Final answer:
The greenhouse effect is responsible for the difference between Earth's actual mean surface temperature of around 15°C and the theoretical -15°C it would be without our atmosphere. Greenhouse gases trap infrared radiation and prevent it from escaping into space, which heats the planet's surface and supports life. This effect is further intensified by human activities resulting in global warming.
Step-by-step explanation:
The greenhouse effect is the primary reason Earth's mean surface temperature is approximately positive 15°C rather than the much colder -15°C it would be without our atmosphere. The greenhouse gases, such as water vapor and carbon dioxide, in the Earth's atmosphere trap infrared radiation from the Sun, which raises the surface temperature. This naturally occurring effect keeps the Earth warm enough to support life as we know it.
Without the greenhouse effect, the energy received from the Sun would simply be radiated back into space, leaving the Earth much colder. The comparison with other planets like Venus, which has a very strong greenhouse effect due to its thick carbon dioxide atmosphere, and Mars, which has a very thin atmosphere with minimal greenhouse warming, highlights the significance of the greenhouse effect in determining a planet's surface temperature.
It is also significant to note that human activities have intensified the greenhouse effect on Earth, leading to global warming. This enhanced greenhouse effect is a concern for the future stability of the planet's climate.