Final answer:
The natural greenhouse effect maintains Earth's temperature by trapping thermal infrared radiation, while the enhanced greenhouse effect, caused by increased greenhouse gas concentrations due to human activities, leads to excessive warming known as global warming.
Step-by-step explanation:
The natural greenhouse effect and the enhanced greenhouse effect both involve the trapping of thermal energy, but they differ in their intensity and impact. The natural greenhouse effect is essential for life on Earth, creating a habitable climate by absorbing thermal infrared radiation emitted from the Earth's surface and re-radiating it, thus maintaining the planet's temperature. In contrast, the enhanced greenhouse effect results from increased levels of greenhouse gases like carbon dioxide (CO2) and methane (CH4) due to human activities. This leads to additional trapping of outgoing infrared radiation, which causes a rise in Earth's average temperature, known as global warming. The two effects do not cancel each other out because greenhouse gases are more effective at absorbing the Earth-emitted infrared radiation, which has a longer wavelength due to the planet's lower temperature, compared to the Sun's radiation that is mostly in the visible spectrum.