The average temperature of the universe is -270°C. Without any source of energy, Earth would also be at this temperature. However, the interior of Earth is hot, because of the initial heat accretion when our planet was formed and because natural radioactive isotopes in the earth continue to release energy. Through geothermic heat flow, this internal energy diffuses slowly toward the surface. If this flow was the only energy source, the temperature of our planet would be -243°C and the air would be a liquid.
We receive a flux of energy from the sun that is 4000 times larger than the geothermal flux. It is this solar radiation that makes our planet inhabitable.
The energy absorbed by Earth’s atmosphere is absolutely critical to life. Without this absorption the average surface temperature on our planet would be very cold at -18°C. Water would not normally exist in liquid form, and life could not have developed in the same way as it has.
Fortunately, our atmosphere contains gases known as greenhouse gases, which, though present in small quantities, are largely transparent to the incident direct solar radiation but capable of absorbing the energy emitted from Earth’s heated surface. This energy is emitted in all directions as longer wavelength infrared radiation. Absorption of Earth’s infrared radiation increases the average temperature from -18°C to +15°C, making our planet a comfortable place to live.
The result is very similar to that employed in greenhouses to grow plants. The greenhouse windows are transparent to visible solar radiation (so solar energy goes in) but absorbed energy reemitted from the soil or other objects located inside the greenhouse remains trapped and increases the internal temperature. Because the atmosphere serves both as a window to admit solar radiation and an absorber to trap part of the reemitted infrared radiation, the subsequent warming effect is known as the “greenhouse effect.”
Labels: Green House Gases