If you consider only the energy balance on earth, you would expect a mean global temperature of . The actual value is . Why? (like carbon dioxide, methane, and ) are transparent to , so they allow most sunlight to pass through the atmosphere without being , causing the earth to heat up. Then the earth's thermal energy should radiate off in the form of . But these gases are opaque in that part of the spectrum, and so they do not allow the thermal energy to escape. Causing the earth to warm on average