CAIRNS • Diving beneath the ocean, Mr Russell Hosp swam towards the limestone bed of the Great Barrier Reef, where he reattached bits of blue staghorn coral. With tourists gone, he was filling the void with this small act of conservation, which took his mind off the uncertain future on land.

"It was a bit surreal," Mr Hosp, a reef guide, said of spending hours at sea unaccompanied by the usual enthusiastic visitors. Aboard the quiet catamaran, he said, he realised just how much the coronavirus "had changed the world".

The pandemic has fast-forwarded a looming reckoning for the tropical city of Cairns, the main gateway to the reef and the base for Mr Hosp and many others whose livelihoods depend on it.

Tour operators there were already fighting a perception that the reef is in its death throes, as warming waters cause repeated mass bleaching that has robbed many corals of their vivid colours. But where climate change has been more of a creeping threat to the reef's survival, and thus to Cairns' tourism lifeblood, the coronavirus has delivered a hammer blow.

Now this city, so linked with the natural wonder just off its shore that it can scarcely imagine life without the visitors who come in droves, has been forced to confront the prospect that it can no longer depend on tourists.

Foreign and local travellers, already deterred by last summer's devastating bush fires and now locked out by Australia's international and domestic travel bans, have all but vanished, and a US$4.6 billion (S$6.3 billion) industry built around the world's largest living structure has ground to a near halt.

The sudden disappearance of visitors feels all the more unreal because the virus itself has barely touched Cairns: The city of 150,000 people in far north-eastern Australia has recorded only a couple of dozen cases and has none currently.

But there is no escaping the reach of the pandemic.

"We'd never stopped running before - the global financial crisis, terrorism attacks, airline strikes; you name it, the world has thrown it at us," Mr Hosp said. "We don't know if we'll ever get back to normal."

In Cairns, visitors who usually cram the jetty every morning as they wait to pile onto boats have dwindled from the thousands to a few hundred, leaving operators out of work, boats moored at the dock, and some hotels and restaurants shuttered.

Storefronts on the main shopping street are for lease, and the esplanade, usually heaving with tourists at dusk, looks like something out of a sleepy beach town.

"It's been so quiet," said Ms Heather Forbes, a Cairns resident, adding that because the city had been dependent on tourism for so long, it was difficult to know how to diversify its economy. "I don't think anywhere should be solely dependent on one thing," she said.

It might seem that there was a silver lining in all this, that the exodus of tourists would be a boon for the health of a reef in critical condition.

But while the abrupt absence of visiting crowds has had surprising effects in other places - monkeys overrunning a Thai city, deer wandering in Japanese cities looking for food - the environmental impact of tourism on the reef is negligible, scientists say, especially when compared with climate change.

The reduction in international travel, and therefore planet-warming emissions, has created only a short-term benefit. The "infrastructure of fossil fuels wasn't affected", said Professor Terry Hughes, a global expert on coral reefs at James Cook University in Townsville, Australia.

In the end, a prolonged downturn in visits to the reef could actually be detrimental to its well-being. "Tourism provides a social and economic rationale for why the reef needs to be better protected," Prof Hughes said.

The situation has prompted the Cairns region to look more critically at its dependence on international travellers, especially those from China, who make up a large portion of reef visitors. China and Australia are engaged in an increasingly bitter diplomatic tug of war that could keep Chinese travellers away even after the borders are reopened.

"We're realising that we can't rely on China," said Ms Samantha Davidson, a travel consultant at the Reef Info Visitor Centre. "It's good," she added, because it's sending a message to those closer to home: 'Hey, come and see us'."

As recent flare-ups of the coronavirus have closed state borders within Australia, some people have taken the opportunity to explore their own (very large) backyards.

"We were supposed to be in Hawaii, but we said we still wanted to take a trip somewhere warm," Ms Alicia Dean said as she lounged in a sarong on the deck of a boat heading out to the reef.

She had travelled within the state of Queensland from Brisbane, the capital, to Cairns, more than 1,600km to the north.

And some foreigners, stranded in Australia, figured they might as well take the time to experience the reef, a World Heritage Site. "My flight keeps getting cancelled," Ms Julia Pape, 27, from Germany, said as she donned her flippers and wet suit, ready to plunge into the tropical waters.

NYTIMES