As Earth started to take its shape from a fire ball to a planet, many gases were released. These gases did not escape into outer space because of the Earth's gravitational pull and it still holds them back. The result? A thin layer of gases surrounds the Earth and provides us several important benefits. For example: the oxygen that we breathe, the ozone that protects us from harmful ultraviolet rays from the sun, the nitrogen that our plants use to make proteins that we need. This layer of gases is the medium through which fresh water is circulated and it also keeps us warm (take a look at the image from Class IX Chapter 4).

Keeping us warm is one of the most important things that the atmosphere does for us. It is like a light, but effective, blanket enveloping Earth. You will recall from Class IX that the atmosphere traps a lot of solar energy that reaches the Earth by preventing it from totally escaping back into space. This is called Green House Effect. This is important and vital for life to survive on this planet. If the Earth did not have this atmosphere, it would be very cold indeed.

However, since the 19th century, the planet has been warming up very rapidly. There is an increasing concern about this. Why this concern? After all, Earth has been through many cycles of warming and cooling (in fact, freezing!). So what is so special about it now?

Earlier, cycles of cooling and warming happened over very long periods of time. This allowed most of the life on Earth the time to adapt to the changes. The problem now is that the heating is much more rapid and could lead to catastrophic changes. Much of the warming that has been occurring since the Industrial Revolution is because of human activities. Hence, the current global warming trend is called AGW (Anthropogenic Global Warming; anthropogenic means caused by humans).

More recently, scientists are discovering large volumes of methane under the frozen tundras of the far northern latitudes (mainly in the vast expanses of northern Russia ).As global temperature increases, the ice in the tundra melts more. The methane that is trapped under the ice escapes into the atmosphere, increasing the global temperatures. In turn, this causes even more ice to melt, releasing even more methane and so on. Methane is said to be even more powerful than Carbondioxide as a greenhouse gas.

AGW is causing many changes in the distribution of heat in the Earth system. Remember how atmospheric and oceanic circulations keep re-distributing the heat in the world? AGW disturbs this and the re-distribution system. This disturbance itself is not so much of a problem. It is the rapidity with which it is happening.