• Life as we know it needs liquid water, and Earth is the only planet to have it: without the Sun, Earth would be an icy rock in space. Even now, Earth is probably the only place in our solar system fit for life: any water on Venus and Mercury would become steam, almost any on Mars or on more distant planets would freeze.

• The source of life-associated energy (and of the food we eat) is sunlight falling on green plants. Almost all of our fuel comes from such plants, or was accumulated by them (in the forms of coal, oil and natural gas) long ago. And today's solar cells convert sunlight energy directly to electricity

Heating the Earth

    Because the ground is nowhere as hot as the Sun, its emission is also much weaker. However, at any location the ground sends out radiation in all directions in the half-sky that is visible, while receiving radiation only from the small solar disk, covering only a small circle in the sky, 0.5 degrees across. Because of this, the total energy any area receives should be equal to the total energy it returns back to space.

    Think it over! If all of Earth's heat comes from the outside (neglecting internal heat), and if it maintains a steady temperature, such equality must exist! Of course, only the average temperature is steady. Actually the ground is heated only in the daytime, but radiates back day and night; so nights, when energy only goes out to space and hardly any arrives from there, are cooler than days.

The "Greenhouse Effect"

• Clouds in the atmosphere reflect some of the sunlight before it reaches the ground, reducing the heating of the ground. This process, hard to estimate, has been monitored by measuring "earthshine", the faint glow from the dark part of the Moon when only a thin crescent is visible.

• The atmosphere absorbs the infra-red (IR) light radiated from the ground and thus delayes the escape of heat to outer space, keeping the ground warmer than it would otherwise be.

• Air can flow, and thus carry its heat from one place to another. That is what produces our weather.

    The 3rd process is discussed further below. It is a big subject--made even bigger by the influence of water vapor, which produces rain, hurricanes and other interesting phenomena--and two additional sections are also devoted to it, starting here, and also one (written later) stressing the energy exchange processes.

    The second process (which keeps us warmer) is stronger than the first (which reduces warming), so the net effect is that like a blanket, the atmosphere helps keep Earth warmer than it would be otherwise. This is called the "greenhouse effect," because the same process operates in greenhouses used for growing vegetables in cold climates. A greenhouse is enclosed and roofed by glass panes, which let sunlight enter, but absorb the IR emitted back by the ground, and thus keep the greenhouse warm.

    The chief absorbers of IR in the atmosphere are not nitrogen and oxygen, the main constituents of air, but a relatively minor percentage of "greenhouse gases" such as water vapor (H₂O), carbon dioxide (CO₂) and methane (CH₄), which are strong absorbers of IR.

    Another molecule, responsible for an important effect even though only a very small amount of it is present, is ozone, a variant of the oxygen molecule--O₃ rather than the usual O₂. It is produced at high altitudes by the action of sunlight on ordinary oxygen and its peak concentration is around 25 kilometers. It is also a greenhouse gas, but more important, it absorbs the Sun's ultra-violet (UV) light, which on can cause skin burns and hurt eyes. The ozone found near the ground and forming part of the urban air pollution comes from a completely different process.

    High altitude ozone is destroyed by the presence of chlorine, and recently attention has been drawn to ozone removal by chlorine from the break-up of escaping refrigerant gases, of the types preferred until recently for use in air conditioners, refrigerators, aerosol cans and also some industrial applications. These gases are very, very stable, and can persist in the atmosphere for many years. Unfortunately, sooner or later their molecules wander into the stratosphere, where the ultra-violet sunlight is capable of breaking them up and releasing chlorine. Because of the damage from these gases to the ozone layer, their use is being phased out.

    The greenhouse effect helps keep Earth at temperatures comfortable for life, but that is a finely balanced situation. In the last half century, the burning of fossil fuels--coal and oil-- has steadily increased the atmospheric content of CO₂. The average temperature of the Earth has also risen, and this rise is believed to be due to the added CO₂.

Further Exploring

Site on the Earth's radiation balance, with several subfiles.
Site on the global solar heat budget.

Additional Exploring

   Strictly for the history-minded: the original 1896 article in which Svante Arrhenius proposed the greenhouse effect. Note that carbon dioxide was then called "carbonic acid."

          Before Arrhenius submitted that article, he spent a year calculating the expected effects of an increase in carbon dioxide. That story is told in "Land of the Midnight Suns" be Fred Pearce, p. 50-51 in the New Scientist, 25 January 2003.  Two recent books also discuss the history of the greenhouse effect:

• Greenhouse: The 200-Year Story of Global Warming,
      by Gale E. Christianson, Constable/Walker 1999
  
• Historical Perspectives on Climate Change,
      by James Rodger Fleming, Oxford Univ. Press, 1998.
  Both books were reviewed by Robert J. Charlson in Nature, vol 401, p. 741, 21 October 1999.