Well, a greenhouse gas is something like carbon dioxide, which has a tendency to promote the greenhouse effect. The greenhouse effect is a trapping of the sun's radiant energy here on Earth, not allowing it to escape back into space. This in turn promotes another effect called global warming, which is an increase in world-wide temperatures. So If we removed the greenhouse gases that exist, that would allow more of the suns radiant energy to escape, which would transfer to lower temperatures overall, canceling global warming. How much? Maybe as much as two to three degrees across the board, globally. That would be enough to preserve the global icecaps, which are currently losing ice at a record pace. That would be good news for all the polar bears, penguins, seals, and other Arctic based wildlife that need cooler temperatures to survive.