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Ten years ago, I wrote a book which I entitled

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"Our Final Century?" Question mark.

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My publishers cut out the question mark. (Laughter)

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The American publishers changed our title

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to "Our Final Hour."

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Americans like instant gratification and the reverse.

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(Laughter)

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And my theme was this:

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Our Earth has existed for 45 million centuries,

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but this one is special —

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it's the first where one species, ours,

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has the planet's future in its hands.

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Over nearly all of Earth's history,

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threats have come from nature —

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disease, earthquakes, asteroids and so forth —

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but from now on, the worst dangers come from us.

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And it's now not just the nuclear threat;

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in our interconnected world,

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network breakdowns can cascade globally;

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air travel can spread pandemics[br]worldwide within days;

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and social media can spread panic and rumor

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literally at the speed of light.

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We fret too much about minor hazards —

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improbable air crashes, carcinogens in food,

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low radiation doses, and so forth —

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but we and our political masters

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are in denial about catastrophic scenarios.

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The worst have thankfully not yet happened.

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Indeed, they probably won't.

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But if an event is potentially devastating,

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it's worth paying a substantial premium

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to safeguard against it, even if it's unlikely,

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just as we take out fire insurance on our house.

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And as science offers greater power and promise,

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the downside gets scarier too.

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We get ever more vulnerable.

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Within a few decades,

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millions will have the capability

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to misuse rapidly advancing biotech,

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just as they misuse cybertech today.

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Freeman Dyson, in a TED Talk,

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foresaw that children will design[br]and create new organisms

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just as routinely as his generation[br]played with chemistry sets.

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Well, this may be on the science fiction fringe,

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but were even part of his scenario to come about,

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our ecology and even our species

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would surely not survive long unscathed.

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For instance, there are some eco-extremists

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who think that it would be better for the planet,

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for Gaia, if there were far fewer humans.

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What happens when such people have mastered

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synthetic biology techniques

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that will be widespread by 2050?

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And by then, other science fiction nightmares

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may transition to reality:

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dumb robots going rogue,

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or a network that develops a mind of its own

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threatens us all.

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Well, can we guard against such risks by regulation?

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We must surely try, but these enterprises

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are so competitive, so globalized,

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and so driven by commercial pressure,

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that anything that can be done[br]will be done somewhere,

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whatever the regulations say.

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It's like the drug laws — we try to regulate, but can't.

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And the global village will have its village idiots,

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and they'll have a global range.

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So as I said in my book,

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we'll have a bumpy ride through this century.

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There may be setbacks to our society —

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indeed, a 50 percent chance of a severe setback.

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But are there conceivable events

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that could be even worse,

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events that could snuff out all life?

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When a new particle accelerator came online,

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some people anxiously asked,

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could it destroy the Earth or, even worse,

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rip apart the fabric of space?

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Well luckily, reassurance could be offered.

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I and others pointed out that nature

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has done the same experiments

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zillions of times already,

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via cosmic ray collisions.

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But scientists should surely be precautionary

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about experiments that generate conditions

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without precedent in the natural world.

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Biologists should avoid release[br]of potentially devastating

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genetically modified pathogens.

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And by the way, our special aversion

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to the risk of truly existential disasters

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depends on a philosophical and ethical question,

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and it's this:

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Consider two scenarios.

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Scenario A wipes out 90 percent of humanity.

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Scenario B wipes out 100 percent.

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How much worse is B than A?

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Some would say 10 percent worse.

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The body count is 10 percent higher.

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But I claim that B is incomparably worse.

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As an astronomer, I can't believe

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that humans are the end of the story.

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It is five billion years before the sun flares up,

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and the universe may go on forever,

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so post-human evolution,

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here on Earth and far beyond,

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could be as prolonged as the Darwinian process

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that's led to us, and even more wonderful.

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And indeed, future evolution[br]will happen much faster,

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on a technological timescale,

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not a natural selection timescale.

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So we surely, in view of those immense stakes,

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shouldn't accept even a one in a billion risk

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that human extinction would foreclose

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this immense potential.

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Some scenarios that have been envisaged

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may indeed be science fiction,

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but others may be disquietingly real.

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It's an important maxim that the unfamiliar

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is not the same as the improbable,

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and in fact, that's why we at Cambridge University

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are setting up a center to study how to mitigate

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these existential risks.

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It seems it's worthwhile just for a few people

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to think about these potential disasters.

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And we need all the help we can get from others,

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because we are stewards of a precious

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pale blue dot in a vast cosmos,

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a planet with 50 million centuries ahead of it.

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And so let's not jeopardize that future.

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And I'd like to finish with a quote

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from a great scientist called Peter Medawar.

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I quote, "The bells that toll for mankind

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are like the bells of Alpine cattle.

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They are attached to our own necks,

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and it must be our fault if they do not make

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a tuneful and melodious sound."

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Thank you very much.

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(Applause)