We quantify historical and projected trends in the population exposure to climate extremes as measured by the United States National Center for Environmental Information Climate Extremes Index (CEI). Based on the analyses of the historical observations, we find that the U.S. has already experienced a rise in the occurrence of aggregated extremes in recent decades, consistent with the climate response to historical increases in radiative forcing. Additionally, we find that exposure can be expected to intensify under the Representative Concentration Pathway 8.5, with all counties permanently exceeding the baseline variability in the occurrence of extreme hot days, warm nights, and drought conditions by 2050. As a result, every county in the U.S. is projected to permanently exceed the historical CEI variability (as measured by one standard deviation during the 1981‐2005 period). Based on the current population distribution, this unprecedented change implies a yearly exposure to extreme conditions for one in every three people. We find that the increasing trend in exposure to the aggregated extremes is already detectable over much of the U.S., and particularly in the central and eastern U.S. The high correspondence between the pattern of trends in our simulations and observations increases confidence in the projected amplification of population exposure to unprecedented combinations of extreme climate conditions, should greenhouse gas concentrations continue to escalate along their current trajectory.

中文翻译：

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到2050年，美国人口在极端气候下的暴露量将增加一倍

我们通过美国国家环境信息中心气候极端指数（CEI）来量化人口在极端气候中的暴露历史和预期趋势。根据对历史观测的分析，我们发现，近几十年来，美国已经出现了极端事件的增加，这与气候对辐射强迫的历史增长的反应一致。此外，我们发现，在代表性浓度途径8.5下，暴露量预计会加剧，到2050年，所有县将在极端炎热的白天，温暖的夜晚和干旱条件下永久超过基线变化。结果，每个县美国 预计将永久超过历史CEI变异性（以1981-2005年期间的一个标准差来衡量）。根据当前的人口分布，这种空前的变化意味着每年有三分之二的人面临极端条件。我们发现，在美国大部分地区，尤其是在美国中部和东部，已经发现了暴露于极端综合风险的增长趋势。我们的模拟和观察结果的趋势模式之间的高度对应关系提高了对预期人口增长的信心如果温室气体浓度沿其当前轨迹继续上升，则暴露于极端气候条件的空前组合之下。这一前所未有的变化意味着每年每三人中就有一个人处于极端条件下。我们发现，在美国大部分地区，尤其是在美国中部和东部，已经发现了暴露于极端综合风险的增长趋势。我们的模拟和观察结果的趋势模式之间的高度对应关系提高了对预期人口增长的信心如果温室气体浓度沿其当前轨迹继续上升，则暴露于极端气候条件的空前组合之下。这一前所未有的变化意味着每年每三人中就有一个人处于极端条件下。我们发现，在美国大部分地区，尤其是在美国中部和东部，已经发现了暴露于极端综合风险的增长趋势。我们的模拟和观察结果的趋势模式之间的高度对应关系提高了对预期人口增长的信心如果温室气体浓度沿其当前轨迹继续上升，则暴露于极端气候条件的空前组合之下。