Greenhouse Effect

Heat transfer also plays an important role in controlling the temperature of the Earth.  The Earth is heated by radiation from the Sun. Sunlight passes through the atmosphere and hits the ground, trees, houses and other things on the surface. Some of the radiant energy bounces off the surface – especially if it is ice or snow which strongly reflect light – and the rest is absorbed, warming the surface. But the absorbed energy doesn’t stay in the soil, rocks, ocean, trees and houses or else the temperature would increase every day.

 

The absorbed energy is re-radiated, but it is not the same kind of energy that was absorbed. The wavelength of radiation depends on how hot the source is. Much of the light from the 5,500°C Sun has short wavelengths – typically 1/10,000^th of a centimeter in length. The ground that re-radiates the energy has a temperature of about 20°C and radiates at longer infrared wavelengths, more like 1/10th of a centimeter. Short wavelength radiation from the Sun passes right through Eath’s atmosphere but long waves don’t so easily. At long wavelengths our air is like a wall that traps some of the radiation within the atmosphere. This is the famous greenhouse effect. The name is from greenhouses that gardeners use. The glass walls and roofs are like certain gases in Earth’s atmosphere – they trap long wavelength energy, resulting in the air inside the greenhouse being warmer than the outside. (Greenhouses also trap heat so it can’t escape by convection.)

 

Not all gases in Earth’s atmosphere trap infrared. Nitrogen and oxygen, the two most common gases don’t. But water vapor, carbon dioxide (CO₂) and methane (CH₄) are greenhouse gases. All of these occur naturally, but burning of coal and oil release buried CO₂ from millions of years ago, causing an increase in atmospheric warming. Strong evidence convinces most scientists that human activites are causing the entire Earth to become warmer.

 

Greenhouse warming from naturally occurring gases is responsible for life being possible on Earth. Without greenhouse warming Earth’s average temperature would be about 30°C colder, and our planet would be locked in a permanent ice age.  Mars has a much thinner atmosphere of greenhouse gases (CO₂) so that it is warmed only an extra 7°C, not enough to keep it above freezing. Conversely, Venus has a very dense atmosphere full of CO₂ which has led to a runaway greenhouse warming of hundreds of degrees – Venus’ temperature is about 425°C, much hotter than the oven that bakes your Thanksgiving turkey. You can understand why climate scientists are concerned about increasing amounts of greenhouse gases being released into Earth’s atmosphere!