Reef relief: how a late summer monsoon helped cool the Great Barrier Reef at just the right time

Reporting by Australian Institute of Marine Science

Australia’s Great Barrier Reef is a true natural wonder: a unique and diverse underwater seascape the size of a small country.

However, cycles of disturbance and recovery threaten to turn its colourful coral gardens into algal-dominated seascapes – then back again. And these cycles are becoming more volatile as our world warms.

At the Australian Institute of Marine Science, we’ve been documenting these changes for the past four decades through our Long-Term Monitoring Program. This is one of the longest records of continuous coral reef data in the world.

Concerningly, we’ve watched the health of the Great Barrier Reef begin to fluctuate dramatically over the past 15 years. The 2024 mass coral bleaching – where a record-breaking marine heatwave bleached coral reefs from the Caribbean to the Indian and Pacific Oceans – was particularly destructive. It caused the largest single-year decline in coral ever measured in two regions.

However, our latest report reveals some promising signs of recovery.

In 2025, the Great Barrier Reef endured its sixth coral bleaching since 2016 – one of only two back-to-back bleachings in the reef’s history.

However, our latest survey data shows the 2025 bleaching was smaller and less severe than in 2024. This means substantially less coral died, showing not all periods of bleaching have the same outcome. A heatwave’s intensity and duration determines whether corals die or survive. If corals get relief from the heat, they can regain their symbiotic algae and survive.

On the Great Barrier Reef, sea surface temperatures generally peak between January and March. During these summer months, marine heatwaves can increase the risk of coral bleaching. Marine heatwaves occur if ocean temperatures remain unusually warm for the location and time of year for at least five consecutive days.

In spring 2025, a marine heatwave swept across the reef, raising ocean temperatures by up to 1.5°C. According to the Bureau of Meteorology’s climate trends, December 2025 was the reef’s hottest December on record since 1900. If the heatwave continued through summer, the unrelenting heat would have likely led to widespread coral bleaching.

Last year, we recorded the largest single-year decline in living coral cover in four decades of monitoring. This followed the devastating 2024 mass coral bleaching. Coral cover refers to how much of the reef surface is covered in live hard coral, compared with algae, sand and other organisms. It’s one indicator of how healthy a reef is.

In good news, our latest report suggests coral cover increased slightly in some parts of the reef from 2025 to 2026. In the northern region (Cape York to Cooktown), coral cover rose from 30% to 35.1%. This figure increased from 28.6% to 31.6% in the central region (Cooktown to Proserpine). This puts both regions above the long-term average of coral cover. However, coral cover in the reef’s southern region (Proserpine to Gladstone) remained below the long-term average, decreasing slightly from 26.9% to 26.4%.

In early 2026, we noticed signs of regional coral bleaching in the northern and central Great Barrier Reef. But a monsoon soon arrived, bringing widespread cloud and rainy weather, helping to cool the reef at just the right time.

Australian monsoon activity typically coincides with the northern wet season, from October to April. During an active monsoon, clouds, rain and winds help cool the surface of the ocean. Breaking waves and turbulence then mix hotter surface waters with deeper, cooler waters. The monsoon season also reverses winds over the tropics, so northern Australian winds blow as moist northwesterly winds.

The monsoon returned in early March 2026, bringing clouds and rain to cool the reef. NASA Worldview - Aqua/MODIS (True Color)