True or False: we're not really sure how much warming has been caused by humans.

• A. True
• B. False
• C. Cannot be determined
• D. Not addressed

Answer: (A)

Attribution of warming to human activities and the uncertainties around that attribution. We can be confident that human activities are the dominant driver of the notable warming since the mid-20th century, because analyses that compare natural-forcing scenarios with those including greenhouse gas emissions reproduce the observed warming pattern far better. Yet the precise amount of warming caused by humans versus natural variability isn’t pinned down to a single exact number; there are uncertainties in how strongly the climate system responds to forcing (climate sensitivity), in the exact historical changes in natural factors, and in how different processes interact and amplify or dampen warming. Because of these uncertainties, it’s reasonable to say we’re not really sure about the exact share of warming attributable to humans, even while we have strong and convergent evidence that human influence is substantial. In practice, scientists express this with ranges and high confidence that human activity is the main driver, rather than a precise, precise figure.