Extreme Heat
The Hong Kong Observatory, which has data stretching back over
100 years, affirms that the city’s climate has been getting
hotter, and at a faster speed. From 1885 to 2020, the average
increase was 0.13 degrees Centigrade per decade. In the past
three decades, this has extended to 0.24 degrees Centigrade.
Using the IPCC’s Representative Concentration Pathway 8.5
(RCP8.5) scenario, which focuses on high greenhouse gas
concentration, the Observatory has projected that these
increases will continue during the 21st century, with the
annual number of very hot days (33 degrees Centigrade or more)
increasing dramatically to approximately 110 per year by the
end of the century from an average of nine between 1986 and
2005
The city’s humid environment is exacerbating the problem. With
the predicted rise in air temperature, the atmosphere will be
able to hold more water. As humidity increases, the
evaporative cooling effect decreases, thus increasing heat
stress. A wet-bulb temperature reading (measuring both
humidity and temperature and a more appropriate measure of
heat stress than air temperature alone) of 35 degrees
Centigrade is commonly used as the threshold for human
adaptability to heat. If this is exceeded, the body cannot
dissipate heat by sweating and hyperthermia ensues.
In 2017, an Observatory team including Tong Hang-wai, Wong
Chau-ping, and Lee Sai-ming showed that the annual number and
maximum length of extremely warm and humid days will
progressively increase throughout the 21st century, in a paper
for the 31st Guangdong-Hong Kong-Macao Seminar on
Meteorological Science and Technology and the 22nd
Guangdong-Hong Kong-Macao Meeting on Cooperation in
Meteorological Operations.
Even under the IPCC’s mid-range scenarios of RCP4.5
(medium-low greenhouse gas concentration) and RCP6.0
(medium-high greenhouse gas concentration), wet bulb
temperatures could exceed 30 degrees Centigrade. Under RCP8.5,
they could rise beyond 31 degrees Centigrade this century. As
a reference, the paper’s authors point out that maximum wet
bulb temperature never exceeded 31 degrees globally from
1999-2008 and 30 degrees Centigrade equates to the most
extreme heat experienced in recent history.
Adding to heat stress, the warm and humid season is projected
to lengthen, occurring as early as late April and extending
into October under RCP8.5. Without adaptive measures, the
implications for Hong Kong for public health could be
significant. The Observatory team pointed out that increasing
the use of air-conditioning is not the solution, as this would
increase greenhouse gas emissions if clean energy is
unavailable.
Hong Kong scientists have developed an extreme heat risk
index to locate high-risk hotspots in urban areas.
Photo: Shutterstock
Extreme hot weather conditions vary greatly in Hong Kong’s
complex urban environment. The higher the population density,
the higher the risk, due to the urban heat island effect.
Researchers from the University of Hong Kong’s (HKU) School of
Architecture, Chinese University of Hong Kong (CUHK)’s
Institute of Future Cities, and the Observatory collaborated
to develop an extreme heat risk index. This was based on an
integration of heat hazards (extreme heat severity, duration,
and frequency), exposure (population distribution), and
vulnerability from 2006 to 2016.
The research, published in 2021 in Sustainable Cities and
Society, found the highest risk locations to be in Sham Shui
Po, Yau Ma Tei, Mong Kok, Choi Hung, Sha Tin city centre and a
part of Wan Chai. These rose at night, with almost 10 more
hotspots in core urban areas. Mortality, especially in urban
areas, increases as the intensity, frequency and duration of
extremely high temperatures rises under climate change,
according to the study.
Dr Hua Junyi , lead author of the 2021 paper and a postdoctoral
fellow in the team of Dr Ren Chao, Associate Professor in HKU’s
Faculty of Architecture, said: “The elderly and poor, who often
live in cramped conditions are particularly susceptible.”
Professor Emily Chan Ying-yang, Assistant Dean in the Faculty
of Medicine at CUHK, has shown that higher temperatures are
already posing a major threat to human health in Hong Kong,
reflected in an increase in mortality and morbidity. “We
studied the relationship between hospital admissions and
temperature in Hong Kong and found an increase in admissions
of 4.5 per cent for every one degree Centigrade increase in
mean daily temperature,” she said.
Rising Sea Levels And Flooding
As a coastal city with roughly 15 per cent of its land close
to sea level, Hong Kong is vulnerable to flooding from rising
sea levels, storm surges, and extreme rainfall, affecting the
human and natural environment.
Several collaborative research projects have investigated the
risk of rising sea levels for Hong Kong and the Pearl River
Delta, which are important for informing long-term adaptive
planning. While there is a consensus that the sea level is
rising and poses a serious risk of increased flooding, the
various scenarios, models, and timeframes used give a range of
projections, from around 30 centimetres by the end of this
decade to nearly two metres by the end of the century.
Global warming is resulting in sea water thermal expansion and
the melting of glaciers, ice sheets, and land water storage
that lead to rises in sea level, a Hong Kong Observatory team
led by He Yuheng explained in a 2016 paper published in the
International Journal of Climatology. Under the IPCC’s RCP8.5
scenario for high greenhouse gas concentration, global mean
sea level would likely rise between 0.45 and 0.82 metres for
the 2081-2100 period, over the levels of 1986-2005.
There are variations due to regional ocean dynamics and
atmospheric changes, as well as land displacement not related
to climatic activities. Based on these factors, regional sea
levels are predicted to rise 0.27 metres for RCP4.5 (an
intermediate scenario when carbon emissions peak around 2040
and then decline), and 0.35 metres for RCP8.5. In Hong Kong,
sea level rise is projected to be 0.67 metres for RCP4.5 and
0.84 metres for RCP8.5. Based on both scenarios, sea level in
the vicinity of the city would continue to rise about 0.2
metres higher than the corresponding global mean values.
Another study, by researchers from the Guy Carpenter
Asia-Pacific Climate Impact Centre at City University of Hong
Kong (CityU), Chinese Academy of Sciences, Macao
Meteorological and Geophysical Bureau, and first authored by
Professor Zhou Wen of CityU’s School of Energy and
Environment, estimates that under the RCP8.5 greenhouse gas
concentration trajectory, coupled with extreme values for land
subsidence and inter-monthly variability, there would be a sea
level rise of 1.94 metres by the end of this century.
Under the paper’s scenario, published in Acta Oceanologica
Sinica in 2015, large areas of the Pearl River Delta would be
threatened, with the part below sea level increasing by
approximately 30 per cent, including Deep Bay.
A further study assessing the damage to people from coastal
flooding in Hong Kong and the Pearl River Delta was published
in Natural Hazards in 2018. This was first authored by Yu
Qiwei, then at Hong Kong University of Science and Technology
(HKUST), and included Professor Jimmy Fung, Associate Head of
the Division of Environment and Sustainability at HKUST, as
one of the authors.
Fung noted the study’s sobering implications for the
community: “It is estimated with a sea level rise of 75
centimetres by 2100, the annual average loss in Hong Kong will
be over 20 deaths and 100,000 displaced people.”
Meanwhile, a team of researchers from Southeast University (Nanjing),
Tongji University (Shanghai), and Florida State University (US) has
demonstrated that sea level rise is already occurring at a greater rate in
the Pearl River Estuary than globally. Along the Guangdong coast,
they project this to be at 30 centimetres and 50 centimetres by 2030 and 2050
respectively, according to the 2017 paper in Ocean
Engineering, first authored by Yin Kai.
Typhoons and Storm Surges
The immediate threat is from storm surges, as generated by
tropical cyclones. Hong Kong lies within the world’s most
active tropical cyclone basin, which produces almost 30 per
cent of these extreme weather events globally. Scientists have
used the IPCC scenarios to project these to increase in
intensity with climate change, resulting in higher storm
surges. Typhoon intensification combined with sea level rise
will lead to more severe flooding events.
The flooding that accompanied Typhoon Mangkhut, which hit Hong
Kong in 2018 with record storm surges, was widespread. But it
could have been much worse. The Hong Kong Observatory analysed
what impact Mangkhut would have had if it had made landfall
during a spring tide. Dr Lee Tsz-cheung, Senior Scientific
Officer at the Observatory, noted that instead of around 3.9
metres, the maximum sea level would have been closer to 4.9
metres in Victoria Harbour and 5.9 metres in Tolo Harbour,
resulting in greater flood damage.
The intergovernmental Typhoon Committee, set up in 1968 under
the Economic and Social Commission for Asia and the Pacific
and the World Meteorological Organization to coordinate
planning and implementation of measures to minimise the loss
of life and damage caused by typhoons, has also conducted
assessments.
An article published in Tropical Cyclone Research and Review
summarises the committee’s findings from more than 180 studies
on projected changes of tropical cyclone activity under a
scenario of two degrees Centigrade global warming. This
affirmed a general consensus for a reduction in typhoon
frequency, by about 10 per cent, but an increase in intensity,
by about five per cent, and increased precipitation of about
17 per cent. Global warming may also lead to changes in
tropical cyclone tracks, and storm surge risk may be increased
by the projected increase in intensity as well as the expected
sea level rise.
In a 2020 paper published in Scientific Reports, a team
including Chen Jilong of the Fujian Agriculture and Forestry
University, Mainland China, and involving CUHK’s
climatologists Professor Gabriel Lau Ngar-cheung and Associate
Professor Dr Francis Tam Chi-yung, investigated the impacts of
climate change, based on the RCP8.5 scenario, on tropical
cyclones and the storm surges they induce.
Peak intensities of cyclones – their maximum surface wind –
are expected to increase by three per cent in the near future
(2015-2039) and 10 per cent in the far future (2075-2099), due
to increased surface warming. Typhoon-induced storm surges
affecting the Pearl River Delta would increase by about 8.5
per cent, resulting in a tide increase of around one metre.
Flooding in the city centre after heavy rain.
Photo: Shutterstock
Extreme Rain
With the climate change-induced warming of the troposphere,
the maximum atmospheric moisture content increases and along
with it the likelihood of more extreme rainfall events. As
with sea level and tropical cyclones, rainfall projections
depend on which IPCC scenario is used.
Given strong indications that the world is on the path of the
RCP8.5 high greenhouse gas concentration scenario, a team of
engineers from the Geotechnical Engineering Office and
meteorologists from the Hong Kong Observatory project that the
number of extremely wet years in Hong Kong will increase from
three between 1885-2005 to around 12 in 2006-2100. Annual
rainfall is expected to increase by roughly 180 millimetres
compared with the average of 1986-2005. The Observatory has
also projected an increase in rainfall intensity and
frequency.
Meanwhile, a 2015 study by Professor Zhou Wen and Richard Li
of the Guy Carpenter Asia-Pacific Climate Centre at CityU,
Shun Chi-ming, a former director of the Observatory, and Lee
Tsz-cheung, also of the Observatory, looks at tropical
cyclone-induced rainfall, a significant contributor to total
rainfall. Using a linear regression method based on historical
records, they found that over the past 50 years there had been
a reduction in typhoon-related rainfall and increase in other
rainfall, the latter also becoming more intense. The trend is
projected to continue, due to the reduction in typhoon
frequency and increase in moisture over Southern China.
Landslides
Hong Kong has always been at risk of landslides because of its
intense seasonal rainfall and mountainous terrain. The
proximity of urban development to hillsides heightens the risk
of human fatalities and property damage when landslides occur.
Since 1948, there have been over 470 deaths in Hong Kong as a
result of landslides. The Hong Kong government’s Geotechnical
Engineering Office (GEO) has substantially reduced landslide
risk since it was established in 1977, using engineering and
non-engineering approaches. However, climate change presents
new challenges.
“We expect the risk of landslides to increase with climate
change because a warming climate is leading to more frequent
extreme rainfall events,” Dr Raymond Cheung, Head of the GEO,
said. “To address this challenge, we are upgrading our
emergency preparedness and applying new technologies for risk
management.” One example is the use of artificial intelligence
to identify areas with rock exposure from aerial images.
According to the Hong Kong Observatory and GEO, correlations
between rainfall and landslide types can be established with
confidence because of the abundance of data in Hong Kong. As
such, rainfall-landslide correlations for man-made slopes and
natural hillsides have been established and used to predict
the severity of landslides with increasing rainfall intensity.
The Observatory and GEO also conducted a study, with findings
published in the book Slope Safety Preparedness for Impact of
Climate Change (2016) , to understand what the impact of
2008’s Typhoon Morakot, the deadliest typhoon to hit Taiwan in
recent history, would have had if it had hit Hong Kong. “Our
study indicated 49,000 natural terrain landslides could occur
with 4,000 to 8,000 impacting on roads or buildings. In
addition 1,000 man-made slope failures could occur with about
620 of these impacting roads or buildings,” Lee Sai-ming,
Senior Scientific Officer at the Observatory, said.
“The current capacity of the emergency landslide system is
about 210 and 360 reported natural terrain and man-made slope
failures, respectively. Therefore, in the case of such an
extreme rainfall event hitting Hong Kong, the emergency
management system would be completely overwhelmed,” he added.
The study affirmed the need for strategies for prevention,
mitigation, preparedness, response, and recovery, the
government’s scientists concluded.
It is critical there is advanced warning for an extreme
weather event that could lead to severe flooding and
landslides. Currently the time for such a notification is
three hours, but a team led by HKUST Chair Professor Charles
Wang Wai Ng hopes to increase this. Using artificial
intelligence and data for precipitation, temperature, and
vegetation, Ng and his team are developing a model to increase
the advanced warning time to six hours. “Our model targets
landslides, but with minor modifications it could be used for
flooding, and heat stress as well,” Ng said.
Such a model could be developed with a user interface geared
towards the non-scientific community, allowing an assessment
of the risk facing a building. By allowing more time to
prepare, the risk of injuries and loss of life could also be
reduced.
Moving Forward
Increased landslides, flooding, increasing intensity of
rainfall, storm surges, and heat stress are just some of the
risks from climate change that government agencies and
scientists have been working to understand and mitigate. Other
risks are less well researched. For example, Professor Emily
Chan of CUHK's Faculty of Medicine identified the need for
more research on climate change and vector-borne diseases
while Dr Margaret Burnett, Programme Director for the
Corporate Environmental Governance Programme at HKU, noted the
issue of food security.
Professor Alexis Lau, Head and Chair Professor of the Division
of Environment and Sustainability at HKUST, highlighted
another risk. “Extended drought combined with extreme heat
could result in hill fires far more severe than have been
experienced in Hong Kong till now.”
Dr Bayden Russell, Associate Director of Swire Institute of
Marine Science and Associate Professor, School of Biological
Sciences at HKU, echoes many scientists on the solution that
is required. “If we stopped using fossil fuels, it would take
away the problem. The concept is simple, but the action
difficult.”
The Croucher Foundation has been funding scientific
research including research on climate change since 1979.
Scientists are able to model the rapidly evolving physical
risks associated with climate change with increasing
accuracy and to link these physical risks with impacts
including economic impact. As Hong Kong studies the risks
associated with climate change, builds predictive models,
and begins to identify science-based solutions, the
Foundation is taking this opportunity to share some of the
directly relevant research.