February 7, 2006

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Home > Greenhouse Effect

The Greenhouse Effect

The sun radiates energy to the Earth. About a third of incoming solar radiation is reflected back to space by the stratospheric ozone layer, as well as by clouds (water vapour), deserts, and snow cover. The remainder is absorbed by the planet's surface during the day which is then re-radiated as infra-red energy at night. It is the greenhouse gases in the atmosphere that absorb and retain this re-radiated energy (like a greenhouse) and prevent extreme temperature variations. At normal atmospheric concentrations of GHGs this balance between retention and loss of energy is maintained. GHGs in most cases occur naturally in our atmosphere, retaining energy from the sun to keep the Earth's temperature close to 15°C, allowing for human existence. The primary man-made GHGs are carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O).

The Intergovernmental Panel on Climate Change (IPCC) states: "A change in average net radiation exiting to space is defined as a radiative forcing. A radiative forcing perturbs the balance between incoming and outgoing radiation and over time climate responds to the perturbation to re-establish the radiative balance. For example, an increase in atmospheric concentrations of carbon dioxide leads to a reduction in outgoing infrared radiation and a positive radiative forcing. This is what is defined as the enhanced greenhouse effect. Other gases that are contributing to an enhanced greenhouse effect are CH₄, N₂O, chlorofluorocarbons (CFCs) and other halocarbons. Some minor atmospheric constituents, such as the nitrogen oxides (NOx) and carbon monoxide (CO), although not important greenhouse gases in their own right, can influence the concentration of some greenhouse gases (tropospheric ozone in particular) through atmospheric chemistry".

Since the Industrial Revolution, sources of GHGs have increased due to burning of fossil fuels for energy; industrial processes; deforestation; modern agricultural practices; altered land use; and rising human and domestic animal populations. Sinks such as forests and vegetation help to balance this increase by consuming carbon dioxide (the most powerful contributor to global warming) during photosynthesis.

In the last 100 years, average global temperatures have increased by approximately 0.5°C while Canada's mean temperature increased by about 1°C. Some models predict that the Earth's average temperature might increase 0.3°C per decade over the next 100 years. A warming of this magnitude could significantly alter the Earth's climate causing sever storm patterns, a rise in sea level displacing millions of coastal residents, as well as regional droughts and flooding. A global increase of temperature by as little as two or three degrees could cause floods, loss of fertile land, and significant shifts in climate. For example, the average world temperature during the last Ice Age was only 5°C lower than it is now.

There is a great deal of uncertainty associated with climate predictions and, although temperature changes during this century are consistent with global warming predictions, they remain within the range of natural variability.