Explain the greenhouse effect and distinguish natural versus anthropogenic enhancements.

• A. Greenhouse gases trap shortwave energy; natural effect warms; human activities reduce warming
• B. Greenhouse gases trap longwave radiation; natural effect maintains habitable temperatures; human activities amplify trapping leading to extra warming
• C. Greenhouse gases reflect sunlight back to space; natural effect warms; anthropogenic enhances cooling
• D. Greenhouse gases absorb all radiation; natural and human both cause cooling

Answer: (B)

The key idea is that greenhouse gases trap longwave (infrared) radiation emitted by Earth, not the shortwave energy from the Sun. Sunlight mostly passes through the atmosphere and heats the surface, which then radiates heat back as infrared. Greenhouse gases absorb much of that infrared and re-emit it in all directions, including back toward the surface, warming the lower atmosphere and the surface.

Naturally, this trapping keeps the planet warmer than it would be otherwise, helping to maintain a climate that can support liquid water and life. When humans add more greenhouse gases to the atmosphere through burning fossil fuels, deforestation, and industrial processes, their concentrations rise, so more longwave radiation is trapped. This amplifies the natural warming, producing extra warming beyond natural variability.

So the description that matches is the one that correctly states longwave radiation trapping, that the natural effect maintains habitable temperatures, and that human activities amplify the trapping, leading to additional warming.