Tracking extreme heat by the hour makes climate change seem even worse

按小时追踪极端高温，让气候变化显得更加严峻

Global warming is leading to more extremely hot weather. There is perhaps no more obvious consequence of climate change. But changing how we analyze this problem can still help scientists think about it in a different way that may yield surprises. 全球变暖正导致极端高温天气频发。这或许是气候变化最显而易见的后果。然而，改变我们分析这一问题的方式，仍能帮助科学家以不同的视角进行思考，并可能带来意想不到的发现。

Generally, studies on heat extremes work with some combination of daily minimum, maximum, and mean temperatures. Sometimes they focus on multi-day heatwaves using those metrics, since the discomfort and health impacts increase for longer events. There are different thresholds you can use to define extreme heat in these analyses, either based on past statistics or on the human body’s physiological limits. A new study led by Weilin Liao and Xuanzong Zhang at Sun Yat-sen University tried breaking down heat extremes to the hourly level instead, which reveals some interesting nuances. 通常，关于极端高温的研究会结合每日最低温、最高温和平均气温来进行分析。有时，研究人员会利用这些指标关注持续多日的极端热浪，因为持续时间越长，带来的不适感和健康影响就越严重。在这些分析中，可以采用不同的阈值来定义极端高温，这些阈值要么基于历史统计数据，要么基于人体的生理极限。由中山大学廖伟林（Weilin Liao）和张宣宗（Xuanzong Zhang）领导的一项新研究尝试将极端高温细化到小时级别，这揭示了一些有趣的细节。

Hourly reports

小时级报告

The researchers calculated each hour of the day separately, comparing 8 am temperatures only to other 8 am temperatures. They defined extreme heat as anything above the 90th percentile of recorded temperatures at that time of day between 1981 and 2010. To put this in context, the global average over the last four decades has been that people experience about 270 hours of “hourly heat extremes” each summer. But that average obscures a very obvious trend: that number has been increasing by 62 hours each decade. 研究人员将一天中的每个小时分开计算，仅将上午8点的气温与其他年份上午8点的气温进行比较。他们将极端高温定义为超过1981年至2010年间该时段记录气温的第90百分位。作为背景参考，过去四十年的全球平均数据显示，人们每年夏天大约会经历270小时的“小时级极端高温”。但这一平均值掩盖了一个非常明显的趋势：这个数字每十年增加62小时。

To project this into the future, the researchers used climate model results from two scenarios of future greenhouse gas emissions—one with medium emissions leading to 3°C (about 5.5°F) warming by 2100, and one with high emissions and 5°C (9°F) warming. Comparing the past couple of decades to 2070–2099, the total number of hourly heat extremes triples in the medium scenario and increases by 4.5 times in the high scenario. For every half a degree C (almost 1°F) of global average temperature increase, the study shows an increase of about 200 hot hours per summer. The trend is worst in the tropics, even though overall warming is stronger at higher latitudes. The researchers say this is because temperature variability increases in the tropics, making extremes more frequent. 为了预测未来，研究人员使用了两种未来温室气体排放情景下的气候模型结果：一种是中等排放情景，到2100年气温上升3°C（约5.5°F）；另一种是高排放情景，气温上升5°C（9°F）。将过去几十年与2070年至2099年进行比较，在中等排放情景下，小时级极端高温的总时长将增加至原来的3倍，而在高排放情景下则增加至4.5倍。研究显示，全球平均气温每升高0.5°C（近1°F），每年夏季的极端高温小时数就会增加约200小时。尽管高纬度地区的整体变暖幅度更大，但热带地区的这一趋势最为严重。研究人员表示，这是因为热带地区的气温波动性增加，使得极端天气更加频繁。

Hot nights

炎热的夜晚

The number of hot hours also increases more at night than it does during the day. This is where the hourly analysis can start to seem more relatable than metrics of temperature change or counts of days. Days that fail to qualify as “hot days” still see an increase of several hot hours—mostly at night. This wouldn’t show up if you just counted the number of hot days. And these shorter hot periods often precede or follow hot days, extending the stress a heatwave puts on your body. 夜间极端高温小时数的增长幅度也超过了白天。在这一点上，小时级分析比单纯的气温变化指标或“高温天数”统计更能让人感同身受。那些未达到“高温天”标准的日期，依然会出现数小时的极端高温——主要集中在夜间。如果只统计高温天数，这一点是无法体现的。此外，这些较短的极端高温时段往往出现在高温天前后，从而延长了热浪对人体造成的压力。

Obviously, those hot days also see an increase in hot hours. Right now, the average hot day contains about 16 hot hours, but that increases to 19 hours in the medium scenario and 21 hours in the high scenario. So the miserable part of the day gets longer, reducing the hours that grant some relief. 显然，那些高温天本身的高温小时数也在增加。目前，平均每个高温天包含约16个极端高温小时，但在中等排放情景下，这一数字增加到19小时，在高排放情景下则增加到21小时。这意味着一天中难熬的时间变长了，而能让人喘息的凉爽时间则相应减少。

The researchers also looked at differences in these trends for countries at different income levels and compared the lifetime total exposure to hot hours for people born in 1981 and 2023. Low-income countries see the strongest increase in hot hours. But even at the global average, those born in 2023 would experience about 1.4 to 1.8 times as many hot hours, depending on the greenhouse gas emissions scenario. 研究人员还观察了不同收入水平国家在这些趋势上的差异，并比较了1981年和2023年出生的人一生中经历的极端高温总小时数。低收入国家的高温小时数增长最为显著。即便按全球平均水平计算，2023年出生的人所经历的极端高温小时数也将是1981年出生者的1.4到1.8倍，具体取决于温室气体排放情景。

While the overall trend in hot weather is well known, the hourly details matter. They highlight that the number of hot hours is increasing even faster than the number of hot days, for example. But the details also help us better represent what this means for human health. The researchers write, “As heat risks continue to intensify, hourly-scale assessments will become essential for anticipating health impacts, informing equitable adaptation strategies and designing responsive early warning systems that reflect the new reality of round-the-clock heat exposure.” 虽然高温天气的总体趋势已广为人知，但小时级的细节至关重要。例如，它们强调了极端高温的小时数增长速度甚至快于高温天数。这些细节也有助于我们更好地理解这对人类健康意味着什么。研究人员写道：“随着高温风险持续加剧，小时级的评估对于预测健康影响、制定公平的适应策略以及设计能够反映全天候高温暴露这一新现实的响应式预警系统，将变得至关重要。”