In a greenhouse, energy from the sun passes through the glass as rays of light. This energy is absorbed by the plants, soil, and other objects in the greenhouse. Much of this absorbed energy is converted to heat, which warms the greenhouse. The glass helps keep the greenhouse warm by trapping this heat.

The earth’s atmosphere acts somewhat like the glass of a greenhouse. About 31 % of the incoming radiation from the sun is reflected directly back to space by the earth’s atmosphere and surface (particularly by snow and ice), and another 20 % is absorbed by the atmosphere. The rest of the incoming radiation is absorbed by the earth’s oceans and land, where it is converted into heat, warming the surface of the earth and the air above it. Particular gases in the atmosphere act like the glass of a greenhouse, preventing the heat from escaping.

These greenhouse gases absorb heat and radiate some of it back to the earth’s surface, causing surface temperatures to be higher than they would otherwise be. The most important naturally occurring greenhouse gas is water vapour and it is the largest contributor to the natural greenhouse effect. However, other gases, although they occur in much smaller quantities, also play a substantial and growing role in the greenhouse effect. These include carbon dioxide, methane, and nitrous oxide.

Without this natural greenhouse effect, the earth would be much colder than it is now – about 33 °C colder – making the average temperature on the planet a freezing -18 °C rather than the balmy 15 °C it is now. The warmth of our climate is crucial because on earth and in the atmosphere, water can exist in all three of its phases – frozen as snow or ice, liquid as water, and gaseous as water vapour. The cycling of water from one phase to another is critical to sustaining life since it is this cycling of water through the land-ocean-atmosphere system that replenishes the water available to life on earth. The water cycle is also an important part of what drives our weather and the climate system generally.