The sun emits a huge amount of solar energy in the form of short-wave visible radiation (light) and ultraviolet radiation. When this energy reaches the Earth's atmosphere, some of it is reflected back into space and some passes through the atmosphere to reach the Earth's surface.

As land and ocean surfaces absorb energy in the form of sunlight, the energy is converted to heat. In physics terms, the energy is changed from short wavelength (light or visible radiation) to longer wavelength (heat or infra-red radiation). These surfaces then emit heat back into the atmosphere.

Radiation and the electromagnetic energy spectrum

Radiation is energy that has characteristics of both waves (light) and particles (photons). There are many kinds of radiation, including radio waves, microwaves, infra-red radiation, visible light, ultraviolet, x-rays, and gamma rays. The difference between them is their wavelength, which is directly related to the amount of energy carried in the waves. All forms of electromagnetic radiation travel at the speed of light and carry a certain amount of energy ­ the shorter the wavelength, the higher the energy. The electromagnetic energy spectrum is what scientists collectively call all the different types of radiation.

If all the long-wave heat energy simply radiated back into space there would be very little warming above the surface, and Earth would be much cooler. However, certain gases in the atmosphere act like the glass panes of a greenhouse. Called greenhouse gases, these gases have physical properties that allow most of the incoming short-wave energy in sunlight to pass through. At the same time, they block or trap much of the outgoing long-wave heat energy (infra-red radiation) and re-radiate it back to the Earth's surface. Without this natural greenhouse effect, the Earth's average surface temperature would be about 33 degrees colder than it presently is.

Climate change is now a concern because human activities are causing the concentration of greenhouse gases in the atmosphere to rise very quickly. This has an effect similar to adding more layers of glass to the greenhouse by trapping more heat energy in the atmosphere.

Click here to see an interactive illustration of the greenhouse effect.

Because it stays so long in the atmosphere, carbon dioxide contributes the most to the greenhouse effect. Therefore, any discussion about climate change and the enhanced greenhouse effect requires an understanding of how carbon moves in the environment. Understanding the carbon cycle is important in helping us make decisions about managing greenhouse gases and protecting and stabilizing climate.

The current relationship between greenhouse gas levels in the atmosphere, average global temperatures and climate, the amount of carbon stored in various forms, and the amount of biological activity by green plants has taken hundreds of millions years to establish. This complex association is part of creating a stable energy balance of the Earth and has led to a flourishing and diverse range of ecosystems and life forms on our planet. When the levels of these greenhouse gases increase beyond natural levels, more energy is trapped in the Earth's atmosphere, leading to higher average temperatures and more atmospheric energy; in other words, global warming and climate change.

The increase in global average temperatures due to rising concentrations of greenhouse gases in the atmosphere from human activities is what scientists and policy experts call the enhanced greenhouse effect.

The main human activities leading to rising concentrations of greenhouse gases include the burning of fossil fuels and the loss of or damage to green plants. which take CO₂ out of the atmosphere through photosynthesis.