From Prof Jeffrey D. Sachs.

Sir, Robert Kagan (Letters, February 7) denies a "world-altering shift in economic weight" in the shares of world income reported by the International Monetary Fund. He should look again.

In 1980, the US share of world income (measured in purchasing power parity prices) was 24.6 per cent. In 2011, it was 19.1 per cent. The IMF projects that it will decline to 17.6 per cent as of 2016.

China, by contrast, was a mere 2.2 per cent of world income in 1980, rising to 14.4 per cent in 2011, and projected by the IMF to overtake the US by 2016, with 18 per cent.

If this isn't a world-altering shift, it's hard to imagine what would be.

Jeffrey D. Sachs,

Director, Earth Institute,

Columbia University, NY, US