McKinnon Karen A, Simpson Isla R, Williams A Park
Department of Statistics and Data Science, University of California, Los Angeles, CA 90095.
Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095.
Proc Natl Acad Sci U S A. 2024 Oct 15;121(42):e2406143121. doi: 10.1073/pnas.2406143121. Epub 2024 Oct 7.
Summer temperature extremes can have large impacts on humans and the biosphere, and an increase in heat extremes is one of the most visible symptoms of climate change. Multiple mechanisms have been proposed that would predict faster warming of heat extremes than typical summer days, but it is unclear whether this is occurring. Here, we show that, in both observations and historical climate model simulations, the hottest summer days have warmed at the same pace as the median globally, in each hemisphere, and in the tropics from 1959 to 2023. In contrast, the coldest summer days have warmed more slowly than the median in the global average, a signal that is not simulated in any of 262 simulations across 28 CMIP6 models. The observed stretching of the cold tail indicates that observed summertime temperatures have become more variable despite the lack of hot day amplification. The interannual variability and trend in the warming of both hot and cold extremes compared to the median can be explained from a surface energy balance perspective based on changes in net surface radiation and evaporative fraction. Tropical hot day amplification is projected to emerge in the future (2024-2099, SSP3-7.0 scenario), while Northern Hemisphere heat extremes are expected to continue to follow the median.
夏季极端温度会对人类和生物圈产生重大影响，极端高温增加是气候变化最明显的症状之一。人们提出了多种机制，预测极端高温的升温速度将快于典型夏日，但目前尚不清楚这种情况是否正在发生。在此，我们表明，在观测数据和历史气候模型模拟中，1959年至2023年期间，全球、每个半球以及热带地区最热的夏日升温速度与全球中位数相同。相比之下，最冷的夏日升温速度比全球平均中位数慢，这一信号在28个CMIP6模型的262次模拟中均未出现。观测到的冷尾拉长表明，尽管没有热日放大现象，但观测到的夏季气温变得更加多变。从地表能量平衡的角度来看，基于净地表辐射和蒸发率的变化，可以解释极端高温和极端低温与中位数相比的年际变率和变暖趋势。预计未来（2024 - 2099年，SSP3 - 7.0情景）热带地区会出现热日放大现象，而北半球的极端高温预计将继续与中位数保持一致。
