He complained of long delays in the process, with Environment Bureau officials often brushing off the group’s expert opinions and ignoring their suggestions.

“I felt I have not been able to fully contribute my expertise, which is why I decided to resign,” he said.

Lam’s exit came as mainland China, the world’s biggest emitter, made a pledge last month to achieve carbon neutrality – which refers to either reducing greenhouse gas emissions to zero, or offsetting emissions with carbon removal – by 2060. It also said its greenhouse gas emissions would peak within the next decade, after which they would start to fall.

Hong Kong should have declared its own target after a public engagement exercise in June last year on formulating a blueprint for carbon cutting measures to combat climate change.

But the results of the exercise, first scheduled for release at the end of 2019, were delayed. The Environment Bureau said it would happen this year, but there has been no announcement yet.

Lam said Hong Kong had already submitted its target to Beijing, but “it does not include public input”.

In 2017, the government pledged to lower per capita carbon emissions to between 3.3 and 3.8 tonnes from the current six. It also aimed to phase out coal in the fuel mix, adding more natural gas and increasing the use of low-carbon energy sources, such as nuclear, to 3 to 4 per cent.

But it stopped short of saying when it would reach carbon neutrality.

The three-month public engagement exercise, which explained the science of climate change and measures to mitigate the impacts of a warming planet, included a questionnaire asking individuals, corporations and organisations if they supported certain carbon-cutting methods.

The responses were to form the basis of a policy recommendation for the government to formulate its climate response as set out under the Paris Agreement to keep earth’s temperature from rising more than 2 degrees Celsius.

Lam said that over the past two years, his group of experts met only three times for less than 10 hours, often without full attendance as the members were only notified by the Sustainable Development Council secretariat a week in advance.

He said one item in last year’s questionnaire triggered a backlash. It asked whether residents supported increasing the proportion of renewable energy as a source of electricity through regional cooperation.

At a time when anti-Beijing sentiment was running high in Hong Kong, some critics took “regional cooperation” to mean working with the mainland.

The result was that only a little more than one in 10 respondents agreed with long-term decarbonisation strategies, he said.

“So the government now faces a paradoxical situation. If they want to listen to public opinion, the long-term decarbonisation plan can be scrapped,” he added.

Lam said officials refused to tell his support group how many responses were received, or say when the results would be released. They also did not let the experts see the responses from environmental organisations beforehand, claiming it would lead to bias among them, he said. 

The officials agreed to provide the expert group with an informal briefing on the results, but when they met in late June, they presented only a four-page brief, not an in-depth report.

A week before the meeting, the experts were informed that their job was done, although the group would not be dissolved until after the report on the climate change strategy was announced.

Lam resigned at the end of last month.

“I wrote to the environment minister complaining about the lack of a concrete timeline, but nothing changed,” Lam said. “I think these officials have no interest in listening to outside opinions, they just want people to conform.”

Both the Environment Bureau and the Council for Sustainable Development, which is in charge of compiling the policy recommendations for the bureau, said they respected Lam’s decision to resign and thanked him for his contributions.

A spokesman for the council said it was in the final stages of preparing its report and would submit it to the government soon.

“We are mindful of the fact that Hong Kong’s carbon emissions had already peaked in 2014 and kept dropping thereafter. The city is therefore moving in the right direction,” the council said, adding Hong Kong was in a position to draw up an “ambitious” long-term strategy.

The bureau also said the council’s work would not be affected by Lam’s departure and the report would be submitted “in the very near future”, but did not disclose whether it had submitted to the central government a date for when Hong Kong would achieve carbon neutrality.

Paul Harris, chair professor of global and environmental studies at the Education University, said it was no surprise that the results of the public engagement had not been released and Lam had stepped down.

“The Hong Kong government is not capable of mustering the internal will to achieve anything akin to fulfilling its obligations as a highly polluting society,” he said. “This is why Lam has quit. We need a wholesale change and the government is entirely preoccupied with political matters such as national security now.”

Edwin Lau Che-feng, executive director of environmental NGO The Green Earth, said he was sorry to hear Lam had quit.

“Lam does not mince words and has always been practical about what needs to be done,” Lau said. “He is a credible figure in the field, so it’s a pity he has resigned.”

Lau thought it was unreasonable for the government to blame the pandemic for delaying the results of public engagement exercise. “Everybody was working from home, but operating as before. There should not have been any impact and they could have still delivered,” he said.

Lam said he still hoped Hong Kong could achieve carbon neutrality, and do it sooner than the mainland’s 2060 target, as the city has barely any industry.

Although no longer involved in government work, he will continue to push for change in the private sector.

“There are a thousand things we can do to reach that goal, especially because Hongkongers have a high awareness of the issue,” he said.