According to MIT's Pr Emanuel and Pr Solomon, the thesis of this article "does not follow from the scientific substance of the essay". Their full argumentation is on The Equation. Here is the extract:

"Koonin notes several key scientific fundamentals. He does not deny that climate is changing, that human activities are at least partly responsible for it, or that policy formulation should take climate change into account. But the headline statement—that not enough is known about climate to warrant significant action given the risks—is a statement of values and does not follow from the scientific substance of the essay."