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The Pentagon ranks it as a national security threat and, left unchecked, climate change is expected to cost the US economy billions of dollars every year – and yet it has proved the great unmentionable of this election campaign.

Amid unprecedented melting of the Arctic summer sea ice, new temperature records in the US and a historic drought, the last of three presidential debates wound up on Monday night without Barack Obama or Mitt Romney ever uttering the words climate change.

It was the first time since 1988, the year Congress was first briefed on the emerging threat by the scientist James Hansen, that there had been no mention of climate change in an election debate.

The question cropped up in the vice-presidential encounter between the Republican Dan Quayle and the Democrat Lloyd Bentsen. Both agreed then it was time to act.

But this year’s vice-presidential contenders, Joe Biden and Paul Ryan, also failed to mention climate change during their single encounter, making for a total of six hours of primetime television debate without a single reference to climate change.

The omission – or “climate silence” – has proved hugely frustrating to campaigners.

It spawned a website which urged Obama and Romney to give climate change the attention it had in the 2008 election and a petition drive before the first debate, which gathered 160,000 signatures demanding debate moderators put climate on the agenda.

The two candidates did engage in a heated back-and-forth about gas prices in their town hall encounter last week. But the exchange saw Obama trying to one-up Romney in his support for oil and coal – fossil fuels rather than renewable sources of power.

CNN’s Candy Crowley, the debate moderator, later told Slate there had been questions from the audience about climate change, but she thought the economy was the priority.