Green house gases include carbon dioxide, water, methane, nitrous oxides, ozone and CFC. When heat from the Sun strikes the Earth a large part of it is reflected back to space. In the absence of an atmosphere and without the presence of greenhouse gases in the atmosphere, the Earth would be cold with a temperature lower than one at which life can survive.
The greenhouse gases help in maintaining the temperature of the Earth to a level that can sustain life by not allowing all the heat that we receive from the Sun to get reflected back to space. The molecules of these gases perform this task by vibrating at a frequency that is the same as that of infra-red radiation.
Though life on Earth has been made possible due to greenhouse gases, it is the increase in these gases due to human activity that has resulted in many problems, like that of global warming. As we burn carbon, the levels of carbon dioxide in the atmosphere has risen to very high levels. The subsequent increase in temperature has led to a meltdown of polar ice, raising sea levels. And is also responsible for changes in weather which include the increased likelihood of storms and hurricanes and droughts and floods throughout the world.
Global warming has now become a problem that has to be dealt with immediately, if life is to be sustained on the Earth.
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.)