In near-term climate projections, which factor is a major source of uncertainty?

• A. Uncertainty in precise future emission trajectories.
• B. Uncertainty in solar forcing variability.
• C. Uncertainty in how the ocean heat uptake changes regional patterns.
• D. Uncertainty in the magnitude of the climate system's response to forcing.

Answer: (D)

The main idea here is that how strongly the climate system reacts to a given amount of forcing sets the amount of warming we expect in the near term. This “response magnitude” is determined by feedbacks in the climate system—things like water vapor, clouds, lapse rate, and albedo—that can amplify or dampen the initial warming from greenhouse gases. Because these feedbacks are complex and not known precisely, different models project different amounts of warming for the same forcing, making this the biggest source of uncertainty in near-term projections.

Solar forcing variability tends to be smaller in impact over decadal timescales and is not the dominant driver of the near-term warming trend. Ocean heat uptake influences where heat goes and how quickly regional temperatures change, so it affects regional patterns, but it does not govern the overall magnitude of global warming as strongly as the climate system’s sensitivity does.