But as the chaotic Copenhagen climate change conference demonstrated last December, it is virtually impossible to negotiate complex global issues among the entire "G192" of UN member states. The G20, however, allows the world's biggest players to shape the contours of a realistic agreement that the entire world can subsequently consider. Viewed in this light, the G20 is not a substitute for the UN but its essential complement.

The major question surrounding the G20 is whether it will become the central hub of major power cooperation or just one "Gx" forum among several. Contrary to expectations a year ago, the G8 has not evaporated. Indeed, US officials remain attached to it as a grouping of "likeminded" states. And yet such nostalgia seems misplaced. Few 21st-century challenges can be resolved in an exclusive Western boardroom. Whether the issue is terrorism, financial instability, climate change or pandemics, global solutions will require action from a diverse set of players.

The biggest challenge the G20 faces is turning its large size and internal diversity into an advantage, rather than a curse. China's leaders seem aware of this need. "In light of the current international context," Hu Jintao has declared, "G20 members must engage in flexible and pragmatic dialogues on the basis of equality and mutual benefits, seeking common grounds while shelving differences."

For China, the G20 offers at least two major strategic benefits. First, it provides a multilateral forum in which to assert China's growing interests as a global power, while reassuring other countries that China's rise will not come at their expense. Second, it gives China a venue to manage its complicated relationship with the US - a venue that eases inevitable bilateral tensions while offering ample opportunities for Sino-American cooperation. By embedding itself in the G20, China both advances its interests and contributes to the global good.

The author is senior fellow and director of the International Institutions and Global Governance Program at the Council on Foreign Relations.