The greenhouse effect is the rise in temperature that the Earth experiences because certain gases in the atmosphere (water vapor, carbon dioxide, nitrous oxide, and methane, for example) behave like the glass panes in a greenhouse and trap energy from the sun.

This happens because greenhouse gases are transparent in visible light but opaque in infrared light (heat radiation):

Solar radiation passes through the atmosphere; the most is absorbed by the Earth’s surface and warms it, while some is reflected by the Earth’s surface, the atmosphere and the clouds  into space.

The warmed Earth’s surface emits infrared radiation. Some of the infrared radiation passes through the atmosphere, some is reemitted in all directions by greenhouse gases and  remains trapped in the atmosphere.

The effect is to warm the Earth’s surface and the lower atmosphere.

• There is a natural greenhouse effect keeping the Earth’s surface about 30°C warmer than the balance between the energy received from the Sun and the heat that escapes into space would suggest. Without the natural greenhouse effect, the Earth would not be warm enough for humans to live.
• But the global mean temperature has increased about  0,74°C over the past 100 years  (Intergovernamental Panel on Climate Change data). Many of the world's leading climate scientists think that the human activities are helping to make the Earth warmer.

Air bubbles trapped in the ice, at depths representing the time when the layer of ice was formed, provides clues about the temperatures and the concentrations of atmospheric gases. 
There is a quite perfect match between the three graphs. The higher the concentration of CO2 (and CH4) is, the higher the temperature is.

The climate sceptics

The climate sceptics argue that the climate has a naturally cyclic behaviour. How do we know that the present warming is not just a part of a natural cycle (glacial and interglacial periods)?
They claim  that the global warming doesn’t depend on the human activities, but it is a consequence of factors such as:

• changes in the solar activity
• changes in the Earth’s orbit around the Sun
• changes in land surface, that vary the way the planet reflects light
• volcanic activities, that inject particles into the upper atmosphere  that block sunlight

 Consequences

Global mean sea level is increasing, both due the fact that warmer water has greater volume and because glaciers are melted

• most glaciers worldwide have retreated since the end of the 19th century
• the sea-ice cover in the Arctic has diminished since satellite measurement began
• there are more frequent downpours and changes in rainfall statistic
• because of the changing habitats, living organisms are forced to move to other regions. As consequence, insects spreading tropical diseases such as malaria could spread into temperate regions. Besides,  alien animals  could cause the extinction of the local ones, as it is happening for some Mediterranean species.

References: