time while others are held fixed. These experiments, which we will refer to as single

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Vijayakumar S. Nair
,
S. Suresh Babu
,
K. Krishna Moorthy
, and
S. S. Prijith

could lead to altering the regional climate through the dynamical feedbacks ( Chung et al. 2002 ; Menon et al. 2002 ; Lau et al. 2006 ). Recently, Bollasina et al. (2011) have shown that spatially heterogeneous anthropogenic aerosol forcing dynamically induce circulation changes and that could explain the decrease in monsoon precipitation over central-northern India. Slowdown of tropical circulation in association with anthropogenic forcing was reported earlier by Vecchi et al. (2006

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Chunhui Lu
,
Ying Sun
, and
Xuebin Zhang

role of aerosols in reducing DTR was made explicit, although the role of greenhouse gases was only implicitly mentioned through the increase of water vapor, which is a result of warming. The observed decrease in DTR over Asia was reproduced by climate models under the combined effect of anthropogenic and natural forcing, with GHG forcing produced larger decrease in DTR for Asia overall. While understanding physical mechanisms involved in the observed decrease in DTR is useful, quantifying influence

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