HKU Bulletin May 2021 (Vol. 22 No.2)

based on the increase in their built-up area (BUA), population growth and increased greening, such as new parks, green spaces and green roofs. This information was combined and compared with the cities’ economic status in World Bank rankings, to reveal trends showing strong links between the different variables. Cities in upper-middle income countries experienced the greatest expansion in their BUAs – 61 such cities expanded by more than 50 square kilometres from 2001 to 2018, compared to 21 lower-middle income cities, 17 high income cities and six low income ones. Upper-middle income cities also showed significant greening, meaning the greening happened during the study period (total vegetation including pre-2001 planting was instead labelled ‘greenness’). Some 325 cities saw significant greening in more than 10 per cent of BUAs and nearly one-third of those were in China. Overall, greening in Chinese cities increased by 32 per cent. Professor Chen explained that many high income places were already quite green to begin with or were in challenging environments, such as the semi-arid conditions around Los Angeles. But in low income cities, the story was different. In Africa, not one city scored highly on greening even though some of them have favourable climates for vegetation growth. “In places like Lagos, their greening decreased dramatically. It’s like bare soil there – they have removed many trees. But somewhere like New York City, although it did not have much greening from 2001 to 2018, its greenness is already quite good. In the Pearl River Delta, total greenness is not as good as New York, but it is steadily increasing and I believe it will increase further,” he said. “The relationship between green space and higher incomes is very clear. People want to live in a good environment. When the economy does well, they can spend money on proper green spaces.” Population challenge Population expansion is another important variable. Some 86 cities in high income countries and 59 cities in upper-middle income ones had negative population growth during the study period, although overall their cities expanded by an average of 100,000 and 300,000 people per city, respectively. Lower- middle income and lower income cities, however, saw much larger growth, expanding by an average of 500,000 people per city. “At the same time, the lower-middle income and lower income cities were substantially lagging behind in BUA expansion and infrastructure development, resulting in serious urban problems such as slums and crowding,” Professor Chen said. The first study to compare urban development, urban greening and population growth in large cities around the world shows how low income cities are lagging behind. THE GLOBAL INEQUITIES OF URBAN DEVELOPMENT The growth in population also meant even fewer people benefitted from whatever greening measures were carried out in low and lower-middle income cities compared with those in wealthier cities, to the detriment of both people and the environment. “Significant greening can help neutralise carbon emissions and mitigate the impact of global climate change in urban areas. A better understanding of the uneven urbanisation in developing countries can give them scientific references for managing urban areas and striking a balance between urbanisation, population growth and environmental changes,” he said. In 1889, Nature magazine carried an article that described China as a treeless country. To Professor Chen Ji of the Department of Civil Engineering, who was corresponding author of a recent groundbreaking study on urban development, the description was revealing of the conditions necessary for people to enjoy pleasant urban surroundings. “Why was it treeless? Because in urban areas, people didn’t have enough resources to develop their environment. Like Lagos in Nigeria today, there is no money for greening, just for building a simple house to live in,” he said. Professor Chen has shown where those conditions exist in the present day in the first global survey to link urban expansion, population growth and greening in large cities. While other studies have addressed individual components, none has provided such a comprehensive picture. The study was published in Nature Communications last October and singled out under its Editors’ Highlights section. “One of our findings is that cities in the lowest income countries face a serious problem with population growth and have an urban expansion path that is below that growth. The population is growing too fast,” he said. “Another big issue is that their urban environment is getting even worse. We hope the study will provide some warning signals to those cities to have more governance over their urbanisation.” The study was based on a rich dataset from 841 cities during 2001–2018. A former PhD student of Professor Chen’s and first author of the study, Dr Sun Liqun, had developed a method to rapidly analyse several terabytes of data from publicly available US satellite data to track development. They also used a vegetation index they had previously applied to analyse the impact of ice storms in southern China and the Wenzhou earthquake in Sichuan, which both occurred in 2008. Income and greenness linked The 841 cities each had areas larger than 100 square kilometres by 2018 and were analysed One of our findings is that cities in the lowest income countries face a serious problem with population growth and have an urban expansion path that is below that growth. The population is growing too fast. PROFESSOR CHEN JI Greening built-up areas in Pearl River Delta (left) and Yangtze River Delta (right) city clusters. The greening in Lagos decreased dramatically after many trees were removed. 19 RESEARCH 18 The University of Hong Kong Bulletin | May 2021