Deal With the Environment Now, And Social Security Later?

Paul Krugman says Robert Samuelson is a hypocrite because Samuelson is tediously, incessantly worried today about the long-run solvency of the social security trust fund, but isn't so worried about the long-run destruction of the planet via climate change. I have the feeling Samuelson wouldn't agree that the two situations are analagous, but let's assume Krugman is right on the merits: You can't play Cassandra with social security and Zeno with climate change. So, if we have an obligation to be consistent about social security and climate change -- if they are analagous long-run problems -- why can't someone just toss the hypocrisy charge right back at Krugman?

Krugman, after all, thinks climate change is a problem we should deal with today. (And I agree with him.) But he doesn't think social security is urgent. Why the difference?