24th May 2024 – (Hong Kong) Hong Kong is bracing for a summer marked by an increased frequency of rainstorm warning signals due to the far-reaching effects of global warming and climate change. This year, residents of the bustling metropolis can expect less sunshine during the summer months, a trend that is likely to worsen in the years to come, according to experts.

The Hong Kong Observatory issued both the Amber Rainstorm Warning Signal and the Thunderstorm Warning earlier today, indicating that more than 30 millimetres of rainfall had fallen or was expected to fall within an hour. This announcement comes on the heels of intense rain and flooding that hit the city in early May, affecting areas such as Tseung Kwan O, Shau Kei Wan, and Clear Water Bay. The scenes of inundation were reminiscent of the “once-in-500-years” storm that battered Hong Kong last September.

Lee Tsz-cheung, a senior scientific officer at the Observatory, described the rainstorm as uncommon and characterised the rainfall as falling in an “extremely uneven” manner across the city. He attributed this variation to the slow movement of the rain clouds. Leung Wing-mo, a former assistant director of the Observatory, noted that recording more than 200mm of rainfall in a single day in May is unusual. While hesitant to state with absolute certainty that Saturday’s rainfall was a direct result of climate change, Leung acknowledged that the occurrence of such events is expected to become more frequent due to global warming.

Historical data reveals that Hong Kong has experienced several instances of above-average rainfall in May. On 30th May, 1889, the city was drenched with 520mm of rain, followed by 324mm on 8th May, 1992, and 273mm on 24th May, 2017. The abrupt nature of these rainstorms can be attributed to the limitations of current meteorological technologies, which can only predict the overall weather patterns favourable for heavy rainfall, according to Leung.

“The Observatory can predict which weather system may cause heavy rain,” he explained. “But with current technology, it is impossible to predict when the rainstorm will come, how heavy it will be, and where it will occur.”

Leung emphasised that the Observatory’s rainstorm signals already provide the public with real-time weather information, with the amber rainstorm warning being issued before the heavy rain commenced and subsequently raised to red as the rain intensified.

The impact of climate change on precipitation patterns is not limited to Hong Kong. Since mid-April, southern China has been grappling with rainstorms that have persisted for days, triggered by a trough of low pressure, upper-altitude disturbances, and divergences. In Guangdong province, many areas have broken local rainfall records for the month of April, with the city of Shaoguan experiencing four times its normal levels of precipitation.

The connection between rising global temperatures and increased rainfall is rooted in atmospheric physics. As Earth’s atmosphere warms, it can hold more water vapour, leading to more intense precipitation events. Liz Moyer, an atmospheric scientist at the University of Chicago, explains that the greenhouse effect, which traps heat in the atmosphere, also compels the planet to shed more energy through evaporation. Consequently, more rain falls as the moisture condenses.

The Clausius–Clapeyron equation, a fundamental principle in atmospheric science, states that for every 1 degree Celsius of warming, the atmosphere can hold 6 to 7 per cent more water. However, this increased moisture content does not necessarily translate to a proportional increase in average rainfall, as other factors, such as the frequency of storms, also play a role.

Yoo-Geun Ham, a climate scientist at Chonnam National University in South Korea, highlights the challenges in distinguishing between natural variability and the influence of climate change on rainfall patterns. Precipitation is inherently complex and variable, making it difficult to detect global warming signals. However, using deep learning models, Ham and his colleagues have concluded that the increased occurrence of heavy rainfall events in East Asia and the Eastern United States this year can be attributed to global warming.

The West Coast of the United States is also expected to experience more intense rainfall due to the influence of atmospheric rivers, which transport moisture across the Pacific Ocean. Rao Kotamarthi, a senior scientist at Argonne National Laboratory, explains that a one-degree increase in ocean surface temperature significantly increases the amount of water carried by these atmospheric rivers, resulting in additional intense rains in California.

The hazards associated with extreme precipitation are particularly acute in mountainous regions and high latitudes, where a higher proportion of precipitation is falling as rain instead of snow. Mohammed Ombadi, a climate scientist at the University of Michigan, emphasizes that this shift has severe consequences, such as flooding and damage to infrastructure. Snow accumulates slowly and can take months to melt, while rain releases water rapidly, increasing the risk of landslides and flash floods.

As Hong Kong and other regions around the world grapple with the growing threat of extreme rainfall, the need for adaptations in urban planning and infrastructure becomes increasingly apparent. Current drainage systems, designed to quickly remove rainwater to prevent flooding, may prove inadequate in the face of ever-bigger deluges. The focus is shifting towards creating “spongier” cities, with more green spaces and fewer impermeable surfaces to allow water to seep into underlying aquifers for later use.

Ombadi stresses the importance of designing new infrastructure that is consistent with the changes brought about by global warming and can withstand the challenges of the coming decades. As Hong Kong braces for a summer marked by frequent rainstorm warnings and reduced sunshine, it serves as a stark reminder of the urgent need for action to mitigate the impacts of climate change and adapt to the new realities it presents.

Meanwhile, Hong Kong must adapt to its wetter future by reevaluating and enhancing its urban planning and infrastructure resilience. This includes upgrading drainage systems, implementing more rigorous building codes, and integrating green infrastructure to manage excess water more effectively.

Furthermore, public awareness and education on the issue are crucial. Understanding the link between climate change and local weather anomalies can foster a more informed and prepared populace, ready to take proactive steps in both prevention and response to flooding and other associated hazards.

Hong Kong’s challenge with increased rainfall is not isolated. It is part of a global pattern where regions around the world are experiencing similar shifts due to climate change. However, the city’s unique geographical and infrastructural context requires tailored solutions that address both immediate needs and long-term sustainability goals.