What will destroy human life on earth?

Originally posted by Soothfast
Plus there's a nonzero probability that, one day, everyone on Earth will simultaneously decide not to have children.

Most developed countries already have a below replacement level of reproduction. It is reasonable to assume that this will become a world wide phenomena as countries get wealthier.
This will result in a continuous reduction in the earths population.
Unless there is a conscious effort at some point to have more children to avoid extinction, it seems likely that humanity would eventually go extinct. But it does not require everyone to simultaneously decide to not have children. It only requires that the average number of children surviving to reproductive age be less than 2 per couple.

Originally posted by twhitehead
Most developed countries already have a below replacement level of reproduction. It is reasonable to assume that this will become a world wide phenomena as countries get wealthier.
This will result in a continuous reduction in the earths population.
Unless there is a conscious effort at some point to have more children to avoid extinction, it seems like ...[text shortened]... res that the average number of children surviving to reproductive age be less than 2 per couple.

It will be found that when the population gets to a certain stage with reduced number of children, the birth rate will go back up. I don't think it is anything more than societal pressure moving populations down.

Originally posted by twhitehead
Most developed countries already have a below replacement level of reproduction. It is reasonable to assume that this will become a world wide phenomena as countries get wealthier.
This will result in a continuous reduction in the earths population.
Unless there is a conscious effort at some point to have more children to avoid extinction, it seems like ...[text shortened]... res that the average number of children surviving to reproductive age be less than 2 per couple.

I think that if the human population keeps going down and down until there is just a few thousand humans left in the whole world, I would imagine that many people would surely start to worry that the human species really would go extinct that way and there would then be the general view that we have a moral obligation to increase the population again.

I for one would be pretty unhappy with the thought the human species will go extinct just a few generations after my death and I for one would demand action to stop such a thing and I bet I would not be alone.

This would then probably start up a rigorous campaign to increase the birth rate back up again by greatly encouraging reproduction by giving all sorts of economical incentives for it as well as moral encouragement for it and perhaps there would even be some political propaganda for it teaching children from a young age that it is their moral duty to reproduce when the population is so low.

Thus I think it unlikely that humanity would go extinct that way.

Originally posted by sonhouse
It will be found that when the population gets to a certain stage with reduced number of children, the birth rate will go back up. I don't think it is anything more than societal pressure moving populations down.

But what are the societal pressures moving populations down, and why would they change? Humy is probably right that there would be deliberate campaigns to change society.

Currently, it seems that the moment child mortality comes down, and birth control becomes available, so does the number of children per woman - to below replacement level - even in the face of cultural pressure to have large numbers of children. There are clearly economic incentives to have fewer children, so if the economic incentive were removed, then the balance may shift the other way. I believe that in some countries there are already financial incentives to have more children.
I for one have chosen to have few children largely for financial reasons.

Many people however do delay marriage simply because child rearing takes so much time, and if women wish to have a career then they want to have children later in life - which means fewer children.

Originally posted by twhitehead
But what are the societal pressures moving populations down, and why would they change? Humy is probably right that there would be deliberate campaigns to change society.

Currently, it seems that the moment child mortality comes down, and birth control becomes available, so does the number of children per woman - to below replacement level - even in th ...[text shortened]... ish to have a career then they want to have children later in life - which means fewer children.

I think anyone would say there are way too many humans on Earth so less than replacement is a good thing up to a point. Obviously if the population got down to 100 million or so, things would look ominous but as it is now, going from 8 billion to 7.9 billion is not a world shaker.

This question begs the question "what is human?" Suppose humans are still around in a hundred million years. They will be genetically very different from us. Are they still "human"? If humans succeed in colonizing other planets, or some global catastrophe throws humans back to the proverbial Stone Age, it is conceivable that separate species evolve. Then who are still the "real" humans?

These considerations aside, you would have to think that it takes some kind of aforementioned global catastrophe to wipe out all humans. War? Unlikely to take out everyone, even if there is some kind of global nuclear war. A meteor is possible, although meteor strikes that are devastating enough are rare. The most likely option in my mind would be extreme climate change, possibly exacerbated by people themselves. People going extinct by not breeding seems like an unlikely option to me - sure, there are some places where birth rates are significantly below replacement level (e.g. Japan, Italy), but most developed nations are now at or near replacement level.

Originally posted by sonhouse
I think anyone would say there are way too many humans on Earth so less than replacement is a good thing up to a point. Obviously if the population got down to 100 million or so, things would look ominous but as it is now, going from 8 billion to 7.9 billion is not a world shaker.

I agree that it is not an imminent danger, the population still has to grow to around 11 billion simply due to better health and an aging population.
However, once it starts to fall, it may be quite rapid. I believe Japans population is falling dramatically - up to 30% reduction per generation.

Originally posted by KazetNagorra
... but most developed nations are now at or near replacement level.

Not so. Many developed nations are shoring up their birth rates because of immigration (recent immigrants have higher birth rates), but even so much of the developed world is below 2. Replacement level is over 2:

http://en.wikipedia.org/wiki/List_of_sovereign_states_and_dependent_territories_by_fertility_rate

http://www.gapminder.org/


China is also below 2.

Originally posted by twhitehead
Not so. Many developed nations are shoring up their birth rates because of immigration (recent immigrants have higher birth rates), but even so much of the developed world is below 2. Replacement level is over 2:

http://en.wikipedia.org/wiki/List_of_sovereign_states_and_dependent_territories_by_fertility_rate

http://www.gapminder.org/


China is also below 2.

Keywords: "most", "at or near."

Originally posted by wolfgang59
Isnt there a threat from oscillating poles and the subsequent degradation of the Van Allen belt?

(Off the top of my head and not googled ... I'll do that tomorrow ... goodnight)

It seems that this theory is not well supported.

Statistical analysis shows no evidence for a correlation between reversals and extinctions
http://en.wikipedia.org/wiki/Geomagnetic_reversal

1. Super volcano
2. Astroid collision.
3. Global warming.
4. Human wars and WMD's.
5. Sun expanding and engulfing the earth in distant future.
6. 4 more years of Obama.
7. A virus
8. Fire and brimstone from heaven

Originally posted by KazetNagorra
Keywords: "most", "at or near."

most: not the case according to the stats.
near: well thats a rather vague word, so I can't exactly say you are wrong.

But consider this:

http://en.wikipedia.org/wiki/Total_fertility_rate

The average total fertility rate in the European Union (EU-27) has been calculated at 1.59 children per woman in 2009

And I believe the average for developed countries is also approx 1.6

That fertility rate results in a loss of 20% of the population every generation.
I do not consider that to be 'near replacement level'.

Now that everyone has chosen scenerios for out demise, let us try to evaluate how much control we have over them.

So what say you? Can mankind save himself? If so, how?

Originally posted by whodey
If so, how?

Isn't it obvious?
Look out for near earth asteroids/comets.
Take steps to stop global warming.
Ban WMDs.
Fight for world peace. (we could start by discouraging religion).
Establish colonies on other planets.
Invest in infectious disease research.
Encourage more children once the population gets down to a manageable level.
Invest in science.
Don't vote republican.

Nuke heaven so we don't get bombarded with fire and brimstone.