Final answer:
The correct answer is 'no protection from ultraviolet radiation'. The ozone layer is vital for protecting Earth from harmful UV rays, and its depletion would increase the risk of skin cancer, cataracts, and harm to various ecosystems.
Step-by-step explanation:
If we did not have an ozone layer, then there would be no protection from ultraviolet radiation. The ozone layer, composed of ozone (O3), acts as Earth's natural sunscreen, protecting life on the surface from harmful UV rays by absorbing the majority of this radiation. As a result of this absorption, the ozone layer also contributes to the thermal structure of the stratosphere. Without it, humans and other organisms would face increased risks of skin cancer and eye cataracts, and ecosystems, including oceanic food webs, would be adversely affected due to the disruption in the nitrogen cycle and increased mortality of plankton species.
In contrast to greenhouse gases like CO2 and methane, which trap heat within Earth's atmosphere and are crucial for maintaining Earth's temperate climate, ozone primarily serves as a UV shield. Thus, the loss of ozone would not lead to 'excessive heat on Earth', which is often associated with the increasing greenhouse gas concentrations and their enhancement of the greenhouse effect.