Extract from ABC News
Delaying efforts to keep global warming below 2 degrees Celsius is projected to have dire consequences for the world's marine fisheries.
Key points:
- Thirteen per cent of important fish habitat could be lost if climate targets are exceeded
- Significant mitigation from 2040 in a worst-case emissions scenario would not be enough to reverse ocean impacts
- Warmer oceans could lead to fewer and smaller fish
New modelling out to 2299 suggests overshooting climate targets could lead to a 13 per cent loss of important fish habitat in the top 200 metres of the ocean.
This zone, known as the epipelagic or "sunlight zone", is home to most of the world's marine fisheries.
Scientists warn fish species in this zone could become less resilient and more susceptible to things like disease with a 2C overshoot.
The decline in habitat was projected by scientists from France's National Centre for Meteorological Research, with help from the CSIRO and other international institutions, in a new study for Nature Communications Earth and Environment.
The researchers used a worst-case climate scenario, SSP5-3.4-OS, where global CO2 emissions continue unabated to 2040, before aggressive mitigation efforts are made to reach net zero emissions in the 2080s.
Surface air temperatures could reach between 2.4C to 4.1C above pre-industrial levels under this scenario, and be brought below 2C by the end of the century.
The ocean takes a lot longer to absorb and release heat, which means there will be a significant delay in ocean cooling, even after CO2 emissions are reduced.
And that could mean long-term impacts for fisheries managers trying to balance food demand with changing stocks.
The CSIRO's Andrew Lenton, a co-author on the study, said modelling showed that even 200 years after an overshoot, the rate of recovery for habitable ocean in the epipelagic zone was low.
"We're talking a lot about reaching net zero and we absolutely should be in line with the Paris Agreement," Dr Lenton said.
"The real risk here will be that we overshoot our emissions ... and as a consequence we'll end up locking in these changes that will be irreversible in our lifetimes.
"We can't really put emissions on a credit card and pay it back later. We're going to pay a pretty heavy interest rate on that."
Warming a 'double whammy' for fish
The United Nations announced in July that the planet was already on track for average air surface warming greater than 1.5C.
So far the average ocean surface level temperatures have increased by about 0.8 degrees compared to pre-industrial levels.
The new study suggests there could be a peak in surface temperature of 19.5C in 2072, based on an average across multiple models, about a decade after atmospheric CO2 peaks.
Surface temperature would drop to 18.8C in 2182 before rising above 19C over the next 100 years.
There is a similar trend for epipelagic waters, while further down (200 to 1,000 metres) in the mesopelagic or "twilight zone", the average ocean temperature stays on an upward trend throughout the projection.
As part of the study, researchers examined how this warming might affect 72 marine species including prawns, scallops, mussels, lobsters, crabs, Atlantic cod and salmon.
As water temperatures increase, so to do the metabolic demands of fish and other sea creatures — meaning they expend more energy when resting and need more oxygen to survive.
Dr Lenton said this situation was compounded by the fact that sea water holds less oxygen when it is warmer.
"You're having an increase in [oxygen] demand and decrease in supply, so you're creating a double whammy for the fish," he said.
This would not suddenly mean underwater wastelands but there could be serious impacts, Dr Lenton added.
"We know life wants to adapt. There will be winners and losers.
"Fish might be smaller to allow for less metabolic demand, [so] you could get a decrease in the size of the fishery and some things [species] won't be viable.
"You'll end up with a new ecosystem or adapted ecosystem that responds to the changes."
Climate change already affecting fisheries
More than 3.3 billion people get 20 per cent of their daily protein intake from fish, with demand set to double by 2050.
Marine finfish made up about 39 per cent of the total aquatic animal production for the world in 2020, according to the Food and Agricultural Organisation of the UN.
But FAO analysis indicates the fraction of fish stocks at biologically sustainable levels had decreased from 90 per cent in 1974 to 64.6 per cent as of 2019.
Research evaluating 124 marine species across 235 populations in 38 ecoregions found their maximum sustainable yield had dropped by 4.1 per cent from 1930 to 2010 due to warming waters.
Nineteen fish stocks, mostly near Northern Europe and Japan, were found to be 8 per cent less productive.
The same study also found instances of cascading effects on zooplankton, forage fish like sardines and demersal fish at the Celtic-Biscay Shelf off Ireland and the North Sea.
Wild-caught Atlantic cod in the North Sea saw its sustainability certification with the Marine Stewardship Council (MSC) suspended in 2019 because of stock declines.
Research from last year claimed 64 per cent of population fluctuations for the North Sea cod fishery could be attributed to climate change while the rest was from the fishing itself.
Closer to home in Australia, Matt Watson of MSC Asia Pacific thinks most commercial stocks are already facing some sort of climate change stress.
Skippers have encountered geographical changes to where they find yellowfin tuna on the east coast of Australia, he said.
"Or over in New Zealand, there are some wobbles in their hoki fishery and there's some arguments that that's driven by climate change as well.
"Management in fisheries can help add a level of resilience.
"You can kind of protect your resource if you have a good understanding of the kind of real-time impacts."
Fish prey also facing decline
Climate change is not only affecting the fish we eat, but the critters they eat too.
Zooplankton — small creatures such as jellyfish and krill — make up about 40 per cent of the world's marine biomass and are eaten by small fish.
And the zooplankton mostly eat phytoplankton, which get smaller in warmer oceans.
University of Sunshine Coast mathematician Ryan Heneghan said this change in the size of phytoplankton could lead to less zooplankton overall.
There could be a shift in zooplankton towards species that have lower carbon content, he added.
"For fish which are eating the zooplankton, that then has knock-on effects in terms of the quality of their diet.
"And then big fish eat the small fish and you keep going up."
Dr Heneghan's research showed that under a high emissions scenario of about 4 to 5C of warming, fish biomass around the world could drop by 10 per cent.
He said the impact of shifts in zooplankton species had been seen already during marine heatwaves.
"There was a big heatwave off the coast of California ... called The Blob in 2016," Dr Heneghan said.
"Temperatures were elevated for a few years ... and then what the follow on from that was not just reduced fish biomass, but it was reproductive failure in fish communities and those sorts of things."
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