Almost 100% of the observed temperature 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 greenhouse gases 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 causes 4-9%, 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.
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 fossil fuels (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: 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)
Fig 3: From which sectors do the major greenhouse gas emissions come from? The lower part of the picture shows the sources individually for the gases carbon dioxide, methane and nitrous oxide, respectively. (Graph from http://en.wikipedia.org/wiki/Image:Greenhouse_Gas_by_Sector.png)
The increase of greenhouse gas concentration (mainly carbon dioxide) led to a substantial warming of the earth and the sea, called global warming. In other words: The increase in the man-made emission of greenhouse gases is the cause for global warming. For the effects of global warming see below.
There are two major effects of global warming:
Increasing global temperatures are causing a broad range of changes. Sea levels are rising due to thermal expansion of the ocean, in addition to melting of land ice. Amounts and patterns of precipitation are changing. The total annual power of hurricanes has already increased markedly since 1975 because their average intensity and average duration have increased (in addition, there has been a high correlation of hurricane power with tropical sea-surface temperature).
Changes in temperature and precipitation patterns increase the frequency, duration, and intensity of other extreme weather events, such as floods, droughts, heat waves, and tornadoes. Other effects of global warming include higher or lower agricultural yields, further glacial retreat, reduced summer stream flows, species extinctions. As a further effect of global warming, diseases like malaria are returning into areas where they have been extinguished earlier.
Although global warming is affecting the number and magnitude of these events, it is difficult to connect specific events to global warming. Although most studies focus on the period up to 2100, warming is expected to continue past then because carbon dioxide (chemical symbol CO2) has an estimated atmospheric lifetime of 50 to 200 years. For a summary of the predictions for the future increase in temperature up to 2100, see here .
The above texts have partially been taken from Wikipedia.org and then adapted and complimented accordingly.