The Southern Ocean is acidifying at such a rate because of rising
carbon dioxide emissions that large regions may be inhospitable for key
organisms in the food chain to survive as soon as 2030, new US research
has found.
Tiny pteropods, snail-like creatures that play an
important role in the food web, will lose their ability to form shells
as oceans absorb more of the CO2 from the atmosphere, a process already
observed over short periods in areas close to the Antarctic coast.
Ocean
acidification is often dubbed the "evil twin" of climate change. As CO2
levels rise, more of it is absorbed by seawater, resulting in a lower
pH level and reduced carbonate ion concentration. Marine organisms with
skeletons and shells then struggle to develop and maintain their
structures.
Using
10 Earth system models and applying a high-emissions scenario, the
researchers found the relatively acidic Southern Ocean quickly becomes
unsuited for shell-forming creatures such as pteropods, according to a
paper
published Tuesday in Nature Climate Change.
"What surprised us was really the abruptness at which this
under-saturation [of calcium carbonate-based aragonite] occurs in large
areas of the Southern Ocean," Axel Timmermann, a co-author of the study
and oceanography professor at the University of Hawaii told Fairfax
Media. "It's actually quite scary."
Since the Southern Ocean is
already close to the threshold for shell-formation, relatively small
changes in acidity levels will likely show up there first, Professor
Timmermann said: "The background state is already very close to
corrosiveness."
A healthy pteropod - fewer of these likely if emissions trends continue.
Photo: NOAA
Below a certain pH level, shells of such creatures become more
brittle, with implications for fisheries that feed off them since
pteropods appear unable to evolve fast enough to cope with the rapidly
changing conditions.
"For pteropods it may be very difficult
because they can't run around without a shell," Professor Timmermann
said. "It's not they dissolve immediately but there's a much higher
energy requirement for them to form the shells."
Given the
sheer scale of the marine creatures involved, "take away this biomass,
[and] you have avalanche effects for the rest of the food web", he said.
Pteropod mollusk found in north Pacific waters.
Photo: Alexander Semenov
As carbon dioxide levels rise, the impacts seen in the Southern
Ocean – and its counterpart regions in the northern hemisphere – can be
expected to spread closer to the equator.
Scientists anticipate
that a halt in the increase in greenhouse gases will take time to have
an impact on slowing the warming of the planet. However, a faster
response can be expected in the oceans to any slowing in the pace of
acidification.
"The corrosiveness of the water is a very strong
function of the atmospheric C02 and there is not much of a delay [to any
changes]", Professor Timmermann said.
The paper's release comes
about four weeks before delegates from almost 200 nations are expected
to gather in Paris, France to negotiate a new global treaty to curb
carbon emissions.
SMH
