What is the 'greenhouse effect'?
While it's not a perfect analogy, some say the atmosphere works like a greenhouse. The sun's rays (shortwave energy) enter a greenhouse through its glass ceiling and walls to warm the interior. The glass makes it hard for the heat (longwave energy) to escape, and heat builds up inside the greenhouse until the heat can escape fast enough.
Certain naturally occurring gases in Earth's atmosphere have a similar warming effect on the surface. This warming is referred to as the "greenhouse effect," and the gases that trap heat are called "greenhouse gases." The most important greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, and ozone. Earth's surface must warm to an average of about 59°F (with present-day concentrations) until enough energy can be emitted by greenhouse gases and escape to space to balance the energy being absorbed from the Sun.
Though these important greenhouse gases occur naturally in the atmosphere in varying concentrations, human activities are directly and indirectly increasing their abundance. In addition, other greenhouse gases not normally found in nature are being added to the atmosphere. The net result is to intensify Earth's greenhouse effect, causing Earth's surface to warm.
Doesn't carbon dioxide in the atmosphere come from natural sources?
There are natural sources of carbon dioxide, such as decomposing biomass, venting volcanoes, naturally occurring wildfires, human and animal respiration, etc. Over geological time spans before the industrial revolution, these natural sources of carbon dioxide were in balance with natural "sinks"—such as the ocean, phytoplankton, and plants on land that absorb carbon dioxide. The only new process on Earth that has been identified that can account for the significant tipping of Earth's carbon balance is humans burning ever increasing amounts of fossil fuels together with other large-scale activities like deforestation, biomass burning, and cement production. Since the industrial revolution, human activities have increased the abundance of carbon dioxide in the lower atmosphere by about 40%.
What is global warming, and how is it different from climate change and climate variability?
"Global warming" refers to an increase in Earth's annually averaged air temperature near the surface. Thermometer readings are collected from many thousands of weather stations around the world—over land and ocean—and then used to produce a global average temperature for each year. The resulting series of annual averages of global temperature from 1880 to 2012 show that Earth has warmed by 1.5°F (0.85°C).Most of that warming has occurred since 1976.
"Climate change" is a broadly inclusive term that refers to a long-term (decades to centuries) change in any of a number of environmental conditions for a given place and time—such as temperature, rainfall, humidity, cloudiness, wind and air circulation patterns, etc. These oscillations and other similar phenomena can influence weather and climate patterns around the globe.
"Climate variability" refers to short-term (weeks to decades) changes in some of these same environmental conditions for a given place and time. Climate variability is often the result of natural oscillations in Earth's climate system — such as the El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), the Pacific-North American Teleconnection Pattern, etc. These oscillations and other similar phenomena can influence weather and climate patterns around the globe.