Climate change

Causes of climate change

Greenhouse effect

The vast majority of the energy which drives Earth's natural processes comes directly from the sun. Around half of the sun's energy that reaches Earth makes its way through the atmosphere and warms the surface of the planet. Most of this solar radiation is absorbed by the land and oceans while the remainder is reflected back into the atmosphere. The solar radiation that is absorbed by the land and ocean is re-emitted as infra-red radiation (heat).

While a portion of this re-emitted heat passes through the atmosphere into space, some of it is trapped by atmospheric greenhouse gases. Greenhouse gases absorb and emit radiation at certain wavelengths within the spectrum of infrared radiation (heat) emitted by the Earth's surface, the atmosphere and clouds. This property of greenhouse gases causes the greenhouse effect.

The greenhouse effect keeps Earth warmer than it would otherwise be, just like a glass greenhouse keeps plants warm. Without this process, the global average surface temperature would be closer to 18^oC below zero (the average temperature on the moon), instead of the current 15^oC.

Greenhouse gases

The primary greenhouse gases in 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 halocarbons and other substances containing chlorine and bromine that are regulated and controlled by the Montreal Protocol.

Human activity

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.

Since the Industrial Revolution, human activities, such as using fossil fuels (coal, oil and natural gas) for energy, agriculture and land clearing, have increased the concentration of greenhouse gases in the atmosphere.

Greenhouse gases such as carbon dioxide, methane and nitrous oxide are naturally occurring while others like sulphur hexafluoride (SF₆), perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs) are anthropogenic. Since 1750, the amount of carbon dioxide in the atmosphere has risen by 35 per cent, and the current concentration is higher than at any time in the last 650,000 years. The amount of nitrous oxide has risen by 17 per cent; methane has increased by 151 per cent. Since 1900, global average surface temperatures have increased by 0.74^oC.

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 time taken to remove this gas from the atmosphere.

In response to these challenges, there are two clear needs:

To reduce greenhouse gas emissions to slow the rate of climate change, and
To prepare strategies to adapt to inevitable climate change.