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KUSA - The future of half of the world's 18 penguin species is in doubt, according to a new study in the Journal of Science.

Warming temperatures are causing a decline in their number one food source: Krill, tiny crustaceans found in the southern ocean supported by ice and frigid waters.

West Antarctica - a major penguin habitat - is one of the regions hardest hit by climate change. The average temperature there has risen by 4.4 degrees in the past 40 years - three times the overall rate of global warming.

Worldwide, temperatures are warmer now than any other time in the past 4,000 years.

For more information, take a look at a special series on global warming at NBCnews.com.