Global warming is a term used to describe an increase over time of the average temperature of the Earth's atmosphere and oceans. The scientific opinion on climate change, as expressed by the UN Intergovernmental Panel on Climate Change (IPCC) and endorsed by the national science academies of the G8 nations, is that the average global temperature have risen 0.6 ± 0.2°C since the late 19th century, and that "most of the warming observed over the last 50 years is attributable to human activities", most prominently the emission of greenhouse gases such as carbon dioxide (CO2).

Only a small minority of qualified scientists still contest the view that humanity's actions have played a role in increasing recent temperatures. However, uncertainties do exist regarding how much climate change should be expected in the future, and a hotly contested political and public debate exists over what actions should be taken in light of global warming.

Based on climate models referenced by the IPCC, temperatures may increase by 1.4°C to 5.8°C between 1990 and 2100. This is expected to result in other climate changes including: rises in sea level and changes in the amount and pattern of precipitation. Such changes may trigger detrimental events such as floods, droughts, heat waves, reduction of agricultural yields or even cause biological extinctions.

GREENHOUSE GASES
Greenhouse gases (GHG) are gaseous components of the atmosphere that contribute to the greenhouse effect. The major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes between 9-26%; and ozone, which causes between 3-7%, methane, nitrous oxide, sulfur hexafluoride, and chlorofluorocarbons. The greenhouse gases, once in the atmosphere, do not remain there eternally. They can be withdrawn from the atmosphere:

- as a consequence of a physical phenomenon (condensation and precipitation remove water vapor from the atmosphere).

- as a consequence of a chemical phenomenon intervening within the atmosphere. This is the case for methane, which is partly eliminated by reaction with the hydroxyl radical, OH·, which is naturally present in the atmosphere, to produce CO2 and water vapor (this effect due to the production of CO2 is not included in the methane GWP).

- as a consequence of a chemical phenomenon intervening at the border between the atmosphere and the other compartments of the planet. This is the case for CO2, which is reduced by photosynthesis of plants, and which is also dissolved in the ocean to form bicarbonate and carbonate ions (CO2 is chemically stable in the atmosphere).

- as a consequence of a radiative phenomenon. For example the electromagnetic radiation emitted by the sun and cosmic rays break molecular bonds of species in the upper atmosphere. Some halocarbons are dissociated in this way which releases Cl· and F· as free radicals with disastrous effects on ozone (halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).

Aside from water vapor near the surface, which has a residence time of few days, the greenhouse gases take a very long time to leave the atmosphere. It is not easy to know with precision how long is necessary, because the atmosphere is a very complex system. However, there are estimates of the duration of stay, i.e. the time which is necessary so that the gas disappears from the atmosphere, for the principal ones.

Duration of stay and warming capability of the different greenhouse gases can be compared:

CO2 duration stay is variable (approximately 200-450 years) and its global warming potential (GWP) is defined as 1.
Methane duration stay is 12 +/- 3 years and a GWP of 22 (meaning that it has 22 times the warming ability of carbon dioxide)
Nitrous oxide has a duration stay of 120 years and a GWP of 310
CFC-12 has a duration stay of 102 years and a GWP between 6200 and 7100
HCFC-22 has a duration stay of 12.1 years and a GWP between 1300 and 1400
Tetrafluoromethane has a duration stay of 50,000 years and a GWP of 6500
Sulfur hexafluoride has a duration stay of 3,200 years and a GWP of 23900.

CAUSES OF GLOBAL WARMIMG
Coal-burning power plants, automobile exhausts, factory smokestacks, and other waste vents of the human environment contribute about 22 billion tons of carbon dioxide and other greenhouse gases into the earth's atmosphere each year. About half of human emissions has remained in the atmosphere. The atmospheric concentration of CO2 has increased by 31% above pre-industrial levels since 1750. This is considerably higher than at any time during the last 420,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 40 million years ago. About three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning.

Adaption stategies accept some warming as a given and focus on preventing or reducing undesirable consequences: for example defending against rising sea levels or ensuring food security.

GLOBAL WARMING AND GLOBAL DIMMING
Some scientists now consider that the effects of the recently recognized phenomenon of global dimming (the reduction in sunlight reaching the surface of the planet, possibly due to aerosols) may have masked some of the effect of global warming. If this is so, the indirect aerosol effect is stronger than previously believed, which would imply that the climate sensitivity to CO2 is also stronger. Concerns about the effect of aerosol on the global climate were first researched as part of concerns over global cooling in the 1970s.

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