SINGAPORE - Increasingly foul air from rapidly growing cities and industry could force South-east Asian nations to speed up the switch to cleaner sources of energy and to meet UN climate goals, says a leading energy specialist.

Dr Ernest Moniz, energy secretary under former US President Barack Obama, said Asean is poised for rapid economic growth but that the current energy policies for most countries in the region risk committing them to highly polluting electricity generation, transport and industry.

The problem is especially acute because rapid urbanisation in the region concentrates people, energy use and pollution.

"Now is the time to make the right choices so that when the population is wealthier they will also enjoy that wealth because you're not going to like it so much if you're living in Los Angeles of the 1960s or the Beijing of 10 years ago," he told The Straits Times on Thursday (June 6) on the sidelines of the Ecosperity Conference, part of Ecosperity Week.

Asean, with more than 620 million people, is one of the fast-growing regions in the world. But it is also ranked as the most vulnerable from the impacts of climate change, including severe storms, rising sea levels, droughts and floods.

Half of Asean's population live in urban areas and by 2025 a further 70 million people will be city dwellers, the Asean Secretariat says. That places ever greater strains on transport, electricity, water and sanitation networks.

Electricity demand in the region is forecast to double by 2025 and with it greenhouse gas emissions are expected to keep growing right at the time that the UN climate panel says nations should be curbing fossil fuel pollution.

This underscores the immense challenge governments in the region face: providing enough energy to drive economic growth and raise living standards without destroying the environment.

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For now, coal remains king but costs of wind and solar energy have plummeted and public awareness of the dangers of air and water pollution is growing, leading to the cancellation of some coal projects.

Asean is at a critical moment, said Dr Moniz, founder and CEO of US think-tank Energy Futures Initiative.

"What this means is the region needs to take advantage of the opportunity of quite literally building major energy infrastructure and to do it in a clean and sustainable way and enhancing the quality of life," he said.

Otherwise, the risk is worsening pollution.

Asean's most populous members - Indonesia, Vietnam and the Philippines - are pumping US$120 billion (S$164 billion) into coal-fired power plants that are under construction or planned, according to a recent study by London-based Carbon Tracker Initiative. Wind and solar investments are still a fraction of this.

A 2016 study co-authored by the Asean Centre for Energy predicted the external costs related to air pollution from fossil fuels could reach US$225 billion annually by 2025.

Dr Moniz said the region had plenty of natural gas resources. While still emitting carbon dioxide, the main greenhouse gas, natural gas was far less polluting than coal and, twinned with greater investment in renewable energy, was one solution for the region while it transitioned to low or zero-carbon energy sources by mid-century.

He suggested Asean nations cooperate more closely on building gas pipeline networks as well as high-voltage electricity lines across the region.

Key, too, was electrifying transport networks in cities in Asean to cut pollution and to reduce car use. This included trains and electric buses, which China has been rolling out on a large scale.

"I think it is a system approach that's going to be needed," he said. That meant closer cooperation of Asean nations to reach low-carbon goals and understanding that each country has different needs.