In February, the average temperature at the surface of the ocean hit an all-time high, climbing above 21°C. It surpassed previous records reached in 2023, which was the hottest year, globally.
Monitoring ocean heat across the globe is a more reliable measure of climate change than average air temperature rise because most of the excess energy from burning fossil fuels goes into the sea. But to observe and assess the impacts of ocean warming – on weather and ecosystems – scientists need local data as well.
Thanks to the foresight of the late marine biologist Bill Ballantine – known for establishing New Zealand’s first marine reserve around Goat Island, north of Auckland – we now have one of the Southern Hemisphere’s longest continuous local sea-surface temperature records, going back almost six decades.
It all started with a bucketful of seawater collected on New Year’s Day in 1967. Ballantine’s daily manual thermometer readings at the Leigh Marine Laboratory have long since been automated, but the unique record he started now shows a dramatic intensification of warming in the Hauraki Gulf.
University of Auckland marine scientist Nick Shears, who recently published the results from the long-term record, says it also tracks the “unprecedented nature of recent marine heatwaves”. Overall, the area experienced 156 marine heatwaves in the past 57 years.
During earlier decades, it was common to have years without marine heatwaves (defined as five or more consecutive days of temperatures above the 90th percentile of 30-year historic values), but the ocean nevertheless continued warming, particularly during autumn and winter.
The data points to 2009 as a step change, which was followed by a period of rapid warming. The highest rates were in May and June. Another change point came in 2012. Every year since then has experienced marine heatwave conditions and their cumulative intensity has been increasing sharply.
This built up until 2022, which had the warmest autumn, winter and spring temperatures so far and a marine heatwave that lasted 313 days. Near continuous heatwave conditions persisted from November 2021 to November 2022, with sea temperatures 1-2°C above average.
“The marine heatwave in 2022 was like nothing we have ever seen before,” Shears says. “Many of the ecological impacts observed were unexpected and concerning.”
Prolonged periods of water temperatures above 20°C now sometimes stretch until May. The ecological changes underway in shallow reef ecosystems are a glimpse of how things could change.
Shears says the warm summer temperatures in 2022 brought on extensive algal blooms. Cold-loving sponges no longer got an autumn respite and “melted”, dying off from heat stress. So did seaweeds and kelp. Warm autumns and winters aided the creeping advance of subtropical species such as a long-spined herbivorous sea urchin that can devastate deep reef environments, and benefited the invasive Caulerpa seaweed, which is now well established in the Hauraki Gulf.
Unlike on land, a very small proportion of the marine environment is protected, with less than 1% of our coastal waters off limits for fishing, Shears says.
“There are many reasons why protecting more of our coastal environment is a good idea, but healthy ecosystems will generally be more resilient to some of the impacts of climate change. The rapid changes we are seeing in the marine environment highlight the urgent need for marine protection and better management of our marine ecosystems and species more generally.
“By better managing the impacts that can be managed – like fishing – we will increase the health and resilience of ecosystems to stressors that can’t be managed in the short-term, like climate change.”
