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Global warming refers to an average increase in the Earth's temperature, which in turn causes changes in climate. Over the past 100 years, the Earth has warmed by about one degree F. The greatest increase has happened in the last 50 years. Scientists agree that greenhouse gases in the air are increasing. This increase makes the Earth warmer by trapping more energy in the atmosphere. Polar icecaps are melting and glaciers are eroding and becoming much smaller. Fish from temperate oceans are migrating to colder waters near the poles as the temperature rises. Birds are also changing their seasonal journeys. Is global warming caused by nature? Is it part of normal cycles in the Earth’s climate—or are humans responsible?