The importance of clouds and aerosols to climate change
Everything, from an individual person to Earth as a whole, emits energy. Scientists refer to this energy as radiation. As Earth absorbs incoming sunlight, it warms up. The planet must emit some of this warmth into space or increase in temperature. Two components make up the Earth's outgoing energy: heat (or thermal radiation) that the Earth's surface and atmosphere emit; and sunlight (or solar radiation) that the land, ocean, clouds and aerosols reflect back to space. The balance between incoming sunlight and outgoing energy determines the planet's temperature and, ultimately, climate. Both natural and human-induced changes affect this balance, called the Earth's radiation budget.

Earth's radiation budget is a balance between incoming and outgoing radiation.

Clouds affect the radiation budget directly by reflecting sunlight into space (cooling the Earth) or absorbing sunlight and heat emitted by the Earth. When clouds absorb sunlight and heat, less energy escapes to space and the planet warms. To understand how clouds impact the energy budget, scientists need to know the composition of cloud particles, the altitude of clouds and the extent to which clouds at different altitudes overlap each other.

Both natural processes and human activities produce aerosols. They either reflect or absorb energy, depending on their size, chemical composition and altitude. The haze layer that is commonly seen in the summertime is one example of an aerosol that primarily reflects sunlight. Soot emitted by diesel engines is an example of an aerosol that absorbs sunlight. The reflection and absorption of energy by aerosols act in a direct way to change the balance between incoming and outgoing energy. These effects are called direct aerosol radiative forcing.

White cloud streaks over the northern Pacific Ocean stem from aerosols emitted into the atmosphere in exhaust from ship engines. Small water or cloud droplets form around these added aerosols, increasing the brightness of clouds over the ship tracks as compared to the surrounding clouds. This example illustrates the indirect effect of aerosols on the Earth's radiation budget. This image was acquired by the Moderate-resolution Imaging Spectroradiometer (MODIS), flying aboard NASA's Terra satellite, on April 29, 2002. (Image courtesy Jacques Descloitres, MODIS Land Rapid Response Team at NASA GSFC).

Smoke plumes, such as those from wildfires in Russia shown above, contain aerosols that directly affect the Earth's radiation budget. This Moderate-resolution Imaging Spectroradiometer (MODIS) image from May 15, 2002 also shows extensive, dark burn scars along with actively burning fires (red dots) on the north side of the Amur River, which separates Russia (north) and China (south). (Image courtesy Jacques Descloitres, MODIS Land Rapid Response Team at NASA GSFC).

Aerosols also can affect the Earth's radiation budget indirectly by modifying the characteristics of clouds. Cloud particles almost always form around aerosols such as natural sea salt particles or human-made sulfate particles. The presence of additional aerosols can change the way clouds radiate energy and the length of time they stay intact. A good example is the way that exhaust particles emitted into the atmosphere by ships can increase the brightness of clouds along their course. These effects are called indirect aerosol radiative forcing.

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CALIPSO

Introduction
The importance of clouds and climate change
A curtain of the atmosphere
The future of CALIPSO

Related Data

Cloud Forcing
Aerosol Index