Does anybody see a problem with the graph on the left?
Here is another graph of the same information (with a few extra years tacked on at the end and the US population added as reference). Do they look at all similar?
The first graph is terribly misleading because the abscissa/x-axis is nonuniformly scaled. I'm not sure more than two consecutive tick marks have the same scale. I've redone this graph using a uniformly scaled abscissa to show true trend. China for some reason cuts their graph off just before their population is projected to begin shrinking. Maybe its just that I'm an engineer, but this sort of wacky scaling drives me up the wall. The benefit of a graph is that it can convey alot of information intuitively. For that reason, if people aren't careful, uneven scaling can be very misleading. This above graph almost makes it look like China had a lull in population growth that has started to pick up again at a rather dramatic pace. This is not the case at all, as the second figure shows. It doesn't take very long to look this information up and create one's own graph. You would think that a reporter would be educated enough to do something like this, especially when they are working for the BBC and not merely some local rag.
The question came up as to why there is such a lag between when fertility rates drop below replacement levels and when population begins declining. Because population growth is the sum of fertility rates and death rates, there is a lag due to the human lifespan. However, this lag is exacerbated because factors associated with more rapidly declining fertility rates are also associated with increased lifespans, namely increased urbanization and development.
The typical age/sex population distribution for an undeveloped country looks like a pyramid. There are lots of children and relatively few adults. However, as fertility rates decline and lifespans increase, the pyramid begins to morph into a rectangle. Zimbabwe, like most underdeveloped countries has recently begun to reduce its population quite dramatically. One can see that 25 years ago, population was still growing exponentially, but great reductions in fertility rates have come since then. One thing that has surprised demographers over the last 25 years is the rapid pace at which fertility rates have declined in the developing world. They have declined much more quickly than they did in the developed world. Now, the population distribution histograms of even underdeveloped countries like Zimbabwe are starting to morph into the rectangle shape indicative of stable population.
Italy is an example of a country that has experienced the transition from increasing population to decreasing population. Instead of transitioning to a rectangle, they are in the process of transitioning to an inverted pyramid. Not only is population growth an exponential function, but population decline is exponential as well. Unless they can reverse this trend in fertility rates or increase their immigration rates, their population will decrease exponentially to a fraction of what it is today.
The US and China are both countries that are in between the pyramid and rectangle stages. The difference between the US and China is that China has experienced more drastic fertility rate reductions and is beginning to transition into the inverted pyramid stage like Italy. However, the US is beginning to look more like the rectangle with a relatively stable population distribution by age and sex. These plots are also interesting, because they can highlight the sex ratio disparities in countries like China and India.
These plots become even more fascinating when one compares one country today to the same country +/-50 years. I think a short course in demograhics should be required for policy makers dealing with plans like social security and medicare, that take from today's worker to pay for today's retirees benefits instead of investing the money.
One last note because the figures ended up being rather small. The horizontal axis is population with zero in the middle and female population in blue increasing to the left and male population in red increasing to the right. Each horizontal red and blue line represents a histogram of the population for a specific age category. The age categories are 5 year age spans proceeding from 0-5 years old on up to 95-100 years old. I'm sure there's a much simpler way to explain this, but I'm tired and it escapes me right now.
Source for my Population Graphs: Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat, World Population Prospects: The 2006 Revision and World Urbanization Prospects: The 2005 Revision, http://esa.un.org/unpp, Wednesday, September 26, 2007;