The discovery is reassuring for a species seen as under threat from global warming and will provide researchers with a benchmark for monitoring the giants of the penguin world in years to come.

Using high-resolution satellite images to study each of 44 colonies around the coastline of Antarctica, experts said on Friday they put the total emperor penguin population at 595,000, or roughly double previous estimates of 270,000 to 350,000.

"It's good news," team leader Peter Fretwell of the British Antarctic Survey said in an interview. "It gives us a bit more confidence not only that there are lots of emperor penguins out there but that we can actually keep track of them as well."

Seven of the colonies studied had never been seen before.

A key advantage of satellites is that they can capture multiple images in one go, whereas visiting dozens of remote colonies in temperatures as low as -50 degrees Celsius (-58 degrees Fahrenheit) would be hugely expensive and time-consuming.

Still, conducting a penguin roll-call from space is not simple. It took a special technique known as pan-sharpening to increase the resolution of the satellite images to differentiate between birds, shadows and penguin poo, or guano.

While some images remained tricky to analyze, Fretwell believes the overall population figure is correct to within a 10-12 percent margin of error.

Scientists are concerned that emperor penguins will be badly affected by climate change, since they form large colonies on the sea-ice, which is fragile and vulnerable to earlier spring warming. Their more northerly colonies are particularly at risk.

The study by Fretwell and colleagues, published in the online journal PLoS ONE (link.reuters.com/byb67s), marks the first time that researchers have counted the entire population of any species by satellite in a single season.

In future, the same technique could also be use to tot up numbers of other wild animals that stand out clearly against their natural habitat, such as flamingos or reindeer.

Counting other types of penguins from space, however, may not be so easy. While emperors are large and contrast sharply against the white snow and ice on which they stand, other species are smaller and tend to breed on dark-colored rock.

(Editing by Alison Williams)

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