Fundamentally, climate is controlled by the long-term balance of energy of the Earth and its
atmosphere. Incoming radiation from the sun, mainly in the form of visible light, is absorbed at
the Earth's surface and in the atmosphere above. On average, absorbed radiation is balanced by
the amount of energy returned to space in the form of infrared "heat" radiation. Greenhouse
gases such as water vapor and carbon dioxide, as well as clouds and small particles (called
aerosols), trap some heat in the lower part of the Earth's atmosphere. This is called the
greenhouse effect. If there were no natural greenhouse effect, the average surface temperature
would be about 34°C (61°F) colder than it is today.