Bold actions by central banks make tail-risk scenarios less likely. Challenges remain for policy makers to avoid setbacks.

In quest of stability in the eurozone: "whatever it takes."The ECB has stepped in with a new bond-purchase programme (OMT) to dispel euro break-up fears.

The Fed will act "as long as needed" to promote a stronger recovery, given downside risks such as fiscal inaction.

Quantitative easing and emerging markets.Its effects will depend on how local policy makers respond.