The question about the definition for global warming or in other words "what is global warming" is relatively easy to answer. We hereby lean at the definitions and explanations given in Wikipedia:
Global warming is the observed and projected increases in the average temperature of Earth's atmosphere and oceans. The Earth's averagetemperature rose about 0.6° Celsius (1.1° Fahrenheit) in the 20th century, see temperature graphs below.
Fig. 1: Definition for global warming: Temp. increase in the last 1'000 years (graph from http://www.globalwarmingart.com/images/b/bb/1000_Year_Temperature_Comparison.png)
Fig. 2: Definition for global warming: Temp. increase in the last 150 years(graph from http://www.globalwarmingart.com/wiki/Image:Instrumental_Temperature_Record_png)
Fig 3: Definition for global warming: Temp. increase in the last 25 years (graph from http://www.globalwarmingart.com/images/a/af/Short_Instrumental_Temperature_Record.png)
Cause and effect for global warming
Cause of global warming
Almost 100% of the observedtemperature increase over the last 50 years has been due to the increase in the atmosphere of greenhouse gas concentrations like water vapour, carbon dioxide (CO2), methane and ozone. Greenhouse gases are those gases that contribute to the greenhouse effect (see below). The largest contributing source of greenhouse gas is the burning of fossil fuels leading to the emission of carbon dioxide.
When sunlight reaches Earth's surface some is absorbed and warms the earth and most of the rest is radiated back to the atmosphere at a longer wavelength than the sun light. Some of these longer wavelengths are absorbed by greenhouse gases in the atmosphere before they are lost to space. The absorption of this longwave radiant energy warms the atmosphere. These greenhousegases act like a mirror and reflect back to the Earth some of the heat energy which would otherwise be lost to space. The reflecting back of heat energy by the atmosphere is called 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 CO2, which causes 9-26%; methane, which causes4-9%, and ozone, which causes 3-7%. It is not possible to state that a certain gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. Other greenhouse gases include, but are not limited to, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons.
Global warming causes by greenhouseeffect
Greenhouse gases in the atmosphere (see above) act like a mirror and reflect back to the Earth a part of the heat radiation, which would otherwise be lost to space. The higher the concentration of green house gases like carbon dioxide in the atmosphere, the more heat energy is being reflected back to the Earth. The emission of carbon dioxide into the environment mainly from burning of fossilfuels (oil, gas, petrol, kerosene, etc.) has been increased dramatically over the past 50 years, see graph below.
Fig. 1: Cause for global warming: Carbon dioxide emissions in million tons per year over the last 200 years. (graph taken from http://www.globalwarmingart.com/wiki/Image:Global_Carbon_Emission_by_Type_png)
Fig. 2: Global warming cause: Concentration of carbondioxide has dramatically increased in the last 50 years (Source: NOAA)
Fig 3: Trends for greenhouse gases: Carbon dioxide (CO2) and nitrous oxide (NOx) concentrations in the atmosphere are still increasing. For the other major greenhouse gases, the steady upward trend has been broken. (Graph taken from http://upload.wikimedia.org/wikipedia/en/b/bb/Major_greenhouse_gas_trends.png)