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Fate of Humanity
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来自 Princeton University 的课程
Imagining Other Earths
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从本节课中
Extinctions
There have been at least 5 mass extinctions in the Earth’s history. These extinctions were events that wiped out most of life on the planet and drove a large fraction of all life forms into extinction in a very short period of time. The most famous of these extinctions “killed’” the dinosaurs when an asteroid hit the Yucatan. We are in the midst of the sixth extinction due to human induced changes in the environment.
与讲师见面
David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
Finally, I want to turn to the question of, are we in the midst
of the sixth great extinction, and what will be the ultimate fate of humanity?
Will we become extinct?
Climate change is obviously having a very big effect on our environment.
We've talked about sea level rise which is certainly bad for
Florida or New York City or Bangladesh or islands in the Pacific.
But what might have more global effects on things like ocean acidification.
The oceans have been absorbing about half the carbon
dioxide that we've been putting up into the atmosphere.
So, as we're putting this carbon dioxide up.
Half stays in the atmosphere, half goes into the oceans.
The carbon dioxide that goes into the oceans makes the oceans more acidic.
It changes the pH of the oceans.
The current best projection suggest that the oceans will be two to two and a half
times more acidic, in 2100 than they were at preindustrial levels.
Though, this level of acidity, has not been seen in, over 300 million years.
This will have significant effect, on things like
plankton, as it will make them more difficult
for them to create shells so
it's going to reduce the abundance of carbonate.
Oceans play a very important role in the carbon dioxide cycle.
Carbon dioxide's absorbed into the ocean dissolved, absorbed into organic material.
Some of it's returned back up into the atmosphere, but
some of the carbon dioxide's incorporated particles, things like carbonate
and organic carbon, and then gets returned, often
in the form of things like calcium carbonate, into rock, into the crust.
So the oceans are pulling carbon dioxide out of the atmosphere,
but in the process of doing, are becoming more acidic.
And this could have potentially a devastating effect on marine
life and be another part of this sixth great extinction.
So if we look at what's going on today, we're seeing
a rate of plant extinction that's a hundred times the normal rate.
There are nearly 20,000 species considered to be high risk for extinction.
Extrapolations that look at what species have already gone extinct.
What ones are threatened and on the endangered species list.
If we assume that over the next 100 years many of these species will co, become
extinct and extrapolate to other life forms that we're not tracking so well.
The current best estimates say that we're on track.
To lose three-quarters of the species over the next century or so.
This enormous extinction rate would make this
the sixth great extinction, and the sixth
great extinction would not be caused by,
you know, an incoming asteroid hitting the Yucatan.
Or a dramatic event in vulcanization or a great cooling, but by humans.
And finally, think about the
question of when will our own extinction occur?
Species are usually around for several million years And we
are a unique species and we are radically changing our environment.
Want to recommend for people who are instead thinking more about this question.
A book by Sir Martin Reese, one
of Britain's leading scientists, called Our Final Hour.
That looks at these different events that might destroy humanity,
or destroy civilization, and perhaps wipe out life on the planet.
Some of these events could be external events.
A nearby star exploding in a supernova
could wipe out the atmosphere on our planet.
Similarly, a nearby gamma ray burst could completely wipe out life.
Other events, like a supervolcano could produce enormous
amounts of carbon dioxide or dust in the atmosphere.
And, of course, there's the favorite alien invasion where they come and wipe us out.
We'll talk a bit more about that possibility in the final lecture.
Of course, there's the possibility of self-imposed destruction,
A nuclear holocaust was a distinct possibility during the Cold War.
Hopefully that risk has now been reduced.
But as we've discussed earlier the effects of nuclear holocaust
would not only be the direct destruction of the cities by
nuclear weapons, but also the dust and material kicked up
into the atmosphere that could potentially have produced a nuclear winter.
We've already touched on climate change.
And its potential effects at the level of
climate change that we're producing at the moment,
it'll have significant effects on the earth but
perhaps not one that will wipe out life entirely.
It'll be, produce the kinds of changes we
have seen in the past, that have been dramatic
but, well life's, life as a whole is
going on, though perhaps not so good for Florida.
Rees speculates about bioterrorist events, where
bioterrorists developed new kinds of viruses or bacteria
that are devastating to human life, and spread and wipe us out.
And finally the possibility of some form of nanotechnology, some kind of,
bot that, self propagates and survives in organic matter and wipes us all out.
Well, hopefully none of these will happen soon.
Rees speculates we'll have a 50% chance of one of
these things happening to us in the next 100 years.
I hope not.
We can think about things we could do to prevent this.
Things like controls on technology.
One of, I think, our great successes over the past 30
years was our ability to stop the destruction of the ozone layer.
International policy has preserved that.
We now see the ozone layer being restored, Nuclear
disarmament has limited some of the risks of nuclear war.
People talk about bioweapons control,
perhaps limiting some of those technologies.
This would reduce the risks by reducing
the kinds of changes that could be devastating.
And then there's the other side of reducing risk, which
is reducing, increasing the number of places where humans live.
If we have human settlement throughout our solar system,
our neighboring solar systems, we're less vulnerable to these events.
Of course when we think about this, we have to
keep in mind that humanity is not designed for space.
We're not designed to live on other planets.
We're not designed for space exploration.
And I think one of the lessons we should draw from our study of evolution is when
you're not evolved for a niche you will change to fit that niche.
And perhaps the fate of humanity is to evolve to become space travelers.
I'll leave you on that note.
And we'll come back and we'll talk about life in the
galaxy and the search for extraterrestrial intelligence in the final two lectures.
Thanks.
