While about 30 percent of the solar radiation directed at the Earth scatters at the outer atmosphere, the remainder is either absorbed by clouds and atmospheric gases or is transmitted to the Earth's surface. The energy that reaches the Earth's surface is absorbed and subsequently reflected back into the atmosphere in the form of infrared radiation, as shown in the diagram below. This thermal radiation from the surface has a much longer wavelength than the solar radiation that was initially absorbed.
The greenhouse effect
A summary of the heat transfer in the Earth's atmosphere.
The majority of gases in the atmosphere, such as nitrogen, oxygen, and argon, cannot absorb this infrared radiation. Gases known as greenhouse gases, including water vapor, carbon dioxide, ozone, and methane, absorb and trap this heat as it tries to escape from the atmosphere. Greenhouse gases then re-radiate this energy back to Earth, elevating atmospheric temperatures even when the surface is not being directly irradiated by the sun. The cloud layer can also absorb infrared radiation and contribute further to the greenhouse effect.
Without this trapping effect, it is estimated that the surface of the Earth would be approximately 30 degrees cooler than current temperatures. The greenhouse effect modulates the temperature at the Earth's surface and makes it hospitable to life.
Human activities have increased greenhouse gas concentrations n the atmosphere through the combustion of fossil fuels, release of methane from farms, industrial emissions, and deforestation. This increase in greenhouse gases is producing the phenomenon known as global warming, a rapid increase in atmospheric temperatures that may produce a number of environmental consequences, such as more extreme weather.