evidence: Water vapor is Earth’s most abundant greenhouse gas. It’s responsible for about half of Earth’s greenhouse effect — the process that occurs when gases in Earth’s atmosphere trap the Sun’s heat. Greenhouse gases keep our planet livable. Without them, Earth’s surface temperature would be about 59 degrees Fahrenheit (33 degrees Celsius) colder. Water vapor is also a key part of Earth’s water cycle: the path that all water follows as it moves around Earth’s atmosphere, land, and ocean as liquid water, solid ice, and gaseous water vapor. [...] The greenhouse gases in the dry air in Earth’s atmosphere include carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons. While making up around 0.05% of Earth’s total atmosphere, they play major roles in trapping Earth’s radiant heat from the Sun and keeping it from escaping into space. Each is driven directly by human activities. [...] Carbon dioxide is responsible for a third of the total warming of Earth’s climate due to human-produced greenhouse gases. Small increases in its concentration have major effects. A key reason is the length of time carbon dioxide remains in the atmosphere.