We live on a planet that has an atmosphere – a protective blanket of air that helps regulate the Earth's temperature.
This blanket of air acts like the glass walls of a greenhouse.
It lets in some of the sun's heat, yet at the same time prevents too many of the sun's damaging rays from coming in.
The process of keeping the Earth warm is called the greenhouse effect.
The atmosphere consists of various gases that are very important in terms of regulating temperatures on the planet.
Some of the gases (such as carbon dioxide and methane) trap the sun's heat. This heat warms the land, air and water.
Some of the other gases, especially in the higher atmosphere (such as ozone), stop many of the sun's damaging rays from coming through.
For a very long time there was very little change in the Earth's atmosphere, and over the long term temperatures stayed fairly stable.
But in the past few decades, global temperatures have been creeping up, along with levels of greenhouse gases such as carbon dioxide.
The increase in greenhouse gases causes the greenhouse effect to be stronger. So more heat from the sun's rays is trapped in the atmosphere.
The increase in greenhouse gases is caused largely by fossil fuels (the stuff that runs cars and heats our homes) and other human activities.
Most scientists now agree there is an increase in levels of greenhouse gases and in global temperatures.
Even a global change of a couple of degrees contributes to changes in weather (drought in some areas, too much rain in others), melting of the icecaps and rising ocean levels.
In southern Ontario, this past winter was unusually warm.
This was partly due to El Nino, an ocean current that starts in the Pacific Ocean near the equator and is unusually warm. This current is so large that it affects the weather in many parts of the world. El Nino happens once in a while, not every year.
So, while the Earth's temperature is slowly increasing (and is a problem), short-term changes in weather are often caused by things like El Nino.
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