Paul Krugman absolutely supports the quantitative (actually, as he correctly notes, qualitative) easing that the Fed’s purchase of long-term bonds will bring. He is clear however, to note the underlying risk:
The problem may come when the economy recovers, and inflation starts to become a problem rather than a hoped-for outcome. Basically, there will come a time when the Fed wants to withdraw that extra $1 trillion of money it created. It will presumably do this by selling the bonds it bought back to the private sector.
But here’s the rub: if and when the economy recovers, it’s likely that long-term interest rates will rise, especially if the Fed’s current policy is successful in bringing them down. Suppose that the Fed has bought a bunch of 10-year bonds at 2.5% interest, and that by the time the Fed wants to shrink the money supply again the interest rate has risen to 5 or 6 percent, where it was before the crisis. Then the price of those bonds will have dropped significantly.
And this also means that selling the bonds at market prices won’t be enough to withdraw all the money now being created. So the Fed will have to sell additional assets; if the rise in interest rates is at all significant, it will have to get those assets from the Treasury. So the Fed is, implicitly, engaged in a deficit spending policy right now.
My back of the envelope calculation looks like this: if the Fed buys $1 trillion of 10-year bonds at 2.5%, and has to sell those bonds in an environment where the market demands a yield to maturity of more than 5%, it will take around a $200 billion loss.
The Fed’s juggling act is far from over and there will be much more quant/qual easing before we’re done.