Scientist: A greenhouse gas, for example, carbon dioxide, forms a transparent layer that traps solar heat beneath it in the earth's atmosphere. Atmospheric levels of carbon dioxide are currently increasing, causing the climate to warm-an effect that is predicted by at least one computer model of the greenhouse effect. But the warming that has occurred is a great deal less than what would be expected based on the model. Therefore, _______.
Which of the following most logically completes the scientist's argument?