Why is the natural greenhouse effect of earth a cause for concern now?

Greenhouse gases, such as water vapor, carbon dioxide, methane etc., move in and out of the atmosphere. They trap some of the heat radiated out from the Earth that would normally move out into space. This is called the greenhouse effect. It is natural, and is caused by the earth's carbon and water cycles and the heat from the sun. Without this warming we would be a cold dead planet.  hope it helps

Its a concern now because the greenhouse gasses are getting in the atmosphere and causing the heat that gets through the atmosphere to be trapped, causing the earth to be significantly warmer all around. This is causing the ice caps to melt and raise the sea level all around and if they melt completely then the world will most likely be flooded.