Greenhouse effect and Global warming

The greenhouse effect is the natural process responsible for warming the Earth's surface and atmosphere -- without it, the planet's average temperature would be around -18 degrees Celsius rather than the actual average of about 15 degrees Celsius. Of the solar radiation reaching Earth, clouds and gases reflect and absorb roughly a quarter of it, while about half reaches and heats the surface directly, with a small portion reflected straight back to space. The heated surface re-emits energy as infrared radiation, but atmospheric gases such as CO2 and CH4 absorb much of this outgoing radiation rather than letting it escape; the molecules of these gases then re-radiate a large part of that absorbed heat back down to Earth, warming the surface further, and this cycle repeats many times over -- much as heat is trapped inside an actual greenhouse or glasshouse, which is why the phenomenon is named after it (see Fig. 15.10 for how the natural cycle compares with the human-enhanced version). The gases mainly responsible are carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs), nitrous oxide (N2O) and water vapour. Burning fossil fuels in industry and vehicles, along with burning agricultural waste, keeps raising CO2 levels; biogas plants, paddy fields and cattle sheds add methane to the atmosphere; and CFCs are released from sources such as fire extinguishers and air conditioners. …