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Because longwave (LW) absorption by greenhouse gases and clouds is more significant than the LW scattering effect by clouds, most climate models neglect cloud LW scattering to save computational costs. Ignoring cloud LW scattering directly overestimates outgoing longwave radiation (OLR). This study included ice‐cloud LW scattering treatment in the Exascale Energy Earth System Model (E3SM) version 2 and ran fully‐coupled simulations, prescribed sea surface temperature simulations, and offline radiative transfer calculations to comprehensively assess the impact of ice‐cloud LW scattering on global climate simulation. The instantaneous effect due to ice‐cloud LW scattering reduces the OLR by ∼1 W/m2 on the global average and 2 W/m2 on the tropical average. Tropospheric warming and high cloud amount reduction act to partially compensate for such instantaneous OLR reduction caused by the inclusion of LW scattering. When the simulation reaches the equilibrium, the surface warms by 0.66 K on average with respect to the simulation without LW scattering, with the Arctic surface temperature differences more than twice as large as that of the global mean. The impact of including LW scattering on the simulated climate change in response to 4 × CO2 is also assessed. While including the cloud LW scattering does not significantly modify radiative forcing and total radiative feedback under such a scenario, it results in a 10% more positive cloud feedback. Plain Language Summary: Clouds are an essential mediator in the climate system because they can reflect solar radiation back to space and block longwave radiation emitted below reaching the top of the atmosphere by either absorbing it or scattering it elsewhere. Such longwave scattering physics is deemed less important and thus neglected in most climate models to save computational time. We incorporated this mechanism into a climate model and ran pairs of simulations, with or without cloud scattering, to see how it would affect the simulated global climate. We found that cloud longwave scattering reduces the longwave radiation that goes to space. Such reduction of outgoing longwave radiation is strongest in the tropics. Compared to the simulation without longwave scattering, the mean‐state surface temperature change is larger in the Arctic than in the tropics, which is primarily caused by the slow response to the inclusion of scattering. We also assessed to what extent the inclusion of cloud longwave scattering can affect the simulated response to abrupt 4 × CO2 increase. We concluded that it can increase the cloud feedback strength by ∼10%, but overall, the impact is not statistically significant. Key Points: Ice‐cloud longwave (LW) scattering leads to warming with a pattern similar to the response to quadrupled CO2 in Exascale Energy Earth System Model version 2Strong Arctic warming is part of the global response to radiative perturbation rather than a local impact of LW scatteringIncluding ice‐cloud LW scattering does not significantly affect the simulated responses to abrupt 4 × CO2 increases [ABSTRACT FROM AUTHOR]

Most climate models neglect cloud longwave (LW) scattering because scattering is considered negligible compared to strong LW absorption by clouds and greenhouse gases. While this rationale is valid for simulating extrapolar regions, it is questionable for the polar regions, where the atmosphere is dry and hence has weak absorption, and ice clouds that have strong scattering capability frequently occur. Using the slab‐ocean Community Earth System Model, we show that ice cloud LW scattering can warm winter surface air temperature by 0.8–1.8 K in the Arctic and 1.3–1.9 K in the Antarctic, while this warming becomes much weaker in polar summer. Such scattering effect cannot be correctly assessed when sea surface temperature and sea ice are prescribed as this effect is manifested through a surface‐atmosphere coupling. Cloud LW scattering is a necessity for the correct simulation of polar climate and surface radiation budget, especially in the winter. Plain Language Summary: Cloud longwave scattering has never been deemed as a necessity in climate models. Out of all climate models in the IPCC fifth and sixth assessments, only three modeling centers have longwave scattering included in their models. Our study explained why the traditional wisdom of neglecting longwave scattering breaks down for the simulation of high‐latitude climate in the fully coupled models. We showed the critical importance of atmosphere‐surface radiative coupling for correctly assessing the role of cloud longwave scattering in the model simulation of climate mean state as well as climate changes, an issue overlooked by all previous studies. We argued that the cloud longwave scattering is a necessity in climate models, not an option. Key Points: Cloud longwave scattering is more important in the polar regions than the extrapolar regionsBy surface‐atmosphere radiative coupling, cloud longwave scattering can warm the polar surface, more in the winter than in the summerCloud longwave scattering is a necessity instead of an option for correctly simulating polar climate and surface energy budget [ABSTRACT FROM AUTHOR]

• MC6 ice and water cloud and THM ice cloud LW optical properties are parameterized. • 2-/4-stream combination is approximately as accurate as 4-stream DISORT. • 2-/4-stream combination facilitates accurate and efficient flux simulations. A radiative transfer model (RTM) that accurately and explicitly accounts for both absorbing and scattering effects requires a substantial amount of computational effort. In the longwave (LW) spectral regime, the atmosphere is optically opaque and absorbs a large portion of terrestrial thermal radiation. To alleviate the computational burden, clouds are assumed to be only absorptive in most general circulation models (GCMs) and their scattering effects are neglected. Using parameterizations of cloud bulk single-scattering properties derived from the latest cloud optical property models for satellite remote sensing, this study analyzes the numerical accuracy and efficiency of a variety of LW RTMs. The approaches considered in this study include the absorption approximation (AA), the absorption approximation with scattering parameterization (ASA), the two-stream approximation (2S), a hybrid two- and four-stream approximation (2/4S), and the discrete ordinate radiative transfer (DISORT) with multiple streams. These approximations are benchmarked against 128-stream DISORT calculation. After evaluating the full ranges of ice and water cloud optical and microphysical properties using these RTMs, we find that neglecting LW scattering effect causes simulation errors as large as ±15% by using the AA method. Among these RTMs, the 2/4S method provides an optimal balance between computational efficiency and accuracy, leading to the maximum cloud emissivity errors within ±5%, 25th to 75th percentile errors about ±1%, and almost zero net bias. Therefore, the 2/4S approximation is computationally accurate and yet affordable option for incorporating cloud longwave scattering effect into the radiation schemes used in weather and climate models. [ABSTRACT FROM AUTHOR]