Mass bleaching now outpaces the five years corals need to start reproducing

Ainsworth says severe disturbance is already happening annually in some places and every few years in others, and that the growth and maturity timeline corals need is now being overwhelmed by stress and disturbance events.

Global hard coral cover increased by 6% from 2017 to 2019, following the third global bleaching event.

Those 2017-2019 gains were reversed by coral loss associated with the fourth global bleaching event, which began in 2023 and continued into 2025, including substantial losses in the 2024 mass bleaching event on the Great Barrier Reef and severe bleaching of Western Australian reefs in 2025.

The fourth global bleaching event produced the largest decline in hard coral cover in the 45-year record, although its full effects are not captured in the report.

Ainsworth says the capacity for recovery depends on retaining healthy, diverse reef areas that can continue supplying new corals to damaged sites, which requires some reefs to escape bleaching, storms and land-based pollution, or at least for some corals to survive and reproduce afterward.

The global 9.5% decline conceals substantial differences between reefs and regions: some locations have experienced far greater losses because they face repeated marine heat waves combined with local pressures such as poor water quality, while other reefs have so far been less exposed to extreme warming.

Ainsworth says inshore coral reefs are affected by both severe stress events and land-based pollution.

Australia's reefs remain in comparatively good condition but have followed the global decline: average Australian hard coral cover fell from 30.2% in the 1980-2009 reference period to 26.8% between 2020 and 2024, a relative decline of 10.9%.

Many corals need at least five years just to mature and begin reproducing.

Ainsworth says the large corals expected on reefs can be decades old, and when they are lost it takes decades for that population structure to return.

The global drop is 2.9 percentage points of cover, and expressed against the 1980-2009 baseline it is a 9.6% relative decline, consistent with the report's 9.5% being a relative rather than absolute figure.

The Australian drop is 3.4 percentage points, which against the same 30.2% baseline gives 11.3% rather than the report's stated 10.9%, a difference consistent with the published cover figures being rounded to one decimal place.

Moving from roughly one major bleaching event per decade to one every five to six years means events arrive about 1.8 times as often.

More than 21 million observations across about 90,000 surveys averages roughly 233 observations per survey.

At a five-to-six-year interval between major bleaching events, the margin over the minimum five years many corals need to reach first reproduction is between zero and one year.

Ainsworth says: "Recovery to the reefs we enjoyed before 2016 is increasingly unlikely under these conditions," citing more frequent bleaching events, more severe storms and more land-based pollution.

Rather than expecting damaged reefs to return to their previous state, scientists are increasingly considering how coral ecosystems might reorganize in response to warmer oceans and more severe disturbances, which Ainsworth describes as restructuring into systems that can withstand higher temperatures and more severe storms.

Ainsworth says coral cover could return but the original reef ecosystem will not, because the mix of coral species, physical structure and ecological relationships is becoming entirely different.