While I find the comments fascinating, in a sense it's all hairsplitting.
Sach's fundamental point seems to need emphasis in this crowd:
world population increase of roughly fifty per cent, with income on a path, barring various disasters, to increase approximately fourfold. Multiplying one and a half by four suggests that the current trajectory would lead to an increase of world economic activity of six times between now and 2050. That is the goal from the point of view of economic development, but think about the paradox, if we already are on an unsustainable trajectory and yet China, India, and large parts of Asia are successfully barrelling ahead with rapid economic development at an unprecedented rate. We are asking our planet to somehow absorb a manyfold increase of economic activity on top of an already existing degree of environmental stress that we've never before seen on the planet.
It is possible that we will not be able to increase sixfold in economic activity with current technologies before the environmental catastrophes would choke off the economic growth. The hardships in water stress, deforestation, hunger, and species extinction, would cause this process to go awry, even before we are able to do more damage to the planet. But that does pose the fundamental question - what will give in the end?
The rest is window dressing. I'd like to see people taking up the core point.