What would most likely happen if e arths atmosphere did not act like a greenhouse?

If Earth's atmosphere did not act like a greenhouse, the planet would be significantly colder, potentially resembling conditions similar to those on Mars. This lack of greenhouse effect would mean that heat from the sun would escape back into space more rapidly, leading to a dramatic drop in average temperatures. Most ecosystems would struggle to survive, resulting in widespread extinction of species and a collapse of food chains. Overall, life as we know it would be severely disrupted.