Mean Heat: Marine Heatwaves To Get Longer And Hotter By 2100

Off the back of Friday's Summer Climate Summary which highlighted this season's record breaking sea temperatures, we bring you new research showing that New Zealand could experience very long and "very severe" marine heatwaves by the end of the century.

Marine heatwaves are already becoming a common experience for New Zealanders. In newly released research from the Deep South Challenge: Changing with our Climate and NIWA , scientists say that by 2100, the 40-odd marine heatwave days we currently see in a normal year will increase to between 80 days (low emissions, best-case scenario) and 170 days (high emissions, worst-case scenario) by the end of the century. For some regions, such as southern tip of the South Island, there is a high chance that marine heatwaves start to last more than a year.

The research also explores the intensity of future marine heatwaves, or just how warm they will be. For coastal waters, average marine heatwave intensities will increase by 20% (best case) to 100% (double, worst case) by the end of the century. For the North Island, this means an average marine heatwave could be between 0.5°C to 2°C more intense than they are today.

Research lead and NIWA ocean modeller Dr Erik Behrens says that the chance of marine heatwaves becoming a permanent fixture is worrying.

"The impacts of climate change are happening all around us and New Zealand isn’t immune. We’re just coming off the back of one of our most intense marine heatwaves, like what we experienced in 2017. Our work indicates that this will start to become the norm as time goes on. Marine heatwaves can have significant impacts both at sea and on land. They kill off corals, disturb ecosystems, and can also pose a problem for fishing and aquaculture, as well as contributing to land heatwaves and climate extremes across the country.

"What is particularly interesting is the disparity between regions, with some coastal areas predicted to experience a much bigger intensity, frequency and duration of warming seas than others. This is important to know so we can focus our efforts in helping marine ecosystems adapt to these changing conditions," says Dr Behrens.

The analysis draws on the New Zealand Earth System Model and its high-resolution ocean grid. The grid helps us understand smaller-scale ocean processes (as opposed to vast oceanic currents, for example), making more accurate regional assessments like this one possible.

"We’re excited that research like this is now possible, giving locally relevant, and even coastal, insights into climate impacts in our oceans," says Kate Turner, Climate Change Knowledge Broker at the Deep South Challenge.

"These projections also tell us we need to start adapting to our changing climate now. Organisations, iwi and hapū, councils and communities up and down the country are experiencing these impacts already. We need to really focus on how we can support their adaptation planning today," she says.

Tony Craig, a partner with marine consultancy Terra Moana says both industry and recreational fishers are already noticing changes in the kinds of species that are caught and where.

"It’s hard to see current fisheries being resilient enough to withstand increases between 80 and 100% of median marine heave wave intensities by the end of the century," he says.

Marine heatwaves occur when water temperatures stay in the warmest 10% of historical observations for at least five days.

This work will be presented during a free webinar on Thursday 17 March 2022, 12-1pm. Find out more here.

The full paper, published in Frontiers in Climate, can be found here.

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