HKBU and CUHK scholars discover human-perceived temperature is rising faster than actual air temperature under climate change

(The research achievement has been published on RGC YouTube Chanel on 22 Jul 2018)

 

Each of the three years from 2014 to 2016 broke the global air temperature record, and 2017 will also turn out to be one of the hottest years ever. To predict how humans will be affected by climate change, geographers and climatologists led by Professor David Chen Yongqin from the Department of Geography and Resource Management at The Chinese University of Hong Kong (CUHK) and Dr Li Jianfeng from the Department of Geography at Hong Kong Baptist University (HKBU) studied the apparent temperature (AP), the temperature equivalent perceived by humans. They found that AP increased faster than air temperature (AT) over land in the past few decades, especially in the low latitude areas, and the rise is expected to continue in the future. This finding was recently published in Nature Climate Change.

 

Scientists have developed and used Global Climate Models (GCMs) to simulate the global climate and make projections of future AT and other climatic variables under different carbon emission scenarios in the 21st century. However, GCMs do not directly project how the change of other climatic factors, such as humidity and wind, affects human perception.

 

Professor Chen remarked, “Among the extensive and far-reaching impacts of global warming, human health and labour productivity are most directly affected by thermal discomfort and heat-related morbidity and mortality. Our study of the faster increases in apparent temperature has produced important findings for this kind of climate change impact assessment, providing a strong scientific support for more stringent and effective climate change mitigation efforts to combat global warming.”

 

Dr Li said the latest research findings give a better understanding of changes in human-perceived equivalent temperature, and indicate global warming has stronger long-term impacts on human beings under both extreme and non-extreme weather conditions, suggesting that climate change adaptation cannot just focus on heat wave events, but should be extended to the whole range of effects of temperature increases. The team will continue to explore the related issues to enhance the scientific knowledge. 

  

The research team used four reanalysis datasets of the past climate and outputs from seven GCMs to estimate the human-perceived equivalent temperature AP, from AT, humidity and wind. Findings showed that the global land average AP increased 0.04℃ per decade faster than AT before 2005, because of the concurrent increases in AT and humidity. This trend was projected to increase to 0.06 ℃per decade and 0.17℃per decade under Representative Concentration Pathway 4.5 scenario (RCP4.5) and RCP8.5, respectively, and reduce to 0.02℃ per decade under RCP2.6. The faster increases in AP are more significant in low latitude areas (tropical and sub-tropical regions) than the middle and high latitude areas. Study also indicated that the number of days with extremely apparent temperature will substantially increase in 2081 to 2100 compared to the period between 1981 and 2000, mainly due to the remarkable increase in the frequency of extremely hot days in summer.

 

Taken together, a key conclusion is that the world, as perceived by human beings, will become hotter than that just indicated by air temperature under global warming. This conclusion clearly implies that cities and communities, especially those located at tropical and sub-tropical regions like Hong Kong, will face bigger threats from hot weather and therefore greater efforts for climate change mitigation and adaptation are vital and urgent.

 

In this study, Professor Chen and Dr Li collaborated with AXA Professor Gabriel Lau Ngar-Cheung from the Department of Geography and Resource Management at CUHK, and Professor Thian Yew Gan from the Department of Civil and Environmental Engineering at the University of Alberta in Canada.

 

全球平均氣溫在2014至2016年間屢創新高，剛過去的2017年亦是有記錄以來最熱的年份之一。為深入瞭解人類在氣候變化下對溫度感受的改變，香港中文大學（中大）地理與資源管理學系陳永勤教授與香港浸會大學（浸大）地理系助理教授李劍鋒博士，聯同多名地理及氣候學專家進行研究，發現過去數十年，陸上體感溫度的上升速度較實際溫度快，尤其是低緯度地區，預測亦顯示這個趨勢會持續。這項研究已在《自然（氣候變化）》期刊發表。

 

現時科學家採用全球氣候模式(GCMs)模擬氣候和預估21世紀氣溫在不同碳排放情境下的變化。不過，全球氣候模式不能直接預估其他氣候因素，例如溫度和風速，對人類體感溫度的影響。

 

陳永勤教授表示:「人類健康和勞動生產力直接受制於熱舒適度，即人類實際感受的熱力，這亦直接影響熱相關疾病的發病和死亡率。研究氣候變化對體感溫度的影響，能夠為推行更嚴格和有效的減緩氣候變化工作提供重要的科學依據。」

 

李劍鋒博士說，是次研究成果協助團隊進一步了解體感溫度的變化，並指出全球暖化在極端和非極端氣象情況下，對人類體感溫度都有更大的影響，因此氣候變化適應措施不能只局限於熱浪等極端事件，需要擴展至溫度上升帶來的整體影響。團隊會繼續探索相關課題，以進一步強化科學認識。

 

研究團隊利用四個歷史氣候的再分析數據集，以及七個全球氣候模式的模擬結果，計算和模擬體感溫度的變化和趨勢，發現2005年前全球陸地平均體感溫度的增速較實際溫度增速每10年快0.04℃，原因是氣溫上升的同時濕度亦增加，人體感受到的熱力因而增加。預測未來在高、中、低三種碳排放情境（即所謂「代表濃度路徑」）下，體感溫度上升將分別較實際溫度每十年快0.17℃（RCP8.5）、0.06℃（RCP4.5）和0.02℃（RCP2.6），低緯度地區 (即熱帶或亞熱帶地區) 體感溫度上升亦較中高緯度地區明顯。研究亦顯示於2081-2100年出現極端體感溫度的日數將會較1981-2000年顯著增加，主要是夏季出現極端熱的頻率大幅上升。

 

總括而言，全球暖化下人類將感受到比實際氣溫更熱的氣候，清楚表明世界各地的城市和社區，特別是地處熱帶或亞熱帶地區的城市，如香港，將面臨酷熱天氣的更大威脅，減緩和適應氣候變化實在刻不容緩。

 

是次研究報告的作者還包括中大AXA安盛地理與資源管理學講座教授劉雅章教授，以及加拿大阿爾伯特大學土木與環境工程系顏天佑教授。