Why is India's future brighter than China's, especially in a warming world? Because India has more good agricultural land per person. That will get more and more important as the temperature goes up.
I first encountered the concept of 'real population density' when I was interviewing people in the Netherlands last year about how the country would fare as the temperature rose. The Dutch were confident that they have the sea level problem under control.
The sea-level experts were also confident that the Netherlands would not face any problems with food when the temperature rises. But I looked up a few agricultural experts, and they explained real population density to me.
'It would take a country three or four times the size of the Netherlands to support our present diet,' said Dr Huib Silvis of the Agricultural Economics Research Institute at Wageningen University. 'If we had to be self-sufficient, we would not be eating meat.'
The real population density of the Netherlands -the number of people per square kilometre of farmland -is 2,205. That is higher than Bangladesh (1,946).
The Netherlands is the second- or third-biggest agricultural exporter in the world, but that is in terms of the cash value of its exports, which are mostly high value-added products such as cut flowers. The Dutch could barely feed themselves from their own resources even now.
Global warming makes matters much worse, because it hits food production very hard. The rule of thumb is that the world loses about 10 per cent of its food production for every rise of one degree Celsius in average global temperature.