The Great Barrier Reef showed rapid but fragile recovery from previous bleaching events.

Oceans absorb 93% of excess heat from climate change. Unlike the atmosphere, global ocean heat content increased considerably, by 6.5% compared to the previous year. This was in line with a steady increase in ocean heat over the last three decades.

Global sea level also continued its steady rise, rising by 5.0 mm in 2021. Sea level has increased 80 mm since 2000 and 103 mm since 1993.

Sea level around Australia has been rising faster than the global average. The fastest rate of rise was in the Tasman Sea, which has risen by more than 150 mm since 1992.

Ocean surface temperatures around Australia were slightly lower than the previous year, but 2021 was still the sixth warmest on record. The mean temperature was 0.52 °C above the 1961–1990 average and 0.09 °C above the 2000–2020 average.

La Niña conditions contributed to a marine heatwave off the coast of Western Australia in early 2021. 

Favourable conditions in the Great Barrier Reef led to rapid but fragile hard coral recovery from the three bleaching events in the previous five years. Later in the year, in December, the warmest sea temperatures on record for that month were observed, but thanks to cooler weather conditions, these did not develop into a bleaching event.

Maximum monthly sea surface temperature across the GBR (BoM)