During 2020, the Arctic is marked by extremely low sea ice coverage and hot climate. September Sea Ice Extent (SIE) was about 2.3 million km below the 1979–2014 mean and the 2nd lowest on the 1979–2020 record, while regional summer (June–August, JJA) mean 2 m air temperature (TAS) was about 1.3 °C above the 1979–2014 mean and was the hottest on record at the time. Locally, September Sea Ice Concentration (SIC) was approximately 70% lower and JJA TAS can be as much as 6.0 °C higher than the 1979–2014 mean. Although the proximate cause for the extreme event was the continuously favorable atmospheric circulation patterns, wind conditions and ice-albedo feedback, the main objective of this paper is probabilistic extreme event attribution studies to assess the anthropogenic influence. Based on the CMIP6 multi-model ensemble products, modeled long-term trends of Arctic sea ice and TAS are consistent with observed trends when including anthropogenic forcing or greenhouse gas (GHG) forcing, while cannot exhibit observed trends with only aerosol or natural forcing. Further analysis reveals that human influence including GHG forcing has substantially increased the probability of occurrence of the 2020-like extreme events, which are rare in aerosol-only or natural-only forcing. The frequencies of 2020-like low SIE increase by 19 times with all forcing and 16 times with GHG forcing than with natural forcing. Future climate simulations under different Shared Socioeconomic Pathway (SSP) scenarios of SSP126, SSP245 and SSP585 show that the 2020-like extreme event that is currently considered rare is projected to become the norm and almost occur 1-in-1 year beyond 2041–2060. The probabilities will be approximately in the range of 0.84–1.00 for SIE and 0.76–0.99 for TAS from low emission of SSP126 to high emission of SSP585.

中文翻译：

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人为活动对北极9月极低海冰和2020年炎热夏季的影响及其未来发生风险


2020年，北极的特点是海冰覆盖率极低，气候炎热。 9 月海冰范围 (SIE) 比 1979-2014 年平均值低约 230 万公里，是 1979-2020 年记录中的第二低，而区域夏季（6 月至 8 月，JJA）平均 2 m 气温 (TAS) 约为 1.3 °C 高于 1979-2014 年的平均值，是当时有记录以来最热的温度。就当地而言，9 月海冰浓度 (SIC) 比 1979-2014 年平均值低约 70%，JJA TAS 可能高出 6.0 °C。尽管极端事件的直接原因是持续有利的大气环流模式、风况和冰反照率反馈，但本文的主要目的是进行概率极端事件归因研究，以评估人为影响。基于CMIP6多模式集合产品，模拟的北极海冰和TAS的长期趋势在包括人为强迫或温室气体（GHG）强迫时与观测到的趋势一致，但不能展现仅气溶胶或自然强迫的观测到的趋势。进一步分析表明，包括温室气体强迫在内的人类影响大大增加了类似2020年的极端事件发生的概率，这在纯气溶胶或纯自然强迫中很少见。与自然强迫相比，2020 年类似的低 SIE 频率在所有强迫下增加了 19 倍，在温室气体强迫下增加了 16 倍。 SSP126、SSP245和SSP585不同共享社会经济路径（SSP）情景下的未来气候模拟表明，目前被认为罕见的类似2020年的极端事件预计将成为常态，并且在2041-2060年后几乎一年一发生。 SIE 的概率约为 0.84-1.00，SIE 的概率约为 0.76-0。TAS 的 99 从低排放 SSP126 到高排放 SSP585。