The surface temperature response to greenhouse gas forcing displays a characteristic pattern of polar-amplified warming 1-5 , particularly in the Northern Hemisphere. However, the causes of this polar amplification are still debated. Some studies highlight the importance of surface-albedo feedback 6-8 , while others find larger contributions from longwave feedbacks 4,9,10 , with changes in atmospheric and oceanic heat transport also thought to play a role [11][12][13][14][15][16] . Here, we determine the causes of polar amplification using climate model simulations in which CO 2 forcing is prescribed in distinct geographical regions, with the linear sum of climate responses to regional forcings replicating the response to global forcing. The degree of polar amplification depends strongly on the location of CO 2 forcing. In particular, polar amplification is found to be dominated by forcing in the polar regions, specifically through positive local lapse-rate feedback, with ice-albedo and Planck feedbacks playing subsidiary roles. Extra-polar forcing is further shown to be conducive to polar warming, but given that it induces a largely uniform warming pattern through enhanced poleward heat transport, it contributes little to polar amplification. Therefore, understanding polar amplification requires primarily a better insight into local forcing and feedbacks rather than extra-polar processes.Polar amplification-commonly defined as the ratio of polar warming to tropical warming 4,10 -is a robust feature of climate change seen in historical observations and climate model simulations [1][2][3][4][5] . Accurate predictions of polar warming are critical given the fundamental role that polar ice plays in the climate system, terrestrial and marine ecosystems, and human society.A key challenge is identifying the roles that local (that is, polar) and remote (that is, extra-polar) processes play in polar amplification within the inherently coupled climate system. Indeed, different conclusions have been reached as to which feedbacks most contribute to polar amplification and whether poleward heat transport plays a significant role 4,5,[7][8][9][10][11][12][13][14]17,18 . These differences may, in part, be due to different analysis methods. For instance: using simulations with prescribed changes in sea-ice and sea-surface temperatures (SSTs), Screen et al. 7 argue that sea-ice loss is the main contributor to Arctic

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The mechanism of polar amplification in the absence of surface albedo feedback is investigated using an atmospheric model coupled to an aquaplanet slab ocean forced by a CO2 doubling. In particular, we examine the sensitivity of polar surface warming response under different insolation conditions from equinox (EQN) to annual mean (ANN) to seasonally varying (SEA). Varying insolation greatly affects the climatological static stability. The equinox condition, with the largest polar static stability, exhibits a bottom-heavy vertical profile of polar warming response that leads to the strongest polar amplification. In contrast, the polar warming response in ANN and SEA exhibits a maximum in the midtroposphere, which leads to only weak polar amplification. The midtropospheric warming maximum, which results from an increased poleward atmospheric energy transport in response to the tropics-to-pole energy imbalance, contributes to polar surface warming via downward clear-sky longwave radiation. However, it is cancelled by negative cloud radiative feedbacks locally. Furthermore, the polar lapse rate feedback, calculated from radiative kernels, is negative due to the midtropospheric warming maximum, and hence is not able to promote the polar surface warming. On the other hand, the polar lapse rate feedback in EQN is positive due to the bottom-heavy warming response, contributing to the strong polar surface warming. This contrast suggests that locally induced positive radiative feedbacks are necessary for strong polar amplification. Our results demonstrate how interactions among climate feedbacks determine the strength of polar amplification.

The polar region has been one of the fastest warming places on Earth in response to greenhouse gas (GHG) forcing. Two distinct processes contribute to the observed warming signal: (i) local warming in direct response to the GHG forcing and (ii) the effect of enhanced poleward heat transport from low latitudes. A series of aquaplanet experiments, which excludes the surface albedo feedback, is conducted to quantify the relative contributions of these two physical processes to the polar warming magnitude and degree of amplification relative to the global mean. The globe is divided into zonal bands with equal area in eight experiments. For each of these, an external heating is prescribed beneath the slab ocean layer in the respective forcing bands. The summation of the individual temperature responses to each local heating in these experiments is very similar to the response to a globally uniform heating. This allows the authors to decompose the polar warming and amplification signal into the effects of local and remote heating. Local polar heating that induces surface-trapped warming due to the large tropospheric static stability in this region accounts for about half of the polar surface warming. Cloud radiative effects act to enhance this local contribution. In contrast, remote nonpolar heating induces a robust polar warming pattern that features a midtropospheric peak, regardless of the meridional location of the forcing. Among all remote forcing experiments, the deep tropical forcing case contributes most to the polar-amplified surface warming pattern relative to the global mean, while the high-latitude forcing cases contribute most to enhancing the polar surface warming magnitude.

An upper-level warming in the Arctic is more efficient at perturbing remote large-scale atmospheric circulations • Near-surface Arctic warming is effectively compensated by local outgoing longwave radiation rather than atmospheric energy transport • Strong sensitivity of longwave at the surface is due to the large emissivity of the polar atmosphere