Will there be enough? There should be, though products in demand such as natural gas will surely give market power to prepared suppliers. And those with low incomes will always tend to be weak in the marketplace, though if the poorest multiply their income eightfold over today, most should be safe from hunger.
Will the fallout harm? Cities will spread and the climate may warm a little, but on balance humans may tread more lightly in nature.At the outset, we recognized the general multipliers, population and GDP. To answer the two basic questions, we now see we must address a third, "Must resource and environmental stresses intensify in unmodified lockstep with these general multipliers?" History suggests technology and science, which lift productivity and efficiency of resource use, are powerful enough divisors to lessen net stresses.
As well as the raising the divisors, people could choose to lessen the multipliers. However, habit favors multiplication, and so for the 21st century we should at least prepare for it.
A world of 10 billion people and $200 trillion will above all handle huge amounts of information, whether in its fields, factories, offices, or homes. Its greatest vulnerabilities may come from failures or rejections of the systems of control for the communication of information. Science is a uniquely effective system of communication for the control of complexity, and the ability to control complexity is effectively the central parameter of the dynamics of evolution. Having come far down the road with science and technology, perhaps we should feel greatest stress from knowing we must keep running.
Acknowledgements: I am grateful for many years of cooperative work with Arnulf Gruebler, Cesare Marchetti, Perrin Meyer, Nebojsa Nakicenovic, and Paul Waggoner and to Kurt Yeager and Chauncey Starr for asking the questions eliciting this paper.