The Greenhouse Planet

Since early in Earth's history, greenhouse gases such as water vapor, carbon dioxide and methane have played an important if not crucial role in the climate of the planet. While about 30% of the energy is reflected back into space, approximately 45% is absorbed directly by the oceans and land and roughly 25% is absorbed by greenhouse gases. Without the heat-capturing properties of these gases, Earth would be an average of about 60 degrees Fahrenheit colder than it is now, or about zero degrees F (-18 Celsius), well below the freezing point of water.

Temperature variability is an important aspect of climate change and can be measured on large regional, continental and global scales. There is often a lag time or thermal response between when heat energy enters the climate system and when the system responds. See Climate Science Diurnal Cycle for an example of how this can occur on a daily time scale.

In recent years, concern has grown that human activities, most particularly the combustion of fossil fuels, are increasing carbon dioxide levels in the atmosphere to levels which haven't been seen in over 400,000 years. See Forecasting the Future and Putting the 20th Century in Perspective for more on the present day greenhouse effect.

Liquid water might still exist under a layer of ice, like on the moon of Jupiter known as Europa, which has an icy shell and a surface temperature of -270 Fahrenheit, but life as we know it on the surface of the planet would not exist. The natural greenhouse effect warms the globe, and permits water to circulate around the planet via the processes of evaporation, transpiration, precipitation and runoff.

The figure below compares the greenhouse planets of Earth and Venus.
Next- Climate & Water Balance

Receiving almost twice the solar radiation as Earth, Venus also has an atmosphere rich in carbon dioxide, creating a very robust greenhouse effect, trapping more heat than on Earth.