As the G20 Toronto Summit nears, there still exist many uncertainties about the global economy, ranging from the struggle of the euro zone and high unemployment rate of the United States to high fiscal deficit with Japan and China's over-reliance on exports.

Rachel Ziemba, a senior economist on China issues at the London-based Roubini Global Economics, discusses the previous G20 summits, the global economy, his prediction of the Canada summit, and China's role in assisting the G20 to reap fruitful results.

Q: How do you assess the effectiveness, achievements and failings of G20 Summits since the financial crisis?