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As the Defense Department keeps its eye on China and Russia, a new rival is also emerging for the military to contend with \u2014 heat.Extreme heat is becoming an increasingly prevalent challenge. A study from the Proceedings of the National Academy of Sciences found that the number of extreme heat days have tripled since the 1980s and are likely to continue to increase.\nLearn more about your ad choices. Visit podcastchoices.com/adchoices