We certainly cannot conclude, as some people want to, that carbon dioxide is not actually a greenhouse gas. More than a century of research thoroughly disproves that claim.
In fact, scientists can calculate how much extra heat should be accumulating from the human-caused increases in greenhouse gases, and the energies involved are staggering. By a conservative estimate, current concentrations are trapping an extra amount of energy equivalent to 400,000 Hiroshima bombs exploding across the face of the earth every day.
So the real question is where all that heat is going, if not to warm the surface. And a prime suspect is the deep ocean. Our measurements there are not good enough to confirm it absolutely, but a growing body of research suggests this may be an important part of the answer.
Exactly why the ocean would have started to draw down extra heat in recent years is a mystery, and one we badly need to understand. But the main ideas have to do with possible shifts in winds and currents that are causing surface heat to be pulled down faster than before.