Global warming and climate change

Global warming, which is a gradual rising of Earth's temperature, is different from all these, representing a scale of threat greater than anything humans have faced in recent history. Unless we tackle the problem soon, it could transform the planet we live on, making the climate (Earth's weather patterns) much more erratic, forcing many species into extinction, and making life much harder—especially for people in developing countries.

Global warming is working a bit like this. Thanks to a variety of things that people do, Earth is getting slightly warmer year by year. It's not really warming up noticeably—at least not in the short term. In fact, since 1900, the whole planet has warmed up only by around 0.8 degrees Celsius. By the end of the 21st century, however, global warming is likely to cause an increase in Earth's temperature of around 2–5 degrees Celsius. There is a 75 percent chance of a 2–3 degree warming and a 50 percent chance of a 5 degree warming, and scientists agree that the warming is most likely to be around 3 degrees. Now even a 5-degree warming might not sound like much to worry about, but 5 degrees is roughly how much difference there is between the world as it is today and as it was during the last Ice Age. In other words, when we came out of the Ice Age, the planet warmed by 5 degrees over about 5000 years. Modern climate change threatens to produce the same amount of a warming in as little as a century! Once something as big as a planet starts to warm up, it's very hard to slow down the process—and almost impossible to stop it completely. Global warming means Big Trouble.

Earth's atmosphere behaves like a gigantic greenhouse, though it traps heat a different way. Gases high in the atmosphere, such as carbon dioxide and methane, behave like a giant piece of curved glass wrapped right round the planet. The Sun's rays (mostly visible light and short-wavelength, high-energy ultraviolet radiation) pass straight through this greenhouse gas and warm up Earth. The warming planet gives off heat energy (longer wavelength infrared radiation), which radiates out toward space. Some of this outgoing radiation does not pass through the atmosphere, but is reflected back down to Earth, effectively trapping heat and keeping the planet about 33 degrees hotter than it would otherwise be. This is called the natural greenhouse effect and it's a good thing. Without it, Earth would be much too cold to support the huge diversity of life that it does.