What would happen on Earth if the sun's energy was absorbed but was not radiated back out into space? Why are scientists concerned with the amount of greenhouse gases in the atmosphere?

Earth's Energy balance describes how the incoming energy from the sun is used and returned to space. If incoming and outgoing energy are in balance, the earth's temperature remains constant. The major Greenhouse Gas, carbon dioxide, emitted naturally and by the burning of fossil fuels, stays in the atmosphere a long time ... Climate scientists are so concerned about carbon dioxide because the more carbon dioxide in the atmosphere, the hotter the earth will become, changing the Earth's climate.