[Nizzle (OP)]

We are all aware of the whole bundle of Damocles' swords hanging over our heads, but which one will fall first?
If you're not too lazy, please also motivate your choice.

EDIT: Whoops, should've been posted in General Science :/

[CliffordK]

It is hard to say...
If you are talking about extinction of many species...

You would have to include over-hunting, habitat destruction, and invasive species, all caused by people.

If you are talking about traumatic changes to mankind.  We could become exposed to a super-virus.  But, I'm doubtful that it could kill more than... say 30% of the people, which would be extraordinary.

However, I also foresee resource depletion as being a risk to humanity... and also to other life as people try to compensate.

[Don_1]

So far as a mass extinction of Man is concerned, an uprising of the masses in revolt of inequality and wealth distribution.

As for other species, I agree with Clifford on the invasion and destruction of natural habitat by Man.

[Nizzle (OP)]

Hmm, I was really thinking more like 90% of all species of plants and animals dying out, like the 5 MEEs that have already occurred..

[grizelda]

I'm going for the nukular. With the ability to eugenically modify their populace, political systems can now rival religious systems with their 99% extinction rate.

[syhprum]

A mass extinction is a massive event although most of the things listed except asteroid impact and expansion of the Sun could cause some readjustmet of species numbers mass extinction is going a bit far.

[Nizzle (OP)]

A mass extinction is a massive event although most of the things listed except asteroid impact and expansion of the Sun could cause some readjustmet of species numbers mass extinction is going a bit far.

Hmm, what?

- Gamma ray burst could sterilize earth, so that's quite massive
- Reversal of magnetic poles comes along with a major decrease in magnetic field strength and the accompanying increase in deadly doses of sunrays to all but the best equiped plants/animals
- Supervolcano eruption (like if Yellowstone National Park would ignite) could have the same atmospheric effects as an asteroid impact.
- Alien invasion (though very² improbable) could terraform our rock and make it uninhabitable for current lifeforms

They're in the list because of their ability to cause a mass extinction (some for laughs)

[syhprum]

Although Noah's flood is told as major event in the good book and the Koran present thinking is that the the raising of the level of the black sea when the waters of the Mediterranean broke thru was a relatively slow event giving both sinners and the god fearing plenty of time to move out.
PS there have been many reversals of the Earths magnetic field but they have not caused mass extinctions

[CliffordK]

I'm going with the current estimate of somewhere on the order of 4 billion years for the eventual death of our sun.  Undoubtedly the Earth will be a different place at that time.

There are estimates that the sun will gradually get hotter before the "end".  However, I believe that we will develop space-mirrors to manage the planet's temperature if it gets either too hot, or too cold.  But, we would have difficulty engineering solutions to deal with a red giant or white dwarf.

It is hard to imagine what will be left of humanity in 4 billion years as the homo sapiens are only about 200,000 years old, and our hominid history is less than 10 million years.

Other cataclysms may occur on the planet before that time, but perhaps we'll at least develop technology to minimize the risk of a large asteroid impact.

[cheryl j]

I sometimes wonder if there was some major world wide disaster how many people would die simply because we are all so specialized and dependant on others that no one one could adequately take care of themselves for a year. People would flow out of cities like rats, with no air conditioning in the summer or heat in the winter, no food left on the shelves of stores or clean water coming out of the fawcets, and garbage piling up everywhere. Where would they go?

[Gordian Knot]

Cliff I think the current estimate for the sun is 5 billion, not 4. Don't take 20% of our time away!  []

Since this poll was about the cause of the next Mass Extinction Event, which as I understand it means one of those 90% loss of all living things on the planet, it narrows down the possibilities. Some like the death of our star are simply too far off to be reasonable. I don't see humankind still being around for a fraction of that time.

Others like asteroid impact, solar flares, super volcanoes are not predictable. Any one of could happen a thousand years from now, or tomorrow. Heck, if there was a huge solar flare happening right at this moment, we wouldn't know about it for what, 7 to 9 minutes from now!

It won't be aliens, as in an advanced alien race as there seems to be no-one nearby, and no plausible way a civilization farther away would spend the time and resources to get all the way here. An off world microbe that brought devastating disease is an interesting possibility though. But that goes into the could happen anytime group.

It won't be the Mayans as the whole Mayan calendar thing has been corrupted by modern misunderstanding. The Mayans never said the earth would be destroyed at the end of the calendar, they said that would start the next cycle. Sort of like moving from one Zodiac sign to the next. Harmless.

It won't be the Guy/Gal upstairs as my admittedly limited understanding of scripture says that after the Flood, He/She said they would not do that again. And if you cannot trust the big G who can ya trust?

As an aside, I've always wondered why it is always referred to as The Good Book. Shouldn't it be called The Great Book? But I digress.....

Although Climate Change will certainly have a significant effect on the future, I think that process is slow enough to cause massive change on the planet, but not necessarily an extinction event.

Bottom line, if one of the could-happen-at-any-time events doesn't occur, my bet is on us. Global thermonuclear war. Less likely than it was a decade ago, but still a real threat. That would do a real job on the planet that would take a verrrrrry long time to undo. That would certainly take care of humanity for good. Not the earth itself necessarily though. Given enough time, even the millions of years it would take for the radiation to wear off, I do believe the earth could regenerate itself again. Which for me, is a comforting thought.

[CliffordK]

Cliff I think the current estimate for the sun is 5 billion, not 4. Don't take 20% of our time away!  []

4 or 5 depends on how you round the numbers, what stage of the solar decay spells doom for Earth, and how much heat we'll be able to divert before "the end".

There are some estimates that we'll have intractable solar mediated warming in the next half billion years, yet I believe that humanity will start engineering our weather and climate before then, creating space technology to reduce the sunlight in some areas and increase it in other areas (which could be troublesome for some species).

Global thermonuclear war. Less likely than it was a decade ago, but still a real threat. That would do a real job on the planet that would take a verrrrrry long time to undo. That would certainly take care of humanity for good. Not the earth itself necessarily though. Given enough time, even the millions of years it would take for the radiation to wear off, I do believe the earth could regenerate itself again. Which for me, is a comforting thought.

I foresee more of a thermonuclear skirmish, than an all-out war. 

Countries like North Korea will never have enough nuclear weapons to make a global impact.  It would take an all-out nuclear war between the USA, Russia, and perhaps China to have a global impact.  Yet, I hope our future politicians will put the brakes on nukes, and perhaps war itself before that happens.

With North Korea, perhaps they'll nuke Seoul South Korea (or even Tokyo) which would lead to a retaliation in Pyongyang North Korea, but overall the actions will be self-limiting.  The North Koreans won't ever have more than a couple of low-yield nuclear bombs in their entire arsenal.

I sometimes wonder if there was some major world wide disaster how many people would die simply because we are all so specialized and dependant on others that no one one could adequately take care of themselves for a year. People would flow out of cities like rats, with no air conditioning in the summer or heat in the winter, no food left on the shelves of stores or clean water coming out of the fawcets, and garbage piling up everywhere. Where would they go?

One can adapt to no AC very quickly, and even to some extent to a lack of heat. 

However, if our electric grid and transportation grids simultaneously collapse.  That means no food in the stores, no clean water, and no flushing of the toilets.  Big cities would become instant cesspools...  and would have major problems.  I suppose that 70 years ago, much of Europe dealt with less than ideal conditions... how did they cope?  Are we to much beyond the what Europeans dealt with in the 40's?

Rural agriculture might take a hit due to loss of mechanization, and thus city-folk, and farmers alike might have to re-learn how to use a scythe. 

[damocles]

Global warming is real, serious, and happening rapidly. What happens next is quite unclear, because there is a lot of uncertainty in our understanding of major Earth systems.

Hottest scenario: rapid climate change will lead to loss of biomass because plants (and especially food crops) not able to adapt quickly enough. Methane released from permafrost, oceanic clathrates, etc. Escalating carbon dioxide level in atmosphere leads to major changes in atmospheric chemistry. Huge loss of food resources results in mass extinctions. Earth as a whole returns to Jurassic type climate and atmosphere.
A runaway greenhouse effect to produce a Venus-like situation where the oceans eventually boil does not look like a serious possibility.

Coldest scenario: mild warming leads to major changes in ocean current circulation, shutting down the Gulf Stream and drastically changing Behring strait flows. A new Ice Age starts in the Northern Hemisphere, with the loss of most agricultural land, and consequent mass extinctions.

Anything in between is up for grabs!

[cheryl j]

One can adapt to no AC very quickly, and even to some extent to a lack of heat. 

  I suppose that 70 years ago, much of Europe dealt with less than ideal conditions... how did they cope?  Are we to much beyond the what Europeans dealt with in the 40's?

AC is a luxury in a lot of places, but we've built entire cities in the desert. Tall buildings with windows that don't even open. And how many people own a wood stove? The difference between now and the 30s or 40s is despite industrialization, a large percentage of people still lived on farms, and still knew how to grow food, preserve it, or ride a horse if they had to, got their water from a well or a cistern. Where do you find a good plow these days? How many steam engines are kicking around?