That one should be obvious, but for some reason almost no one ever says it. This is why it is very nice to see Eduardo Porter's piece making the point in the NYT today.
The basic point is probably too simple for economists to understand, but if we have 20 percent fewer people in 2050 than in a baseline scenario, then they all can emit 20 percent more greenhouse gas (GHG) emissions in that year and have the same amount of total emissions. Alternatively, if we have the same amount of per capita emissions, we will have 20 percent less total emissions.Restraining population growth is not going to solve the problem. We have to sharply reduce the amount of GHG emissions per person, but reaching whatever targets we set will be much easier with a smaller population.
It is remarkable how frequently news stories decry evidence of slowing population growth or shrinking populations as implying some sort of catastrophe. This is nonsense. It simply implies a tighter labor market with a rising ratio of capital to labor. In this scenario, workers switch from low productivity jobs (e.g. restaurant work, house cleaning, and retail clerks) to higher productivity jobs. This is a problem for the people who want to hire cheap labor, but will likely be seen as good news by almost everyone else.