Climate projections for the twenty-first century suggest an increase in the occurrence of heat waves. However, the time at which externally forced signals of anthropogenic climate change (ACC) emerge against background natural variability (time of emergence (ToE)) has been challenging to quantify, which makes future heat-wave projections uncertain. Here we combine observations and model simulations under present and future forcing to assess how internal variability and ACC modulate US heat waves. We show that ACC dominates heat-wave occurrence over the western United States and Great Lakes regions, with ToE that occurred as early as the 2020s and 2030s, respectively. In contrast, internal variability governs heat waves in the northern and southern Great Plains, where ToE occurs in the 2050s and 2070s; this later ToE is believed to be a result of a projected increase in circulation variability, namely the Great Plain low-level jet. Thus, greater mitigation and adaptation efforts are needed in the Great Lakes and western United States regions.

对二十一世纪的气候预测表明，热浪的发生增加了。然而，人为气候变化（ACC）的外部强迫信号相对于背景自然变异性（出现时间（ToE））出现的时间很难量化，这使未来的热浪预测变得不确定。在这里，我们结合了当前和未来强迫下的观测结果和模型模拟，以评估内部变化和ACC如何调节美国的热浪。我们显示，ACC主导了美国西部和大湖地区的热波发生，而ToE分别发生在2020年代和2030年代。相反，内部可变性控制着大平原北部和南部的热浪，而ToE发生在2050和2070年代。后来的ToE被认为是预计的环流变化增加的结果，即大平原低空急流。因此，在大湖区和美国西部地区需要作出更大的缓解和适应努力。