Critics have condemned the government for spending HK$10 million on a study, due to be completed by 2019, which will consider the feasibility of building housing for elderly residents on 20-hectare sites on the edge of Tai Lam and Ma On Shan country parks.

They argue the study is biased because it is being led by the Housing Society, a statutory non-profit group which has a vested interest in building housing.

Environmentalists maintain that country parks provide an essential balance with Hong Kong’s urban areas, namely by counteracting our carbon footprint. The government has already pledged to reduce total carbon emissions by 26 to 36 per cent by 2030 as part of efforts to meet targets set by the Paris agreement on climate change.

The city’s carbon emissions have been gradually rising since the 1997 handover, and in 2009, it had the world’s second highest carbon footprint per capita.

It remains difficult for scientists to accurately assess the extent to which Hong Kong’s country parks offset the city’s carbon emissions.

In 2002, a study by think tank Civic Exchange found the city’s natural vegetation stored 15 tonnes of carbon per hectare per year. Researchers Lisa Hopkinson and Rachel Stern estimated there were 20,000 hectares of woodland and forest in Hong Kong, meaning vegetation captures 300,000 tonnes of carbon per year, equivalent to absorbing carbon emissions from 63,829 cars each year.

The report suggested country parks were therefore worth between HK$1.8 billion and HK$6.5 billion annually, but emphasised they could not simply be replaced by “depositing money in a bank account”.

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Jude Wu, managing director of NGO Conservation International Hong Kong, said the government and its partners needed to make “a very careful assessment” of the value of country parks in considering whether to build on certain sites.

She said her organisation was working on its own report about the conservation of green belt land.

“It’s difficult to put a dollar value on country parks,” she said. “They benefit us in so many different ways – providing recreation space, landslide prevention, reducing flooding on our streets, mitigating the effects of air pollution and helping to ensure fresh water reaches our reservoirs. Many people don’t know about all these benefits.”

Although lawmakers generally appear focused on plans to build on brownfield sites on the fringes of country parks, which are of lower ecological value than green belt areas, critics have warned this could be the start of more destructive development.

Wong Fook-yee, former assistant director at the Agriculture, Fisheries and Conservation Department, has warned such developments would set a dangerous “precedent”. Campaigners have also warned that some smaller, non-residental developments on green belt or brownfield sites are occurring and are not being recorded.

Six months before his term ended in June, former chief executive Leung Chun-ying suggested Hong Kong needed to “think out of the box” in order to increase the land supply, as he advocated building on “the periphery of country parks”.

His successor Carrie Lam Cheng Yuet-ngor, Leung’s former chief secretary, also pledged on the campaign trail to increase the city’s housing stock by developing new land. Since she took office on July 1, she has not responded to calls from environmental groups to scrap the Housing Society study.

The government has set itself a target of building 460,000 flats within the next decade, which it so far seems unlikely to achieve.

The target for private housing has consistently been exceeded because developers remain attracted by the increasing value of housing. But experts predict there will be a shortfall of 44,000 public housing flats by 2026-27. As of May this year, there were 275,900 families and individuals waiting for public housing.

Last week, further controversy was sparked after a report by two University of Hong Kong real estate and planning academics suggested the city’s second-largest reservoir, Plover Cove, should be drained to make way for 300,000 flats. They argued there would be limited environmental costs because the original marine ecology had already been killed off by the reservoir’s development in the 1960s.

If Hong Kong can make better use of this supply, a quarter of which comes from rain that falls on our hillsides, then it may become less dependent on its increasingly expensive water supply from the Dongjiang – or East River – in Guangdong province. Hong Kong currently pays HK$13.4 billion for 820 million cubic metres of water per year, representing about 80 per cent of its total supply.

Country parks are also important for maintaining Hong Kong’s ecosystems. In January, local campaign group The Green Earth issued a statement on behalf of 20 environmental groups in Hong Kong, urging the government to think again about building on the fringes of country parks because of the ecological impact.

“All the plants, flowers and birds in the country parks have their own meaning and value to the natural environment. Ecological value cannot be measured in objective criteria,” it said.

In terms of recreation, scientists have found parks and green spaces provide essential opportunities for urban dwellers to exercise, reducing their chances of developing various health problems including cancer, heart disease and depression. The number of visitors to country parks has steadily increased from 12.5 million in 2007 to 13 million last year.