Explainer: After another ‘hotter than usual’ summer, how much is Hong Kong heating up?
Hong Kong Free Press
Hong Kong is getting hotter. The city has just sweated through a summer that was, in the words of its government forecaster, “much hotter than usual,” after experiencing its hottest summer on record last year.
The Hong Kong Observatory has been conducting meteorological observations since 1884, except for a period during World War Two from 1940 to 1946. Over the decades, the city’s hot and humid summers have only become more uncomfortable – particularly for the most vulnerable residents, including outdoor workers, the elderly, and those living in substandard accommodation.
Rising temperatures globally are a result of increased carbon dioxide emissions in the atmosphere and other human activity. In 2015, 196 members of the UN Framework Convention on Climate Change adopted an agreement to keep the mean global temperature rise below 2 degrees Celsius above pre-industrial levels, and preferably to limit the increase to 1.5 degrees. China is among the signatories.
See also: ‘It’s happening to us as well’ – Hong Kong climate advocates sound alarm at COP
In June, a report from the EU’s Copernicus Climate Change Service found that average global temperatures had been 1.5 degrees higher than they were in the late 1800s for 12 consecutive months. According to climate data analysis platform Berkeley Earth, Hong Kong had warmed 1.7 degrees by 2022.
While the 1.5 degrees barrier does not signal a permanent breach of the critical limit – which scientists say is measured over decades – it sends a clear warning that climate change is intensifying much more rapidly than expected.
Average annual temperature
The mean annual temperature, which has been logged since 1885, has risen by a rate of 0.14 degrees Celsius per decade from 1885 to 2023, according to the Observatory’s analysis. From 1994 to 2023, the rate of increase was 0.30 degrees Celsius per decade.
Hong Kong’s hottest year to date was 2021, when the mean temperature was 24.6 degrees Celsius, while the city’s three hottest years have occurred since 2019.
Very hot days
Days when the maximum temperature reaches or exceeds 33 degrees Celsius are considered “very hot” by the Observatory. Before 2020, their number had never risen beyond 40, with the highest number of 37 recorded in 1967.
Since 2020, the annual number of very hot days has not dipped below 47. As of August 31, there have already been 38 days with temperatures of 33 degrees or above in 2024.
From 1961 to 1990, the climatological normal (average) number of very hot days in the city was 13.37 annually. For the period from 1991 to 2020, that had risen to 17.47.
Extremely hot days
The Observatory has also since 1884 logged the number of “extremely hot” days – when the maximum temperature reaches or exceeds 35 degrees Celsius.
The year with the highest number of extremely hot days was 2022, when 15 were logged. There has been one extremely hot day in 2024 so far.
Hot nights
“Hot nights” are recorded when the daily temperature does not dip below 28 degrees Celsius.
In August, Hong Kong experienced a total of 18 hot nights, the highest on record for the month, which brought the cumulative total for 2024 to 44 as of August 31.
From 1884 to 1913, the average number of hot nights in Hong Kong was 0.6; by 1994 to 2023, that had risen to 27.3 per year.
According to projections from the Hong Kong Observatory under different scenarios for greenhouse gas emissions, residents could expect to see between 59 and 65 hot nights on average per year between 2021 and 2040.
Cold days
In Hong Kong, the Observatory considers “cold days” as those when the temperature does not reach 12 degrees Celsius.
As the heat in Hong Kong has increased, there has been a downward trend in the number of cold days.
What could this mean for Hongkongers?
The Observatory offers projections of future climate scenarios depending on the levels of emissions released into the atmosphere. If emissions are “unabating,” the government forecaster predicts that the number of hot nights and very hot days “are expected to increase significantly in the 21st century,” while the number of cold days are expected to fall further.
Rising temperatures are of particular concern for those most exposed to them, with the risk of heat stress in Hong Kong exacerbated by the humidity.
High humidity changes the way that sweat, the body’s natural cooling mechanism, evaporates and inhibits the body’s ability to regulate its temperature. Not being able to cool down can put people’s health at severe risk, and can lead to heart and respiratory problems, dehydration, heatstroke, high blood pressure, and sleep deprivation.
This leaves those who work outside – often in low-income jobs – those who cannot afford to run air conditioning, those in poor living environments, and the elderly particularly vulnerable to illness and injury as a result of exposure to heat.
Last year, biological scientist Michael Boyle at the University of Hong Kong explored the relationship between the median monthly salary of Hong Kong districts and their climate conditions. Boyle found that residents of the poorest neighbourhoods were at greater risk of exposure to heat considered dangerous to human survival.
“Projecting into the future using global climate change models, the wealthiest districts will have roughly 30 years longer than less wealthy districts before temperatures that are considered dangerous to human survival are reached,” Boyle told HKFP.
“Based on my findings, combinations of temperature and humidity considered dangerous to human survival may be experienced in Hong Kong as a normal part of the climate by the end of the century.”
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