The defining challenge of the 21st century will be to face the reality that humanity shares a common fate on a crowded planet.

All of the incessant debate about development assistance, and whether the rich are doing enough to help the poor, actually concerns less than 1% of rich world income. The effort required of the rich is indeed so slight that to do less is to announce brazenly to a large part of the world: 'You count for nothing.' We should not be surprised, then, if in later years the rich reap the whirlwind of that heartless response.

The longer you wait, the less fun. If you wait until the bitter end, the whole economy can be destroyed.

We need to defend the interests of those whom we've never met and never will.

The world got side-tracked from development issues during the post-9/11 crisis period.

This is our greatest challenge: learning to live in a crowded and interconnected world that is creating unprecedented pressures on human society and on the physical environment.

It's the American leadership that has not played the role it should be playing and that leaders in other countries have been playing.