Just when you thought you had most of the threats from climate change covered.
We are looking on now as warming oceans stress the world's coral reefs, prompting them to turn white, including our Great Barrier Reef.
We also know that our oceans have become about 30 per cent more acidic since pre-industrial times as they absorb the billions of tonnes a year of carbon dioxide released from our burning of fossil fuels and forests, making it harder for shellfish and crabs to form shells.
But now, some of the first evidence is emerging of what scientists have been expecting for decades: oxygen levels in some oceans are beginning to fall and widespread evidence of the trend should be evident from 2030 onwards.
Warming seas absorb less oxygen at the surface. Another effect of a changing climate is that oceans turn over less, so that oxygen at the surface has less chance of moving deeper.
Matthew Long, lead author of a study published in the American Geophysical Union's journal, Global Biogeochemical Cycles, said deoxygenation poses a major threat to marine life and is one of the most serious side-effects from a warming atmosphere.
"Oxygen is a necessary ingredient for marine life, for all sorts of marine organisms," Dr Long, a scientist with the US's National Centre for Atmospheric Research and based in Colorado.
"The extent we care about marine ecosystems for their intrinsic value, we should care," Dr Long told Fairfax Media. "We're also reliant on these systems for food - fisheries will be vulnerable."
According to the models, the process is likely to be underway in the southern Indian Ocean and parts of the eastern tropical Pacific and Atlantic.
The study found that eastern Australia, eastern Africa and south-east Asia may be relatively spared, with impacts likely to be delayed until the next century.
The effects of lower oxygen levels will compound other harmful trends for wildlife such as oceans becoming more acidic.
"We're driving pretty massive changes in the environment - and we're not just changing one variable," Dr Long said. "We're changing a suite of variables to which marine organisms are sensitive, and basically putting significant demands on their adaptive capacities."
CSIRO cuts
Dr Long said it was important for governments to invest in long-term, consistent research to help predict and manage the impacts. This role was particularly vital given observation records for much of the world's oceans are limited.
He also questioned the decision by Australia's CSIRO to cut climate monitoring and modelling programs.
The agency has trimmed the number of scientists to go from two key research programs from almost 100 to about 45, a move that has drawn wide criticism from researchers at home and abroad.
"I don't understand it, and it does seem short-sighted in my view," Dr Long said.
In the longer term, the world had to curb carbon impacts and limit global warming.
"If the carbon dioxide-driven warming continues, the trend in ocean deoxygenation is basically an inexorable component associated with that warming," he said.
Work 'in jeopardy'
Richard Matear, a senior research scientist at CSIRO, said a 2001 paper that he and a team of researchers had worked on had detected some changes to oxygen levels south of Tasmania. The new paper, though, suggests, natural fluctuations may be the key there.
"We're just slowly getting a handle on how much natural variability there is in the system," Dr Matear said.
The Southern Ocean to Australia's south has relatively high oxygen levels compared with other oceans, and a strengthening of the circumpolar winds with climate change may foster more mixing of oxygen-rich waters, he said.
Still, changing circulation would alter which seas become more oxygenated - and at what depths. Those shifts could impact more mobile species such as tuna and squid, whose larger ranges require more oxygen to match their higher metabolism.
The current cuts to CSIRO climate science will remove about 13 researchers from Dr Matear's program. His unit has yet to determine which projects will be scaled back or dropped, to cope with reduced funding.
Some of his colleagues have just left on a research voyage to conduct "repeat lines" of an area studied in the Pacific, to detect changes.
"We have no funding going forward to do that cruise again in the next five to 10 years," Dr Matear said, hindering the ability to track changes to oxygen and other elements.
Of the other key areas being monitored, cruises to the region south of Tasmania may continue with funds cobbled together from other sources. The region south of Perth is another area where monitoring may be disrupted, he said.
"In the current situation, there's really no funding to support them," Dr Matear said of the cruises.
"We're an important player in the southern hemisphere and the Southern Ocean, and that work is in jeopardy," he said.
The evidence suggests that the combination of rising CO2 and falling oxygen levels "will be worse than each one acting independently", Dr Matear said.
CSIRO is a principal sponsor of a conference in Hobart next week on "The oceans in a high CO2 world".
