Scientists working in Antarctica have discovered an alarming rise in sea temperature that threatens to disrupt populations of penguins, whales, seals and a host of smaller creatures within a few decades.

The new study shows the ocean west of the Antarctic Peninsula has warmed by more than a degree since the 1960s - confounding computer models and experts who believed that a combination of ice, winds and currents would keep the water cool and shield fragile marine creatures from the effects of climate change. This is the first evidence that the key Southern Ocean is getting warmer: a finding with potentially severe implications for wildlife.

Lloyd Peck, a marine biologist with the British Antarctic Survey, said: "The sea temperature is going up in a way that wasn't predicted and this makes me more worried for the marine animals. The evidence we've got and the models we've been looking at said sea temperature was not likely to change much in the Antarctic. A one degree increase puts us into the region where the animals are pushed to one end of their biological, physiological and ecological capabilities."

Animals that live on the seabed around the Antarctic Peninsula, where summertime water temperatures currently peak at about 0.5C, are sensitive to small shifts in temperature. In water just two degrees warmer, molluscs become unable to bury themselves in seabed sediment, limpets cannot turn over and scallops lose the ability to swim. These changes would make them more susceptible to predators, disrupting the food chain and quickly endangering larger animals and birds.

"If the warming goes on at the same rate for 50 years or 100 years then lots of populations of animals I work on, and maybe entire species, would be at risk," Professor Peck said.

The climate of the Antarctic Peninsula, which reaches north from the frozen continent towards South America, is the most rapidly changing in the southern hemisphere. Air temperatures there have risen nearly 3C since 1951 and sea ice cover around it has dropped 20% since 1979. Now, polar experts Michael Meredith and John King, also with the British Antarctic Survey, have shown that sea temperatures are on the rise.

There are few long term analyses of conditions in the Southern Ocean, making temperature trends difficult to monitor. Instead, Dr Meredith and Dr King combined several sets of satellite data, historical records and measurements taken from ships to reconstruct the temperature in the upper layer of the sea over the past few decades. They found the average sea temperature off the peninsula during the summer rose by 1.2C during the period 1955 to 1994.

The amount of salt in the top layer of water has also increased: a crucial discovery as dissolved salt lowers the freezing point of water and helps to make it more difficult for the insulating cover of sea ice to form in winter. Dr Meredith said less ice would form on warmer seas in winter, which in turn would increase the warming effect because sea ice reflects sunlight and protects the water from the warmer air.

"Both the temperature and salinity trends are in a direction that will act to reduce future sea ice production. Since a reduction in ice cover was important in the instigation of these trends, they constitute positive feedbacks," the scientists write in the journal Geophysical Research Letters.

The rising sea temperature will also speed up the melting of glaciers. "The melt rates of glaciers depend critically on the temperature of the ocean they are in contact with. Our findings of strong surface warming and positive ocean feedbacks have significant implications for the future of the ice sheet in this region," they say.

Rising temperatures and greater losses of sea ice could also spell big problems for krill. A study published last year showed krill numbers had fallen by 80% since the 1970s. Experts linked its collapse to shrinking sea ice.

Prof Peck said increased loss of sea ice would also hit species which rely on it to reproduce.

Antarctic creatures are particularly vulnerable to rising temperatures because the roughly circular Antarctic continent stops them moving south to escape warmer conditions. "Our animals appear to be very sensitive to temperature change. They don't appear to have the physiological make-up to cope," Prof Peck said.

Footnotes

Antarctic Peninsula

Home to the British research bases in Antarctica, the peninsula has the mildest climate on the continent and a string of mountains believed to be a continuation of the South American Andes

Computer models

These souped up weather forecasts help scientists predict climate. A shortage of data for the computer modelling, combined with complicated winds, currents and icy conditions mean they perform poorly in Antarctica

Positive feedbacks

Climate change can get locked into cycles where the warming generates effects that speed up warming. Loss of sea ice is a classic example because exposed water absorbs more of the sun's heat

Krill

Shrimp like crustaceans that grow up to 6cm (2.4 inches) long. They are a staple food for everything from fish to whales and their decline could have dramatic consequences for the food chain

Moving south

The easiest way for species that like cold conditions to cope with global warming is to move closer to the poles. Marine creatures around Antarctica cannot do this - the circular solid continent blocks them