What is Climate Change?

Factors affecting global climate

The Earth intercepts the Sun's radiation, which is the energy that drives our weather and climate. About one third of this solar radiation is reflected back into space. The rest is absorbed by different parts of the climate system: the atmosphere, oceans, ice, land and various forms of life.

The Earth then sends its energy out into space, or re-radiates, in the form of long wave radiation. Some of this energy is again absorbed and re-emitted through a process known as the Greenhouse Effect. The rest of the energy is lost to space. There is a delicate long term balance between the outgoing radiation and incoming solar energy. Any change in the factors that affect this process of incoming and outgoing energy, or change the energy distribution itself, will change our climate.

1. Natural Factors

Over the history of the Earth, the climate has changed. The ice ages and intervening warm periods are examples. Some changes are global in scale, while others have been regional or hemispheric. There are a number of natural factors that contribute to changes in the Earth's climate over various time scales. It is important to understand these factors when attempting to detect a human influence on climate:

• Changes in Solar Output. The amount of energy radiating from the sun is not constant. There is evidence of an 11-year solar cycle (variation in energy output) in the temperature record of the Earth. Longer period changes may occur as well.
• Changes in the Earth's Orbit. Slow variations in the Earth's orbit around the Sun change where and when solar energy is received on Earth. This affects the amount of energy that is reflected and absorbed. These orbital variations are believed to be a factor in initiating the ice ages.
• The Greenhouse Effect. When energy from the sun enters the Earth's atmosphere, about a third of it is reflected back to space. Of the rest, the atmosphere absorbs some, but most of it is absorbed by the surface of the Earth. The Earth emits energy at longer wavelengths. Some of this energy escapes to space but some of it is absorbed again and re-emitted by clouds and greenhouse gases such as water vapour, carbon dioxide, methane and nitrous oxide. This helps to warm the surface and the troposphere (lowest layer of the atmosphere), keeping it 33 °C warmer than it would otherwise be. This is the natural Greenhouse Effect, and vital to life as we know it.

Solar energy enters the atmosphere: some is reflected. This energy is absorbed by the Earth and reradiated back to space at longer wavelengths. Greenhouse gases absorb this energy and reradiate much of it back to the surface, much like an insulating blanket.

• Aerosols. These are very fine particles and droplets that are small enough to remain suspended in the atmosphere for considerable periods of time. They both reflect and absorb incoming solar radiation. Changing the type and quantity of aerosols in the atmosphere affects the amount of solar energy reflected or absorbed.

2. Human Factors

Enhancing the Greenhouse Effect. Naturally occurring greenhouse gases (e.g. water vapour, carbon dioxide, methane, nitrous oxide) keep the Earth warm enough to support life. Scientific studies show that a variety of human activities release greenhouse gases. These include the burning of fossil fuels for producing electrical energy, heating and transportation. By increasing their concentrations and by adding new greenhouse gases like CFCs, humankind is capable of raising the average global temperature.

Land Use Change. As humans replace forests with agricultural lands, or natural vegetation with asphalt or concrete, they substantially alter the way the Earth's surface reflects sunlight and releases heat. All these changes also affect regional evaporation, runoff and rainfall patterns.

Atmospheric aerosols. Humans are adding large quantities of fine particles (aerosols) to the atmosphere, both from agriculture and industrial activities. Although most of these aerosols are soon removed by gravity and rainfall, they still affect the radiation balance in the atmosphere. Whether this effect adds to or offsets any warming trend depends on the quantity and nature of the particles as well as the nature of the land or ocean surface below. The regional effects, however, can be significant.