David Attenborough has highlighted Australia as an “extraordinary” example of a country where people in power remained climate change deniers despite the country facing some of the worst effects of global heating.

Appearing before the UK parliament’s business, energy and industrial strategy committee on how to tackle the climate emergency, the celebrated broadcaster and natural historian was asked about claims people were overreacting to the threat of a climate emergency.

He replied: “I am sorry that there are people who are in power … notably, of course, [in] the United States but also in Australia [who are climate change deniers], which is extraordinary because Australia is already facing having to deal with some of the most extreme manifestations of climate change.

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“Both [in] Australia and America, those voices are clearly heard and one hopes that the electorate will actually respond to those.”

Asked to recall his most vivid impressions of humans’ impact on the planet, Attenborough said it was returning to the Great Barrier Reef, where he had first dived in the 1950s.

“I will never forget diving on the reef about 10 years ago and suddenly seeing that instead of this multitude of wonderful forms and life that it was stark white. It had bleached white because of the rising temperatures and the increasing acidity of the sea,” he said.

Attenborough said radical action was needed to tackle the climate emergency – “we cannot be radical enough” – but also called for pragmatism in working out what was possible and how best to convince the public of the need for change.

He said he did not think opposing voices should be suppressed. “It’s very, very important that voices of dissent should have a place where they’re heard and the arguments between the two sides can be worked out in public, and compared and analysed in public,” he said.

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He found reason for optimism among young people who were “already making themselves and their voices very, very clear”.

“I’m OK, and all of us here are OK, because we don’t face the problems that are coming. But the problems in the next 30 years are really major problems that are going to cause social unrest, and great changes in the way that we live and what we eat. It’s going to happen.”

Attenborough recently presented a primetime BBC documentary, Climate Change – The Facts, which was watched live by almost 3 million people in the UK. It is yet to be shown in Australia.