Projections of future carbon-dioxide emissions are complicated. How will energy consumption habits change as societies get richer or more urban? What mix of sources will we be getting our energy from?
My friend and colleague Emilo Zagheni decided we should make the calculus that much more complicated (and informative) by also asking how the aging of the population will influence carbon outputs. A demographer's demographer, which Emilio surely is, is never happy with any calculation that does not include age-structure in one way or another.
This article in the Economist summarizes what he did and what he found. As people get older, they tend to consume more and more, emitting more and more carbon, until 65ish, at which point consumption tends to start declining. See the graph, and the analysis, in the Economist, or the original in the journal Demography (2011) pp 371-399. The punchline for the carbon-watcher is that the changing age-structure will tend to increase carbon emissions until about 2050, after which point such a large portion of the population will be above 65 (I'll be 73) that the age effect will begin to marginally decrease emissions.
I should finish with a quote from Ron Lee, Professor of both Demography and Economics at UC Berkeley, who both Emilio and I studied under. Ron was one of the inventors of the widely used Lee-Carter method* (1992) for forecasting future mortality patterns. Seventeen years later, I asked Ron how well his forecasts for the first 17 years matched what had actually happened in those years. He cocked his head slightly to the left, sighed sagely and said, "Well, demographers are well aware that our projections don't always fair so well in a complex world. But we console ourselves with the knowledge that we do much better than the economists."
* The original article has been cited more than 1000 times in the peer-reviewed literature, and modifications are used by the US Census Bureau, the UN, and so forth.