What is the greenhouse effect, and how does increasing atmospheric CO2 influence global temperature?

• A. The greenhouse effect is a misnomer; greenhouse gases do not influence temperature.
• B. The greenhouse effect is the natural trapping of heat by greenhouse gases; increasing CO2 enhances heat retention, increasing global temperatures.
• C. Greenhouse gases cool the surface by reflecting sunlight away.
• D. Only water vapor, not CO2, traps heat.

Answer: (B)

The main concept being tested is how greenhouse gases trap heat in Earth's atmosphere and what happens when atmospheric CO2 increases. The greenhouse effect is a natural process in which sunlight heats the Earth's surface, and the surface then emits infrared radiation. Greenhouse gases such as carbon dioxide, water vapor, and methane absorb some of that infrared radiation and re-radiate it in all directions, including back toward the surface. This trapping of heat keeps the lower atmosphere and surface warmer than they would be otherwise.

When CO2 levels rise, more infrared energy is absorbed and less escapes to space. This strengthens the atmospheric blanket, so more heat is retained near the surface, leading to higher global temperatures over time. CO2 is a long-lived gas, so its increased concentration continuously shifts the balance of the energy budget toward warming, even though water vapor also acts as a greenhouse gas and can amplify the warming as temperatures rise.

The other statements don’t fit this understanding. One is false because greenhouse gases do influence temperature. Another is incorrect because heat trapping is due to infrared absorption, not primarily reflection of sunlight. The last statement is incomplete because CO2 also traps heat; water vapor is important, but CO2 plays a crucial role in setting long-term climate trends.