1.	Short-wave ultraviolet light and visible light passes through the atmosphere.
	2.	Most of the light is absorbed by the earth and converted into heat in water and land.
	3.	Some of the light is reflected back to space.
	4.	The warm surface of earth radiates the heat back to space as long-wave infrared light.
	5.	Some gases such as water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs) absorb infrared light and re-emit the heat radiation in all directions.

The gases which absorb infrared light in atmosphere allow ultraviolet and visible light from the sun to pass but tend trap the heat radiation (Infrared light) from the Earths surface, just like the glass in a greenhouse.  These gases are often referred to as “greenhouse gases”. The temperature of the earth is determined by the balance between the input of light from the sun and the output of heat radiation from the earth's surface, which in turn is determined by the composition of greenhouse gasses in the atmosphere. The concentrations of greenhouse gases are continuously increasing  in the atmosphere due to human activities 1) by emissions of naturally occurring greenhouse gasses such as (CO₂), methane (CH₄) and nitrous oxide (N₂O) and 2) by emissions of  new substances such as halogenated carbons (CFCs, HCFCs and PFCs). The possible increase of the earth’s temperature due to these emissions is often referred to as the “greenhouse effect” or “global warming."

Is global warming for real? The earth’s climate regulation is complex and for many years, there has been much doubt and discussion among researchers about the earth’s temperature development and about the significance of humans' contributions to the greenhouse effect. The discussion continues, but according to the latest “Summary for Policy Makers” from Intergovernmental Panel on Climate Change (IPCC) the doubt is decreasing: “An increasing body of observations gives a collective picture of a warmer world and other changes in the climate system” and “there is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities." The possible dramatic consequences of global warming (see below) and the facts that human emissions of greenhouse gasses probably play a role in the earth’s climate, lead many people to the conclusion that human-induced global warming must be taken seriously.

Some of the compounds, which contribute to greenhouse effect: Carbon dioxide (CO2) is emitted from all kinds of combustion, degradation and the metabolic processes of organic material such as wood, leaves, coal, diesel, plastic, meat, leather, rice, etc. Carbondioxide is a part of the “carbon cycle" and only CO2 emissions from fossil sources (oil, coal, natural gas) and processes such as forest burning with no tree replacement are increasing the CO2 concentration in the atmosphere and hence, contributing to the greenhouse effect. Methane (CH4) is emitted from coal and oil extraction processes, anaerobic degradation of organic waste in landfills, anaerobic degradation of organic matter in rice fields, and from enteric fermentation in cattle (anaerobic decomposition of food in the animal’s stomach). Nitrous oxide (N2O) is emitted from land and from farmland in particular due to anoxic decomposition of organic matter in the soil and from certain industrial processes. Volatile halogenated organic compounds include a series of volatile organic compounds which contain flourine (F), chlorine (Cl) and bromine (Br) in various positions. The group include substances such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), bromochlorodiflouromethane (CBrClF2) and sulfur hexafluoride (SF6).  These substances are emitted from cooling aggregates (e.g. refrigerators, freezers and air-conditioners), old fashioned fire-fighting equipment and a variety of outdated industrial degreasing processes. Sulfate aerosols contribute negatively to greenhouse effect by reflecting light from the sun. Sulfate aerosols are produced during the combustion of sulfur-containing fuels such as coal and oil. Ozone (O3) in the stratosphere contributes negatively to greenhouse effect and ozone in the troposphere contributes positively to greenhouse effect. The extent of the ozone’s contribution is currently uncertain.

What are the possible consequences of greenhouse effect? Without the greenhouse effect, the earth’s average temperature would have been around 30oC lower than present, and as such the greenhouse effect is desirable. The problem is the increased greenhouse effect induced by human activity and the possible climate changes and consequences to environment and society. According to IPCC, global warming and climate change is already a reality. Some of the effects, which have been observed, are described below with projection of possible effects into the 21st century. Increased global average temperature The global average surface temperature has increased over the 20th century by about 0.6oC and the temperature increase is projected to accelerate in the coming century. Increased average temperatures on earth are expected to raise sea levels primarily due to thermal expansion of the earth’s water-masses and the melting of glaciers and ice caps. This, in turn, is expected to lead to permanent flooding and damage of large land areas including agricultural land, cities, wetlands and forests, damage to marine ecosystems such as coral reefs, and salt intrusion into freshwater in rivers, lakes, wetlands and ground. Higher maximum temperatures and more hot days and heat waves It is likely that higher maximum temperatures and more hot days is already a reality in nearly all land-areas, and it is very likely that this trend will continue during the 21st century and it is likely that increased summer continental drying and associated risk of drought will appear in the 21st century. In certain areas, this may lead to increased incidence of death and serious illness in older age groups and urban poor, increased heat stress in livestock, wildlife and increased risk of damage to crops in certain areas. Increased precipitation It is likely that precipitation has increased in the 20th century and that there has been an increase of heavy precipitation events and it is likely that the increase of precipitation and the trend of more intense precipitation events will continue in the 21st century together with larger year to year variations. In certain areas, it is very likely, that this may lead to increased flooding, landslide, avalanche, and soil erosion risk. Global warming can lead to more storms Global warming can lead to melting of polar ice Global warming can lead to more flooding The effects of climate change vary from region to region in the world and so does the sensitivity to changes and the capacity of societies to adapt. Specific impacts and concerns about various parts of the world can be found below. All statements in the following are cited directly from Ahmad O.K. et al., 2001 (see below) with permission from IPCC. Africa Asia Australia and New Zealand Europe Latin America North America Polar Small Island States References: Statements and predictions in this section are all based on following two references: Albritton, D.L. et al. (2001): "Climate Change 2001: The Scientific Basis - Summary for Policy makers”. A report of Working Group I of the Intergovernmental Panel on Climate Change, IPCC. Ahmad Q.K. et al. (2001):"Climate Change 2001: Impacts, Adaptation and Vulnerability - Summary for Policymakers". A report of Working Group II of the Intergovernmental Panel on Climate Change, IPCC.