Which statement best captures a major source of uncertainty in how the climate system responds to forcing, including cloud feedbacks?

• A. Uncertainty in the exact timing of historical volcanic eruptions.
• B. Uncertainty in the measurement of current atmospheric carbon dioxide.
• C. Uncertainty in how the climate system responds to forcing, including cloud feedbacks.
• D. Uncertainty in the accuracy of long-term climate records.

Answer: (C)

Understanding how the climate responds to external forcing hinges on feedbacks that can amplify or dampen the initial change. Clouds sit at the heart of this because they influence the planet’s energy balance in two opposite ways: they reflect sunlight, which cools the surface, and they trap heat, which warms it. The net effect depends on cloud properties—coverage, altitude, droplet size, and phase—which are not yet reliably predicted as conditions shift. This cloud response is where models disagree most, leading to a wide range of possible warming for the same forcing. In short, the big uncertainty is how the climate system reacts to forcing, especially through cloud feedbacks. Other uncertainties, like volcanic timing or precise CO2 measurements, matter but do not govern the fundamental way the system responds to forcing in the same central way.