The first study of its kind to look at deep ocean marine heatwaves has found they have a bigger impact on the organisms that live there.
It has New Zealand researchers worried about the impact on our fish species.
Until now most research has focussed on sea surface temperatures, but a new international analysis of data from 1993 to 2019 at depths of up to 2000 metres shows the highest heatwave intensity is actually at the subsurface - between 50 and 250 metres deep.
It also found that harmful impacts can be felt for up to two years after the surface cools down again - a finding that Victoria University Marine Biologist Prof James Bell said we should take seriously.
"We really should be concerned that these impacts are not just in the shallow waters and could be impacting species in the deep", he said.
Marine heatwaves are a direct result of climate change, and the impacts of our warming oceans are being felt across the world - such as fish dying, species moving to different habitats, and the bleaching of millions of sea sponges.
But discovering the deeper impacts is challenging.
"These environments are much harder for us to explore and monitor so it's much harder for us to tell if these impacts are occurring," Prof Bell said.
New Zealand is no stranger to marine heatwaves, with surface temperatures soaring by up to 6 degrees in some places over the past few years and scientists said it'll take some time to see the full effect.
"It's very concerning in terms of biological implications, marine organisms," NIWA ocean modeller Erik Behrens said.
"Temperature extremes in subsurface, below surface, can be much higher than what we see on satellites, which is something exciting."
It's exciting because the study is the first of its kind to dive this deep.
"We could see knock-on impacts for commercial species that rely on deeper water environments," Prof Bell said.
Something they want to learn more about as marine heatwaves are expected to become more frequent, intense and longer lasting.
