The world's coral reefs are at a "make or break point" and every nation should think twice about developing new coal mines, including those planned for Queensland, the United Nations environment chief Erik Solheim said.
Mr Solheim, in Australia on a whirlwind trip to meet coral experts and visit the Great Barrier Reef, said coral reefs were one of the planet's most important ecosystems, home to a quarter of the oceans' biodiversity in just 1 per cent of the area.
However, recent severe coral bleaching caused by warming waters was among their leading threats.
"Reefs are in peril," Mr Solheim told Fairfax Media at the start of the International Year of the Reef, aimed at lifting awareness of their plight.
"We have seen a huge amount of bleaching incidents in reefs all over the planet and their rapid decline," he said. "We are at a make or break point where we can take possible steps forward or [oversee] the decline of the reefs."
All countries should reconsider opening new coal regions - such as Queensland's Galilee Basin where India's Adani Group is among firms planning mega-mines - not least because solar and wind ventures were now highly competitive.
"Those who open up a lot of coal now may not only have an environmental problem but very soon [also] an economic problem because coal is more costly than renewables," he said.
He noted New York City, where the UN has its headquarters, had not only divested $US5 billion ($6.27 billion) in fossil fuel assets, but begun suing Shell, ExxonMobile and other oil giants for their current and future climate damage.
"You see activists suing and big investors asking 'why we are putting our money into something which can be an economic liability and a political liability for the future?'"
2017, he said, had been a mixed year, with "Mother Nature really hitting back at humans". Extreme weather events included huge cyclones in the Carribean and big floods in South Asia.
On the plus side, big carbon emitters China and India had accelerated efforts to install renewable energy, while nations such as Botswana and Rwanda had boosted wildlife protection.
This year had also begun with Fiji seeking nomination of its Great Sea Reef - the third-largest in the southern hemisphere - for added protection as a Ramsar site. "That's a huge step forward," Mr Solheim said.
He also hailed the decision on Tuesday by the European Commission to pass its first Europe-wide plan, a move that would force all plastic packaging to be recyclable by 2030.
"The ability of humans to change is enormous," he said, citing his reason for remaining optimistic about the global environment.
