Egret in cypress swamp
Great egret in Everglades National Park
NPS/Rodney Cammauf
Earth's temperature fluctuates naturally over tens of thousands to millions of years. Scientists take these fluctuations into account when they assert that we are currently experiencing unique conditions in Earth's history. Scientists have found only one variable to explain the relatively recent rapidity of global warming: an increase in greenhouse gases in the atmosphere due to human activity. By burning fossil fuels, humans have increased carbon dioxide in