Comment by WildUtah | Hacker News Reader

Comment by WildUtah | Hacker News Reader

No rainforest in going to be preserved in a nation that continues on a path of exponential growth of population like Liberia. The nature of exponential growth is that it always looks sustainable until the catastrophe hits and everything collapses and there's nothing that can be done to save anything and probably mass starvation.

The fertility rate is 5.16 in Liberia which corresponds to doubling the population about once every 25 years. Then if fertility ever drops back to steady state (2.1), you double the population at least twice more because of population momentum.

The hopeful factor in Liberia is that the fertility rate is dropping. IT was 5.85 in 2000. If it doesn't drop fast, no payment for forests will help. If it does drop fast, the locals will enjoy their environment and work to preserve it.

Right now Liberia has about the population and land area of Virginia, which means it's already overpopulated. Around mid-century it will have at least quadruple the population. Then -- if fertility drops very fast down to replacement level -- it will quadruple again. That will be sixteen times the density of Virginia. And that is the best case scenario.

>Right now Liberia has about the population and land area of Virginia, which means it's already overpopulated.

First, that assumes Virginia is 'optimally populated'.

Second, according to Google, the population of Liberia is 4.294 million (2013) and the population of Virginia is 8.186 million (2012), so using your Virginia-baseline, Liberia can about double its population before being considered over-populated. The Indian state of Kerala, meanwhile, has 34.8 million (2012) people in a smaller area (~38k mi^2 vs ~43k mi^2).

> Then if fertility ever drops back to steady state (2.1), you double the population at least twice more because of population momentum.

What? Can you explain this? Sorry if it's obvious, but I can't seem to grasp what you mean by population momentum.

Say the population has been growing fast so most people are aged 0-20, and then the fertility rate drops to 2.1. Over the next 25 years you still have a lot of kids born while those people grow up, while there will not be many deaths because there will not be many old people to die off.

Suppose the population has been growing at a rapid clip, like in Liberia.

Then you will have a situation where the grandparent generation will have less population than the parents and the parents will be fewer than the children, e.g.:

2 80 year olds 6 60 year olds 15 40 year olds 40 20 year olds 100 babies -------- 163 people

One woman 80 year old had six babies with her husband. The three women in the next generation bore 15 babies. With a fertility rate around five, you will have a population pyramid where the babies outnumber the 80 year olds by about 50 to 1.

Then if fertility suddenly drops down to replacement level, the babies will eventually grow up to be 80 year olds in a country where the size of each generation is equal, e.g.:

100 80 year olds 100 60 year olds 100 40 year olds 100 20 year olds 100 babies ----- 500 people

So there's one more quadrupling (approximately) built in to the system even after the birth rate gets under control. Since the birth rate might not get under control until the resources run out in famine, war, or epidemic, we may be looking at a situation where high fertility countries are destined to have not only more people than they can support, but four times as many as they can support and chronic disasters for humanity. The only relief is thinking ahead and family planning, which is not necessarily our strong point as a species.

Hans Rosling explains this quite elegantly with boxes: http://www.youtube.com/watch?v=ezVk1ahRF78&t=11m

Even with the 16x growth that you're assuming, the population density of Liberia would only be 33% higher than New Jersey.

Reply on news.ycombinator.com