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Effects & Impacts: Marine heatwaves

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Marine heatwaves

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Summary

Climate change has contributed to observed increases in the frequency, intensity and duration of marine heatwaves over recent decades. Recent marine heatwaves in all oceans have been longer and more intense [causing] widespread impacts on marine species with changes in distribution, loss of biodiversity, collapse of foundation species including coral, kelp and seagrass and the ecosystems they support, and declines in fisheries and cultural values. Ongoing climate change will lead to additional increases in marine heatwave frequency and intensity, further threatening marine life and the ecosystem services they provide to human societies. – Marine Heatwaves International Working Group.

“Heat uptake has accelerated dramatically since the 1990s, nearly doubling during 2010–2020 relative to 1990–2000.” – Li et al, 2023

Because we are surrounded by so much ocean, we [thought] we were protected a bit by warming effects, this [data] is saying that’s not true.“- NIWA, 2024

Phytoplankton – microscopic algae that form the base of marine food-webs – is tending to decrease in New Zealand’s warmer northern waters. – Stats NZ 09 July 2024

  • To date, the oceans have absorbed ~93% of global warming (Fig. 1) and heating up 40% faster than the IPCC estimated.
  • The oceans cover 70% of Earth and the deepest areas are almost 4,000m; a huge area to store heat. Additionally the ocean is dark, so it has a very low albedo meaning that it can absorb much more heat than the land. 
  • Deeper ocean heatwaves are not felt at the surface, so about half are not being recorded:

Annual number of subsurface marine heatwave shows a significant increase in response to subsurface mean-state warming during the past three decades. Our findings reveal the limitation of identifying marine heatwaves solely based on the sea surface temperature and underscore the necessity of subsurface observations for monitoring marine heatwaves. – Sun et al, October 2023

  Effects

Other sections

Home > Climate wiki > Effects > Marine heatwaves

Summary

Climate change has contributed to observed increases in the frequency, intensity and duration of marine heatwaves over recent decades. Recent marine heatwaves in all oceans have been longer and more intense [causing] widespread impacts on marine species with changes in distribution, loss of biodiversity, collapse of foundation species including coral, kelp and seagrass and the ecosystems they support, and declines in fisheries and cultural values. Ongoing climate change will lead to additional increases in marine heatwave frequency and intensity, further threatening marine life and the ecosystem services they provide to human societies. – Marine Heatwaves International Working Group.

Heat uptake has accelerated dramatically since the 1990s, nearly doubling during 2010–2020 relative to 1990–2000. – Li et al, 2023

Because we are surrounded by so much ocean, we [thought] we were protected a bit by warming effects, this
[data] is saying that’s not true. – NIWA, 2024

Phytoplankton – microscopic algae that form the base of marine food-webs – is tending to decrease in New Zealand’s warmer northern waters. – Stats NZ 09 July 2024

  • To date, the oceans have absorbed ~93% of global warming (Fig. 1) and heating up 40% faster than the IPCC estimated.
  • The oceans cover 70% of Earth and the deepest areas are almost 4,000m; a huge area to store heat. Additionally the ocean is dark, so it has a very low albedo meaning that it can absorb much more heat than the land. 
  • Deeper ocean heatwaves are not felt at the surface, so about half are not being recorded:

Annual number of subsurface marine heatwave shows a significant increase in response to subsurface mean-state warming during the past three decades. Our findings reveal the limitation of identifying marine heatwaves solely based on the sea surface temperature and underscore the necessity of subsurface observations for monitoring marine heatwaves.– Sun et al, October 2023

Fig. 1: Click on the image to be taken to the 'Climate Shift Index' portal where you can see absolute and relative temperature changes. Image: screen grab 31 August 2024.
Fig. 1: Click on the image to be taken to the ‘Climate Shift Index’ portal where you can see absolute and relative temperature changes. Image: screen grab 31 August 2024.
Fig. 2: Average daily sea surface temperatures across the globe in 2024 are shown in red. Orange is 2023. By December 2024, temperatures were lower as we head into a weak La Niña. To see daily updates, click on the image.
Fig. 2: Average daily sea surface temperatures across the globe in 2024 are shown in red. Orange is 2023. By December 2024, temperatures were lower as we head into a weak La Niña. To see daily updates, click on the image.
Fig. 3: Based on IPCC AR4 5.2.2.3 for the period 1993-2013. (Image: Skeptical Science).
Fig. 3: Based on IPCC AR4 5.2.2.3 for the period 1993-2013. (Image: Skeptical Science).

“During austral [southern hemisphere] summer 2017/18, the New Zealand region experienced an unprecedented coupled ocean-atmosphere heatwave, covering an area of 4 million km2. Regional average air temperature anomalies over land were +2.2 °C, and sea surface temperature anomalies reached +3.7°C in the eastern Tasman Sea… The event persisted for the entire austral summer resulting in a 3.8 ± 0.6 km3 loss of glacier ice in the Southern Alps (the largest annual loss in records back to 1962)… The best match suggests this extreme summer may be typical of average New Zealand summer climate for 2081–2100, under the RCP4.5 or RCP6.0 scenario.”  – Salinger et al 2019

Rather than a glimpse into what summers might be like after 2081, the warming over the ocean increased the following year (Fig. 4).

Fig.4: Changes in ocean temperatures around New Zealand 2010 - 2019 (Image: NIWA). Warmer oceans means there's more water vapour over the water. And warmer air can carry more moisture. This powers tropical cyclones, so they may reach New Zealand more often, bringing greater risks of destructive winds and flooding.
Fig.4: Changes in ocean temperatures around New Zealand 2010 – 2019 (Image: NIWA). Warmer oceans means there’s more water vapour over the water. And warmer air can carry more moisture. This powers tropical cyclones, so they may reach New Zealand more often, bringing greater risks of destructive winds and flooding.

Then in the summer of 2019/2020 something extraordinary happened:

“In an event that is unprecedented in 40 years of record-keeping, temperatures over Antarctica rose rapidly, causing the polar vortex over the Southern Hemisphere to break down and even reverse direction. This had cascading effects on weather patterns.” – Andrew Freedman, January, 2020

Marine heatwaves occurred again in late 2020, 2021 (Fig. 5) and have continued each year since then.

Fig. 5: Marine heatwaves are becoming annual events. This is changing our climate as well as our marine life that cannot live in higher temperatures (Image: NIWA).
Fig. 5: Marine heatwaves are becoming annual events. This is changing our climate as well as our marine life that cannot live in higher temperatures (Image: NIWA).

Once global average temperatures exceed 1.5°C (this occurred in the last few months of 2023) extreme marine heatwaves that occurred on average once per century, are more likely to occur ever decade. At +3.5°C, the worst case scenario and our current climate trajectory:

“…the number of marine heatwave days is projected to increase by a factor of at least 40. At this level of warming, marine heatwaves have a spatial extent that is over 20 times bigger than preindustrial levels… Under high future emissions, by the late 21st century, much of the global ocean may reach a permanent state of marine heatwave, relative to a fixed pre-industrial threshold.” –  Marine Heatwaves International Working Group PDF of the Science Brief for COP26 (Glasgow, November 2021).

Video 1: New Zeland National Science Challenge presentation on marine heat waves (2022)

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