21-2 The Earth’s Natural Greenhouse Effect

What Role Does the Natural Greenhouse Effect Play in the Earth’s Temperature and Climate? A Giver of Life

In addition to incoming sunlight, a natural process called the greenhouse effect warms the Earth’s lower troposphere and surface. Some of the energy from the sun warms the Earth’s surface, causing it to radiate infrared energy back toward space. Clouds, water vapour, carbon dioxide, and other gases in the lower troposphere are heated when they absorb some of this outgoing infrared energy. These clouds and gases (called greenhouse gases) then radiate heat as longer-wavelength infrared radiation in all directions. Some of the released energy is radiated into space and some warms the troposphere and the Earth’s surface.

A natural cooling process also takes place at the Earth’s surface. Large quantities of heat are absorbed by the evaporation of liquid surface water, and the water vapour molecules rise, condense to form drop- lets in clouds, and release their stored heat higher in the troposphere.

What Are the Major Greenhouse Gases? Two Important Molecules

The two greenhouse gases with the largest concentrations are water vapour, controlled by the hydrologic cycle, and carbon dioxide (CO2), controlled by the carbon cycle. Carbon dioxide is a greenhouse gas we have added to the troposphere in huge quantities.

The coal, oil, and natural gas that support the world’s economy all contain carbon that plants and sunshine converted to organic compounds hundreds of millions of years ago. Under high pressures and temperatures these buried organic compounds were converted to fossil fuels. Extracting and burning these storehouses of carbon releases carbon dioxide into the atmosphere.