What are greenhouse gases?

Find out what greenhouse gases are and how they cause global warming in this video.

[robinmeade]

Something is off with the math in all of the videos. They claim current emissions are something like 40 billion tons of CO2 a year. Which would require something like 800 Billion trees to absorb it a year. But according to estimate there is something like 3 TRILLIONS of JUST trees, then there is grass, brushes, oceans, probably other things that absorb all that pollution so what does that mean?