Because he's been around the block enough times to know that it's not so simple. There are a ton of variables, confounding factors and second order effects in p

Because he's been around the block enough times to know that it's not so simple. There are a ton of variables, confounding factors and second order effects in play when it comes to things like forecasting climate and nutrition. There are plenty of examples where we got things terribly wrong, even when there were high levels of confidence (e.g., dietary recommendations). In the case of candy, he probably thinks sugar isn't good in and of itself and doesn't eat it with a spoon. But it's the overall effect of candy that matters. If it reduces stress, then maybe it's not so bad or even a net good in some amounts or for some people. Warren himself made it to 89 (so far) eating quite a bit of junk food, drinking coke and is still sharp and has enough energy to be working, traveling and so on. Many examples like that, so indeed it's not so simple. With CO2, he probably thinks there is or can be too much in the atmosphere. But again it's not the whole picture. For one thing, you need to be sure that building and operating the solution (making machines, transporting, maintaining, powering them) doesn't exceed their net benefit. It's easy to find yourself in a situation where the cure is only marginally better or even worse than the disease. Which happens a lot with complex systems. Warren is exceptionally skilled at getting to the ground truth. When he says he doesn't know about something that you feel is obvious or basic, then it may be worth re-evaluating your level of confidence.