the Greenhouse Effect

A greenhouse is a house with transparent plastic or glass roof and walls that is built to grow vegetables, flowers or other plants in temperate and colder countries. A greenhouse protects and provides heat to plants: its roof and walls allow sunlight to enter and prevent heat from escaping. This effect is known as the "greenhouse effect".

The earth's atmosphere contains some gases known as greenhouse gases, which occur naturally: water vapor, carbon dioxide, methane (CH4), nitrous oxide (N2O), and ozone (O3). The layers of these gases naturally present in the atmosphere acts as the roof of a greenhouse and trap heat close to the earth's surface. As a result, it maintains the mean temperature of the earth's surface at around 16 ^oC, which is crucial to ensuring climatic conditions that can support life of animals and plants on earth.

Since the industrial revolution, human activities have caused substantial increases in concentrations of greenhouse gases in the atmosphere. The main GHGs and their emission sources are:

• Carbon dioxide from burning fossil fuels (coal, oil, natural gas), and from deforestation;
• Methane from rice paddies, livestock, waste dumps, domestic sewage, coal mining;
• Nitrous dioxide mainly from chemical fertilizers used in intensive farming, and from fossil fuel combustion;
• Ozone in the lower atmosphere indirectly from automobile exhaust fumes;
• Chlorofluorocarbons (CFCs) from aerosol sprays, air conditioner and fridge coolants.

The increased concentrations of greenhouse gases in the atmosphere make the blanket or the "roof" around the earth thicker, which in turn prevents more and more heat from escaping into space. This disturbs the balance of heat exchange and causes the air temperature to rise and that of the earth's surface to rise.

In 1990, the Inter-governmental Panel on Climate Change (IPCC), which consist of about 2,500 international scientists, confirmed that human activities have contributed to climate change. It also concluded that if current greenhouse gas emission trends continue, the mean global temperature will increase by 3^oC before the end of the 21st century.

Climate change is having serious impacts on agricultural production, water resources, human health, coastal areas, forest and ecosystems. Increasing floods, droughts, windstorms and other climate change related disasters, both in frequency and intensity, have caused enormous damages to many countries throughout the world.

the Ministry of Environment, (2010),  Climate Change and Clean Development Mechanism, Phnom Penh