Joe Biden has vowed that US commitment to defend Israel against Iran was “ironclad” as concerns rose in Washington that a “significant” Iranian strike could happen within days, in retaliation for the bombing of an Iranian consular building in Damascus.

US and allied officials fear that a strike is imminent and could come in the form of a direct missile launch from Iran, rather than an attack through a proxy like Hezbollah in Lebanon. Israel has vowed to respond in kind to such a direct strike, raising the prospect of a regional war, which US officials now believe is more likely than at any point since the beginning of the Gaza conflict on 7 October.

Biden’s pledge of support to Israel at the White House, intended as a deterrent, came a few hours after Iran’s supreme leader, Ayatollah Ali Khamenei, repeated a threat to strike back against Israel over the Damascus bombing that killed 12 people, including Gen Mohammad Reza Zahedi, a senior figure in Iran’s Islamic Revolutionary Guard Corps, and six other guard members.

“When they attacked our consulate area, it was like they attacked our territory,” Khamenei said, in remarks broadcast by Iranian state TV. “The evil regime must be punished, and it will be punished.”

Israel’s foreign minister, Israel Katz, responded in a post on the X social media platform, vowing that: “If Iran attacks from its territory, Israel will respond and attack in Iran.”

Israel has not formally taken responsibility for the 1 April bombing, but Israeli and US officials have made clear it carried out the strike.

Israel and Iran have been trading blows in Lebanon and Syria for months, but Biden administration officials fear that the 1 April Damascus bombing on an Iranian diplomatic building, which Tehran considers its own territory, has significantly raised the threat of the Gaza war widening into a broader conflict.

Since the Damascus bombing, Tehran has sent Washington messages attributing ultimate blame for the attack on the US and warning the US to stay out of its confrontation with Israel.

Biden’s pledge to Israel on the White House lawn, in a joint appearance with the Japanese prime minister, Fumio Kishida, appeared to be a response to that warning, insisting the US would not stay on the sidelines.

“We also want to address the Iranian threat to launch a significant – they’re threatening to launch a significant attack in Israel,” Biden said. “As I told Prime Minister Netanyahu, our commitment to Israel’s security against these threats from Iran and its proxies is ironclad. Let me say it again, ironclad. We’re gonna do all we can to protect Israel’s security.”

The Biden administration is seeking to head off a direct Iranian attack by messaging that Tehran cannot assume that US forces in the region, reinforced significantly since the start of the Gaza war, would stay out of a conflict with Israel.

“We’ve been clear that we do not want this conflict to escalate or spread further in the region. We’ll continue to undertake diplomatic efforts to ensure that remains the case,” the spokesperson said. “We also retain a military presence in the region to deter those who seek to take advantage of the conflict.”

Secretary of state Antony Blinken reiterated that message on Wednesday in a call with Israeli defence minister Yoav Gallant, in which he made clear the US would stand with Israel against any threats by Iran, the state department said.

Alon Pinkas, a former Israeli diplomat and adviser to prime ministers Ehud Barak and Shimon Peres, said: “The prevailing conventional wisdom is that because the attack in Damascus was directly against Iran, then that means that Iran will have to respond to retaliate directly, rather than via a proxy.

“From what I’m hearing here, the most telling sign is that Khamenei has mentioned the need to retaliate twice in the last week in his sermons or whatever,” Pinkas said. “Usually, they don’t do that. Usually they are much more opaque and only commit to a response one day at the right moment and in the right place.”

Among the possible targets are Israeli embassies around the world, and they have been taking extra security precautions in the wake of the Damascus bombing, but US officials also believe that a direct strike on military or government targets on Israeli territory is also a significant possibility.

The US and Israeli militaries and intelligence agencies are in constant contact about the threat. Axios reported that the head of US Central Command, Gen Erik Kurilla, is due in Israel on Thursday to discuss coordination with his Israeli counterparts and the defence minister, Yoav Gallant.

The ability of the Biden administration to rein in an Israeli response to an Iranian attack would very much depend on the specifics. If Iranian retaliation comes in the form of an assault on an Israeli embassy, or if an incoming Iranian missile or drone is intercepted, it may be possible to prevent escalation, officials said, but if an Iranian strike caused multiple casualties inside Israel, it would be very much harder.

The world’s coal power capacity grew for the first time since 2019 last year, despite warnings that coal plants need to close at a rate of at least 6% each year to avoid a climate emergency.

A report by Global Energy Monitor found that coal power capacity grew by 2% last year, driven by an increase in new coal plants across China and a slowdown of plant closures in Europe and the US.

About 69.5 gigawatts (GW) of coal plant capacity came online last year, of which two-thirds were built in China, according to the report. There were also plants built in Indonesia, India, Vietnam, Japan, Bangladesh, Pakistan, South Korea, Greece and Zimbabwe.

Meanwhile, a slowdown in coal plant shutdowns in the US and Europe led to more than 21GW retiring last year. This resulted in a net annual increase of almost 48.5GW for the year, the highest since 2016.

The authors of the report said coal plants needed to shut at a faster pace, and that China needed to adopt stricter controls on its expansion of capacity.

Flora Champenois, a Global Energy Monitor analyst, said: “Otherwise we can forget about meeting our goals in the Paris agreement and reaping the benefits that a swift transition to clean energy will bring,.”

Climate scientists have said all coal plants should be shut by 2040 – unless they are fitted with effective carbon-removal technology – if governments hope to limit global heating to within 1.5C of pre-industrialised levels.

This would require an average of 126GW of coal plants to retire from the current fleet of 2,130GW every year for the next 17 years, according to the report, or the equivalent of about two plants a week.

Champenois described last year’s coal plant expansion as an “anomaly” as more signs point to it “reversing course”. “But countries that have coal plants to retire need to do so more quickly, and countries that have plans for new coal plants must make sure these are never built.”

Research by Capital Economics suggests China’s appetite for coal power may reduce as low-carbon options accelerate.

The report found that a resurgence in hydropower this year, combined with China’s expansion of wind, solar and nuclear power, could mark the start of a steady decline in its coal demand.

Coal plant retirements were expected to pick up speed across Europe and the US this year, and fewer countries were beginning construction of new plants, Champenois said.

The report found that construction started on less than 4GW of new projects outside China last year, a quarter of the annual average between 2015 and 2022 for the same set of countries.

Concern that the Great Barrier Reef may be suffering the most severe mass coral bleaching event on record has escalated after a conservation group released footage showing damage up to 18 metres below the surface.

Dr Selina Ward, a marine biologist and former academic director of the University of Queensland’s Heron Island Research Station, said it was the worst bleaching she had seen in 30 years working on the reef, and that some coral was starting to die.

The Great Barrier Reef Marine Park Authority last week said aerial surveys of more than 1,000 individual reefs revealed more than half were rated as having high or very high levels of bleaching, and a smaller number in the south – less than 10% of the total – had extreme bleaching. Only about a quarter were relatively unaffected.

It confirmed the 2,300-kilometre reef system was experiencing its fifth mass bleaching event in eight years. The authority said sea surface temperatures had been between 0.5C and 1.5C hotter than expected for this time of year.

The Australian Marine Conservation Society on Thursday released video and photos that it said showed bleaching on the southern part of the reef extended to greater depths than had been previously reported this year.

Ward said the impact of bleaching had been extensive across 16 sites that she visited in the reef’s southern section, affecting coral species that had usually been resistant to bleaching. Some coral had started to die, a process that usually takes weeks or months after bleaching occurs.

“I feel devastated,” she said. “I’ve been working on the reef since 1992 but this [event], I’m really struggling with.”

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What is coral bleaching?

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Coral bleaching describes a process where the coral animal expels the algae that live in their tissues and give them their colour and much of their nutrients.

Without their algae, a coral’s white skeleton can be seen through their translucent flesh, giving a bleached appearance.

Mass coral bleaching over large areas, first noticed in the 1980s around the Caribbean, is caused by rising ocean temperatures.

Some corals also display fluorescent colours under stress when they release a pigment that filters light. Sunlight also plays a role in triggering bleaching.

Corals can survive bleaching if temperatures are not too extreme or prolonged.  But extreme marine heatwaves can kill corals outright.

Coral bleaching can also have sub-lethal effects, including increased susceptibility to disease and reduced rates of growth and reproduction.

Scientists say the gaps between bleaching events are becoming too short to allow reefs to recover.

Coral reefs are considered one of the planet’s ecosystems most at risk from global heating. Reefs support fisheries that feed hundreds of millions of people, as well as supporting major tourism industries.

The world’s biggest coral reef system – Australia’s Great Barrier Reef – has suffered seven mass bleaching events since 1998, of which five were in the past decade. 

Ward said sea temperatures at two of the sites she visited were the same at the surface and 20 metres below the surface. This was “very unusual”, and reinforced the need for rapid action to reduce greenhouse gas emissions, she said.

“What are we doing to stop the reef from being lost?” Ward said. “We cannot expect to save the Great Barrier Reef and be opening new fossil fuel developments. It’s time to act and there are no more excuses.”

Coral bleaching occurs when the coral becomes heat stressed and ejects the tiny marine algae, known as zooxanthellae, that live in its tissue and give most of its colour and energy. With the zooxanthellae gone, the coral starves and its bone-white calcium skeleton becomes visible.

If the elevated temperature doesn’t last long, the coral can recover. Otherwise, it starts to die. In the most severe cases, the bleaching is skipped and the coral dies almost immediately, usually turning a dirty brown.

Terry Hughes, an emeritus professor at James Cook University and longtime reef bleaching researcher, said the aerial surveys showed “the most widespread and most severe mass bleaching and mortality event ever recorded on the Great Barrier Reef”.

He said the scale of the damage was comparable to 2016, the worst previous year experienced, but there were now fewer individual reefs untouched by bleaching between southern Queensland and the Torres Strait. He said the area south of Townsville had been particularly badly hit this year.

“We’re already seeing extensive loss of corals at the time of peak bleaching,” he said. “It’s heartbreaking to see damage as severe as this as soon as this.”

Hughes said every part of the reef system had now bleached at least once since 1998. Some reefs had bleached three or four times. He said the cumulative damage made it harder for reefs to recover and more likely they would succumb.

The Intergovernmental Panel on Climate Change in 2018 found that most tropical coral reefs would be lost if global heating was limited to an average of 1.5C above pre-industrial levels and 99% were likely to be lost of heating reached 2C. They found they would be at high risk at 1.2C, a level that may have already been reached.

Dr Lissa Schindler, an ecologist and the reef campaign manager with the Australian Marine Conservation Society, called on the Great Barrier Reef Marine Park Authority to release maps showing the extent and severity of the bleaching so the public had a true picture of the scale of the impact.

Schindler also urged the authority, which she described as the reef’s custodian, to play a greater role in advocating for stronger action on emissions.

She said in the past the authority had called for “strong and fast national action” to deal with the climate crisis, but a more recent climate statement it issued focused on global action and did not say anything about Australia needing to increase what it was doing.

“If the Albanese government is serious about its commitment to Unesco to protect the reef then it must commit to net zero emissions by 2035 and stop approving new fossil fuel projects,” she said.

Scientists have said the government’s emissions reduction targets – a 43% cut compared with 2005 levels and net zero by 2050 – are consistent with global action that could lead to 2C of global heating.

Interviewed on ABC’s Radio National on Wednesday, the environment minister, Tanya Plibersek, said the government was “very concerned about the bleaching that we see at the moment, sadly, not just on the Great Barrier Reef, but right around the world”.

She said the government was doing “whatever we can” to get to net zero emissions. “We need to protect the reef because it’s … unique in the world and also 64,000 people rely on it for their work,” she said.