In the 1970s many people were concerned about a "population explosion." Dystopian fiction--already a well-established sub-genre--featured visions of a grossly overcrowded future earth, along the lines of "Solyent Green." However, something happened on the way to such nightmares: birth rates in developed countries and large developing ones like China slowed in tandem with rising incomes. Instead of a world of 12 billion by 2100 or sooner, long-term estimates in the last decade, including from the United Nations, began to focus on an eventual plateau around 9 billion.
Now it appears those lower forecasts might have been too optimistic, particularly with regard to birth rates in sub-Saharan Africa. The analysis in a paper published in Science last month suggests that growth will continue beyond the end of the current century. The authors expect global population in 2100 to reach 9.6 to 12.3 billion. That could have significant implications for energy demand and climate change, among other environmental and development issues, while in turn being influenced by them. Nick Butler, who writes on energy for the Financial Times, looked at this from the perspective of oil and other energy sources and concluded, "None of the current technologies...offer an adequate answer."
I would take Mr. Butler's observation a step farther. It's extremely challenging to say anything confidently concerning how much energy the world of 2100 might need, or where it will come from. Forecasts are rarely accurate beyond a few years, and even scenario methods struggle to cope with the unknown-unknowns involved in such time frames.
Recall that in 1928--as far removed from today as 2100-- world oil production was less than 5 million barrels per day, and the first chain reaction making nuclear power possible was still 14 years in the future. Natural gas was mainly viewed as a low-value byproduct of oil production, while wind power was considered quaint. And with a global population of just over 2 billion at the time, meeting the energy needs of today's 7 billion might have seemed even more daunting than supplying 11 or 12 billion does to us.
It's also worth keeping in mind that more than three-fourths of today's oil is consumed by countries with 60% of the world's population. The curve drops off steeply from there, leaving roughly 2 billion without modern energy services. So the energy implications of an extra two billion people by the turn of the century depend heavily on whether their energy demand looks more like today's top 4 billion or bottom 2 billion energy consumers. The upcoming "Africa Energy Outlook" from the International Energy Agency (IEA) could shed some useful light on that question.
For that matter, even if an expansion of fossil fuel production on that scale were possible, due to the technology that is currently unlocking oil and gas from source rock rather than conventional reservoirs--a.k.a. the shale revolution--it would bypass any notions of a "carbon budget" that might constrain the projected global temperature increase to a manageable level. So it's a reasonable bet that however many people are alive in 2100, they will use less fossil fuels per capita than we do.
Consider what some of today's mainstream forecasts say about the future energy mix. The main "New Policies" scenario of the IEA's 2013 World Energy Outlook sees renewable energy growing from 11% to 18% of total primary energy by 2035, while its more aggressive "450" scenario has them supplying 26%, with commensurate reductions in fossil fuels. Shell's current long-range scenarios envision divergent futures in which fossil fuels still supply 50-60% of nearly doubled energy demand by 2060, but shrink to around 20% or less by 2100.
One big trend that could help facilitate that kind of change is electrification, which will increasingly displace liquid fuels from illumination, cooking, and even transportation. That's important because while we have few practical large-scale alternatives to petroleum for liquid fuels, we have many ways to generate electricity and could accommodate more, including the long-awaited arrival of practical nuclear fusion or some currently unanticipated energy source. Eight decades would be more than sufficient for an entirely new generating technology to become significant.
Reconciling the energy needs of a large, growing population with preventing dangerous global warming--referred to by some as the "energy dilemma"--thus appears to require a sustained, protracted transformation of the entire energy economy. That shouldn't be a surprising insight. The bigger question is whether such a transformation can be achieved through the gradual evolution of the energy technologies available today, or whether it will require revolutionary developments. That remains a matter of considerable debate in energy circles.