The presence of trace atmospheric gases make the earth warmer than would direct sunlight alone. These gases (carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), tropospheric ozone (O3), and water vapor (H2O)) allow visible light and ultraviolet light (short-wave radiation) to pass through the atmosphere and heat the earth's surface. This heat is re-radiated from the earth in form of infrared energy (long-wave radiation). The greenhouse gases absorb part of that energy before it escapes into space. This process of trapping the long-wave radiation is known as the greenhouse effect. Scientists estimate that without the greenhouse effect, the earth's surface would be roughly 54 degrees Fahrenheit colder than it is today too cold to support life as we know it. See GLOBAL CLIMATE CHANGE.