The statistician didn't say it point-blank, but by using the word "essentially," he's making it clear that the Sun is the only variable that matters in controlling land temps on Earth.

In using the word "essentially," the stimulus is saying that Sun is pretty much the only variable that controls T. Maybe there are other factors that also affect T, but the stimulus is pretty clear in conveying that the Sun is the only variable that matters. So, we can assume that the statistician is giving us a complete explanation.

Since the statistician is giving us a complete explanation, A is descriptively wrong.

One of the clearest and most helpful posts on LR. Thank you for sharing (and congrats on your LSAT score - obviously very well-deserved)!!