How big will the human population get on this planet? They’re always fiddling with the predictions, but by some estimates, we’ll top out at around 9 billion by 2050 (see this Economist video and article). More dire predictions can be found, but these seem reasonable to me, given current demographic trends.
Slowing population growth is a good thing, but there’s another factor to consider. To what extent will the aggregate needs of the human race grow even as its population begins to shrink? Geoffrey West, a physicist who studies cities, has asked an interesting question. What are the energy needs of the human animal? According to his calculations, a human being at rest runs on 90 watts. Pretty remarkable, eh? I know light bulbs that eat more than that. But then West takes it a step farther. Suppose we rolled up the energy needs of your light bulbs and your car and your house and so on, and we pinned all that on you… in other words, what are your energy needs not as an animal in a box, but as a civilized human going through a normal day? He comes up with something like 11,000 watts. That’s an energy obesity multiplier of 120! He continues.
What kind of animal requires 11,000 watts to live? And what you find is that we have created a lifestyle where we need more watts than a blue whale. We require more energy than the biggest animal that has ever existed. That is why our lifestyle is unsustainable. We can’t have seven billion blue whales on this planet.
As a person’s appetite for energy grows, the infrastructure required to feed them must expand correspondingly. Thus a population that’s getting richer and smaller can grow and shrink at the same time.
In literal terms, think about weighing everybody on the planet on one giant scale. Given enough fat people, you’ll obscure the fact that some skinny people have died. Big bellies hide many mouths. Consider this BBC article: Global weight gain more damaging than rising numbers.
So one projection has the human population peaking in 2050. What I’d like to see is a projection of when the aggregate human energy needs will peak. It will certainly be after peak population. But how much longer? Only then will the human footprint on the planet truly start to recede.
Rambles Blog at Star Chamber 
I’m not sure that our energy needs will ever peak. There are 6 billion people on this world with a standard of living somewhere between “mud hut” and “concrete apartment building in mumbai”. They want (and it would not be right to deny) the lifestyle of the other billion, a project that’s going to take 2-5 generations at least.
As for the other billion: maybe there’s some hope in the transfer of physical products to virtual goods and time-shared services. But the Jevons Paradox has yet to be disproved: every time we come up with a more efficient way to do something, we just use the savings to build and do more stuff.
Great TED talk by Hans Rosling, regarding world population:
Regarding insatiable energy needs, here’s a good talk by “Do the Math” blogger Tom Murphy: http://fora.tv/2011/10/26/Growth_Has_an_Expiration_Date
In it, he touches on some of the thermodynamic limits to energy usage. At some point, even with free magical energy, rejecting heat from the planet becomes a serious problem. Can we build a space-elevator planet radiator? Better get started now…
I can’t get over how much a lot of that sounds like the macroeconomic forecasts of Jeremy Grantham (Mr “G” of GMO, a Boston-based hedge fund).
If you go to the expected home page of a company called GMO (note clever reference rather than just plain url as a clever spam filter avoidance strategy) you can find links to some of his quarterly stuff, by the way.
In fact, I think the reason Grantham is a success as a hedge fund hotshot, and a big part of the reason why I can know he’s an actual success and not just lucky, is that his forecasts sound a lot like that and not many other economists’ forecasts do. (And also why he feels like he can publish a forecast and believe it to be a net plus to his bank account.)
Now if I just had the courage to actually act on his forecasts rather than just believing in them.
Yours sincerely etc.
–JMike