how did global warning happen?

Answer 1

One day Al Gore woke up and said: "I have a scam".

Answer 2

Man-made activities and partially nature.

Everywhere you see there's cars, factories, and more cars. Or wasteful energy hungry consuming appliances (fossil fuel is burned also to generate electricity). All of their tailpipe and smokestack emissions are carbon dioxide, which is a greenhouse gas. Meaning it rises to the atmosphere and traps the sun's heat(Venus is even more severe), therefore gradually warming the planet by several degrees. With the rates of deforestation and rapid development, the carbon emissions can't be absorbed that fast. Consequently, the climate will begin to change by the offsetting. We've all heard of the melting ice on the poles (caused by the warming oceans-the oceans also absorb the sun's energy and carbon emissions, causing it to warm and become acidic) that increases the sea level by a few feet that may pose a threat by flooding parts of the world. The melting of the ice also speeds up global warming, since the ice acts as a reflector for the sun's energy.

Of course, not all emissions that contribute to global warming is manmade. Even nature produces greenhouse gases. For example, active volcanoes and volcanic activity is a large contributor-belching out probably thousands of cars worth of emissions. Animal manure such as those from cows, and even trees produce greenhouse gases (methane, I believe).

While warming and cooling periods have occurred in time over thousands of years, we may be speeding up the process to the point of being unnatural...

Rapidly developing countries might also speed it up even more. Whenever something is burned (especially if it's fossil fuel), it will create carbon emissions. I've heard of even landfills generating greenhouse gases.

Answer 3

To answer this a different way, take a look at the satellite composite image of earth at night. Do you see the impact humans have on the planet? With the exception of the Sahara, the Gobi desert and what remains of the Amazon rain forest lights everywhere. Once you understand that we are everywhere, you can see how our activities of deforestation and burning of oil and coal would have a serious impact on the planet.