Global warming is the warming of the Earth’s atmosphere due to increased concentrations of greenhouse gases emitted from human and other activities. It occurs as an amplification of a natural phenomenon, the Earth’s greenhouse effect, whereby a delicate balance of greenhouse gases helps keep the temperature of the Earth at a level that supports life. When the sun heats the Earth, some of this heat escapes back to space. The rest of the heat is trapped in the atmosphere by clouds and greenhouse gases in the form of infrared radiation. While this “greenhouse effect” occurs naturally, human activities in the past century have substantially increased the amount of greenhouse gases in the atmosphere, causing the atmosphere to trap more heat and leading to changes in the Earth’s climate. The major greenhouse gases emitted into the atmosphere through human activities are carbon dioxide, methane, nitrous oxide and fluorinated gases.
The addition of greenhouse gases to the atmosphere from human activities has warmed the atmosphere – the average temperature of the Earth’s surface increased by about 1.4 degrees Fahrenheit over the past 100 years, with about 1.0 degrees Fahrenheit of this warming occurring just over the past three decades. This global warming has resulted in observed changes to the weather and climate, sea level rise, melting of ice and glaciers, and other impacts. According to a 2013 overview of the evidence for and causes of climate change prepared by the National Academy of Sciences, “It is now more certain than ever, based on many lines of evidence, that humans are changing Earth’s climate.”