在相對較少的額外氣候變暖的情況下，預計致命的熱應激每年會影響數億人。 該 巴黎協定 commits the international community to limit global warming to no more than 2? above pre-industrial (late 19th century) air temperatures, with an aspirational target of 1.5?. In our latest research, which looked at the impact of global temperature rises on magacities, 我們發現 that even if 1.5? is achieved, large increases in the frequency of deadly heat are expected. 
通過2050關於350m，每年生活在特大城市的人們可能會遭受致命的高溫。
Humans become “heat stressed” when the body absorbs more heat than is tolerable. If core body temperature rises just a few degrees above 37?, deadly 心髒病 可以導致。 通過使用其冷卻系統 - 出汗 – the human body can maintain a safe temperature even if air temperatures rise above 37?. This mechanism works better in a drier atmosphere (which is why steam rooms feel hotter than saunas – even at the same air temperature). The 熱度指數 is a measure that combines this humidity effect with air temperature to provide a “feels like” temperature. A heat index in excess of about 40.6? is considered dangerous to human health.
隨著全球氣溫上升，氣候模型的觀測和實驗表明大氣含水量也在上升。 這意味著熱指數（以及感覺的熱度）比空氣溫度上升得更快。 而且，因為在較高溫度下大氣可以保持的水分量增加得更快，所以熱指數也上升得更快（非線性響應）。
限制全球變暖的強烈動機
This non-linear response carries over to the definition of “global heat stress burden” used in our research, which we define as the average number of days per year over land areas with a daily heat index above 40.6?. Using a large number of climate model simulations, we found that this quantity increases faster and faster as global average air temperatures rise. This sharp rise in global heat stress burden has important consequences.
First, any increase in global heat stress from climate warming to date will be smaller than that caused by the same additional warming in the future. (We have seen a 0.8? rise in global temperature; another 0.8? of warming could be expected to lead to a greater increase in heat stress than caused by the first 0.8?.)
Second, there may be progressively heavier global impacts if the Paris targets are breached. Our analysis suggests that for 1.5? warming, the global heat stress burden will be almost six times greater than experienced during 1979-2005. But heat stress is 12 times greater if warming reaches 2?. With 4? warming – which could happen if mitigation efforts fail – our analysis suggests that the global heat stress burden could be more than 75 times larger.
如此大的熱應力增加可能很難想像，因此我們使用最近的熱浪來幫助傳達可能存在的影響。
For example, in 2015, Karachi and Kolkata in India experienced lethal temperatures. Our analysis suggests that in a 2?-warmer world, both cities could experience these deadly conditions at least once a year. If global warming reaches 4?, the record heat of 2015 would be commonplace – more than 40 days a year. Other regions would not be immune. With only 1.5? of warming, twice as many global megacities (cities with a population greater than 10m, including Lagos, Nigeria, and Shanghai, China) could start to regularly experience heat stress. At 2?, Tokyo (the world’s most populous city), may be affected. New York City joins the list at 4?.
If the global population grows as anticipated this century, it could drive up global heat stress even more. The situation in Lagos illustrates this well. If global warming reaches 1.5? by the end of the century (at which time the population of Lagos may have increased elevenfold and dangerous heat may be 100 times more common) the heat stress burden could be more than a thousand times greater than the recent past.
Across all megacity regions, if the 1.5? limit is breached by the 2050s, as many as 350m people globally could be regularly exposed to dangerous heat stress. This is more than a fourfold increase compared with 1979-2005.
Heat stress sensitivity to global temperature rise and the potential human impacts – even at 1.5? above pre-industrial levels – provide a strong incentive for limiting global warming. Warming associated with the Paris targets may sound modest enough for the urgency of the situation to be lost. Our analysis shows that even if ambitious mitigation targets are met, the need to adapt to extreme heat will remain. The high concentration of people and heat in urban environments make cities an important focus for these adaptation efforts.
關於作者
Tom Matthews，自然地理學講師， 利物浦約翰摩爾斯大學
相關書籍
at InnerSelf 市場和亞馬遜
