Why did the Earth getting hotter?

When we noticed, from time to time the earth's temperature was getting hotter and hotter. Why did this happen? That is what we called global warming.  Global warming is the increase in temperature of the earth because of the influence of greenhouse gases in the atmosphere such as water vapor, carbon dioxide, nitrous oxide, and methane. In the normal amount, these gases are useful to maintain the temperature of the earth keep warm. In excessive amounts, these gases will cause the greenhouse effect.

The greenhouse effect can be explained as follows: The sun's rays penetrate the earth's atmosphere, in which greenhouse gases are also located, toward the surface of the earth. Most of the solar energy is absorbed by the earth and causing the Earth's surface temperature increases. Some is reflected back into the atmosphere in the form of infrared rays. 

Small portion will go through back to space but most is absorbed and emitted in all direction by greenhouse gases, including the earth's surface. The greater the number of greenhouse gases in the atmosphere the more heat energy is trapped and reflected back to the earth's surface. Rising concentrations of greenhouse gases in the atmosphere is generally caused by burning fossil fuels in industry and motor vehicles. This is what the greenhouse effect.  And consequently there is an increase in global temperature of the Earth.