Climate Change and causes

Greenhouse Effect

Much of the energy which drives the earth's natural processes comes directly from the sun. Around half of the sun's energy that reaches the earth breaks through the atmosphere, warming the surface of the planet. Some of this solar radiation is reflected back into the atmosphere by the land and oceans.

A portion of this reflected heat passes through the atmosphere, but some of it is trapped by atmospheric greenhouse gases. Greenhouse gases absorb and emit radiation at specific wavelengths within the spectrum of infrared radiation (heat) emitted by the Earth's surface, the atmosphere and clouds. It is this property that causes the greenhouse effect.

The greenhouse effect keeps temperatures higher than they otherwise would be, just like a glass greenhouse keeps plants warm. Without this process, the global average surface temperature would be closer to 18 degrees Celsius below zero, instead of the current 15 degrees Celsius.

Greenhouse Gases

The primary greenhouse gases in the earth's atmosphere are: water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3). These are all naturally occurring, but humans can make them too.

In addition to these there are a number of entirely human-made (anthropogenic) greenhouse gases in the atmosphere, such as the halocarbons and other chlorine and bromine containing substances, dealt with under the Montreal Protocol.

Human Activity

Since the Industrial Revolution human activity - particularly the burning of fossil fuels (coal, oil and natural gas), agriculture and land clearing - has increased the concentration of greenhouse gases in the atmosphere.

This includes both naturally occurring greenhouse gases such as carbon dioxide, methane and nitrous oxide, as well as anthropogenic greenhouse gases such as sulphur hexafluoride, perfluorocarbons and hydrofluorocarbons. Since 1750, the amount of carbon dioxide in the atmosphere has risen 35%, and the current concentration is higher than any time in the last 650,000 years. The level of nitrous oxide has risen 17% and methane is up by 151%. Since 1900, global average temperatures have increased 0.74 degrees Celsius.

The Intergovernmental Panel on Climate Change (IPCC), established by the World Meteorological Organisation and the United Nations Environment Program, has concluded that it is very likely* that human activities are responsible for most of the observed increase in globally averaged temperatures since the mid-20th century.

Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere.