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Without the so-called greenhouse gases, including carbon dioxide, methane, nitrous oxide, and water vapor, Earth would be too cold to inhabit. These gases in Earth's atmosphere absorb and emit heat energy, creating the greenhouse effect that keeps our planet's temperature livable.
Water vapor is the most plentiful greenhouse gas on the planet, accounting for about 60% of the current greenhouse effect. Even ozone helps trap some of the heat that makes life on Earth possible, but the "ozone hole" is a separate issue not directly related to global warming.
Since the industrial revolution, people have burned vast amounts of coal, petroleum, and other fossil fuels to create heat and power. This releases carbon dioxide, the most plentiful human-produced greenhouse gas, into the atmosphere. The result: more heat is trapped in Earth's atmosphere instead of radiating out into space.
On average, carbon dioxide lasts more than a century in the atmosphere. As a result, CO2 is well mixed around the globe. Measurements collected atop Hawaii’s Mauna Loa and other locations show a steady rise in global carbon dioxide concentrations since 1958. These concentrations have increased by more than 35% since preindustrial times, according to the World Meteorological Organization. Other, less prevalent greenhouse gases have increased at different rates. Methane, for example, virtually leveled off after 1999 at 155% above its preindustrial level, but began climbing again in 2007.
The relationship between Earth's water cycle and global warming creates a well-known feedback loop. Warmer temperatures cause more water to evaporate from land and oceans into the atmosphere. The added water vapor then contributes to warmer temperatures, completing the feedback loop. This is just one of many feedbacks in the Earth system that climate scientists are studying to improve projections of future climate change.
Explore these topics in depth on Windows to the Universe: The Greenhouse Effect & Greenhouse Gases.