An alarming rise in temperature in the Southern Ocean threatens seals, whales and penguins as well as krill, which play a crucial role in the food chain.
Many Antarctic organisms depend on krill
The ocean west of the Antarctic Peninsula has warmed by more than a degree since the 1960s - contradicting the results of computer models.
Sea animals in the region are highly sensitive to changes in temperature.
UK scientists predict whole populations and species could disappear from the region as a result of further warming.
In the journal Geophysical Research Letters, Michael Meredith and John King of the British Antarctic Survey write: "Marine species in this region have extreme sensitivities to their environment."
They add that "population and species removal [are] predicted in response to very small increases in ocean temperature."
In the summer, water temperatures around the Antarctic Peninsula peak at around 0.5C. At about 2C, Antarctic scallops lose the ability to swim and at around 4-5C, clams lose the ability to burrow into the seabed.
Krill is considered a keystone species, an organism upon which many others in the region depend; but it is already under pressure.
A study published last year showed krill numbers had fallen by 80% since the 1970s and experts linked the collapse to shrinking sea ice (the crustacean feeds on algae under the ice).
Professor Lloyd Peck, a polar expert also at Bas, commented: "It is the first paper to show a temperature change in the Southern Ocean that could have ecological significance and possibly global importance."
Computer models had suggested that a combination of ice, winds and currents would keep the Antarctic water cool and shield many marine creatures from the effects of climate change.
"Air temperatures on the Antarctic Peninsula have gone up by three degrees in the last 50 years or so and none of the computer models show that either," Professor Peck told the BBC News website.
"So I think you have to accept that the ability of the models to characterise polar regions is relatively poor."
The amount of salt in the top layer of water has also increased. This is important as dissolved salt lowers the freezing point of ice. This makes it more difficult for sea ice to form in winter.
Ice is a powerful reflector of sunlight, so reducing its area at the poles could increase the warming effect both on polar regions and globally.