There is alarming evidence that increased levels of carbon dioxide in the atmosphere are harming ocean-based animal life.
When CO2 enters the atmosphere it is absorbed by the ocean, causing increased acidity in the water.
A new study has found a link between this process and a decrease in the shell-making ability of microscopic marine organisms.
Dr Will Howard of the Antarctic Climate and Ecosystems Cooperative Research Centre and colleagues report their findings today in the journal Nature Geoscience.
"The ocean is currently taking up somewhere in the neighbourhood of a third of our fossil fuel emissions," the palaeo-climatologist said.
Scientists have predicted this absorption of carbon dioxide would boost the acidity of oceans and interfere with the ability of some marine species to build their shells.
"It was predicted and now we're seeing it," Dr Howard said.
"This is not an issue for the future any more. This is an issue for now."
The researchers studied the shells of a protozoa called foraminifera (Globigerina bulloides), in the Southern Ocean south of Tasmania .
They collected current-day foraminifera as they fell to the bottom of the ocean and compared them to foraminifera dating back in time, taken from cores taken from the sea bed.
"[The method] gives us access to times when the carbon dioxide chemistry in the atmosphere and the ocean has changed measurably and noticeably, and we can see how the organisms responded," Dr Howard said.
He and colleagues linked changes in shell formation with the amount of carbon dioxide around at the time.
"[The foraminifera] have more heavily calcified shells during intervals when carbon dioxide content was lower and thus the pH of the ocean was higher," Dr Howard said.
By contrast, the researchers found modern shells were 30 to 35 per cent lighter than those formed prior to the industrial period.
"When you're looking at the modern counterparts, they're making thinner shells than anything in the cores," he said.
He says the team has evidence that modern shells are thinner than at any time in the past 200,000 years.
Fossil fuel signature
Dr Howard says previous research has confirmed the carbon dioxide affecting the shells has a unique isotopic signature which confirms it is from fossil fuels.
"We know independently that the carbon dioxide that they're picking up is at least partly anthropogenic," he said.
He says calcium carbonate shells of foraminifera, which fall to the bottom of the deep sea when they die, are a major vehicle for removing carbon from atmospheric circulation.
"One of the things we're concerned about is that if their shell-making ability is compromised that we might be reducing the efficiency of the ocean carbon pump," he said.
Dr Howard says that over time, the ocean may be able to counteract acidity by dissolving accumulated shells of dead marine organisms on the ocean floor, thus raising ocean pH and its ability to take up CO2.
But he says this will take a long time and come at the cost of living marine organisms.
"The buffering mechanisms in the ocean are quite slow compared to the rate at which we are putting fossil fuel carbon into the atmosphere and into the ocean.," he said.
The research was funded by the Australian Government's Department of Climate Change.