After some research I found out a very surprising fact about greenhouse gases.

Most of us think greenhouse gases are just damaging the Earth but in actually fact without greenhouse gases the Earth will be cold and dead , no life form can ever survive. 

Surprisingly , water vapor is a greenhouse gas as well. So in a way trees are contributing to global warming but taking in CO2 will overall help the Earth.

Greenhouse gases are useful in some ways such as help maintain the Earth's average temperature.

Human activities have made a big rise in green house gases in the 20th century. That will explain the rise in the Earth's average temperature because greenhouse gases emit and absorb the heat from the Sun. 

Interesting Fact : When you fart you are contributing to global warming as methane is a greenhouse gas.