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[It's time to start talking
about engineering our climate]

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What if there was a way
to build a thermostat

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that allowed you to turn down
the temperature of the earth

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anytime you wanted?

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Now, you would think if somebody
had a plausible idea about how to do that,

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everybody would be very excited about it,

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and there would be lots
of research on how to do it.

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But in fact, a lot of people
do understand how to do that.

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But there's not much support
for research in this area.

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And I think part of it

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is because there are some real
misunderstandings about it.

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So I'm not going to try to convince you
today that this is a good idea.

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But I am going to try to get
your curiosity going about it

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and clear up some
of the misunderstandings.

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So, the basic idea of solar geoengineering

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is that we can cool things down

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just by reflecting
a little bit more sunlight

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back into space.

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And ideas about how to do this
have been around literally for decades.

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Clouds are a great way to do that,
these low-lying clouds.

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Everybody knows it's cooler under a cloud.

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I like this cloud because it has exactly
the same water content

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as the transparent air around it.

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And it just shows that even a little bit
of a change in the flow of the air

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can cause a cloud to form.

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We make artificial clouds all the time.

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These are contrails,
which are artificial water clouds

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that are made by the passing
of a jet engine.

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And so, we're already changing
the clouds on earth.

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By accident.

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Or, if you like to believe it,
by supersecret government conspiracy.

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(Laughter)

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But we are already doing this quite a lot.

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This is a NASA picture of shipping lanes.

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Passing ships actually cause
clouds to form,

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and this is a big enough effect

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that it actually helps reduce
global warming already by about a degree.

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So we already are doing solar engineering.

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There's lots of ideas
about how to do this.

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People have looked at everything,

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from building giant parasols
out into space

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to fizzing bubble waters in the ocean.

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And some of these are actually
very plausible ideas.

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One that was published recently
by David Keith at Harvard

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is to take chalk and put dust
up into the stratosphere,

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where it reflects off sunlight.

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And that's a really neat idea,

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because chalk is one of the most
common minerals on earth,

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and it's very safe -- it's so safe,
we put it into baby food.

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And basically, if you throw chalk
up into the stratosphere,

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it comes down in a couple of years
all by itself, dissolved in rainwater.

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Now, before you start worrying
about all this chalk in your rainwater,

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let me explain to you
how little of it it actually takes.

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And that turns out to be
very easy to calculate.

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This is a back-of-the-envelope
calculation I made.

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(Laughter)

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(Applause)

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I assure you, people have done
much more careful calculations,

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and it comes out with the same answer,

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which is that you have to put chalk up
at the rate of about 10 teragrams a year

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to undo the effects of the CO2
that we've already done --

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just in terms of temperature,
not all the effects, but the temperature.

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So what does that look like?

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I can't visualize 10 teragrams per year.

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So I asked the Cambridge
Fire Department and Taylor Milsal

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to lend me a hand.

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This is a hose pumping water
at 10 teragrams a year.

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And that is how much

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you would have to pump
into the stratosphere

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to cool the earth back down
to pre-industrial levels.

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And it's amazingly little;
it's like one hose for the entire earth.

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Now of course, you wouldn't
really use a hose,

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you'd fly it up in airplanes
or something like that.

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But it's so little, it would be like
putting a handful of chalk

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into every Olympic
swimming pool full of rain.

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It's almost nothing.

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So why don't people like this idea?

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Why isn't it taken more seriously?

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And there are some
very good reasons for that.

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A lot of people really don't think
we should be talking about this at all.

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And, in fact, I have some
very good friends in the audience

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who I respect a lot,

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who really don't think
I should be talking about this.

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And the reason is that they're concerned

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that if people imagine
there's some easy way out,

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that we won't give up
our addiction to fossil fuels.

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And I do worry about that.

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I think it's actually a serious problem.

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But there's also, I think,
a deeper problem,

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which is: nobody likes the idea
of messing with the entire earth --

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I certainly don't.

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I love this planet, I really do.

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And I don't want to mess with it.

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But we're already changing our atmosphere,

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we're already messing with it.

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And so I think it makes sense
for us to look for ways

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to mitigate that impact.

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And we need to do research to do that.

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We need to understand
the science behind that.

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I've noticed that there's a theme
that's kind of developed at TED,

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which is kind of, "fear versus hope,"

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or "creativity versus caution."

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And of course, we need both of those.

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So there aren't any silver bullets.

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This is certainly not a silver bullet.

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But we need science to tell us
what our options are;

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that informs both
our creativity and our caution.

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So I am an optimist
about our future selves,

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but I'm not an optimist
because I think our problems are small.

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I'm an optimist because I think
our capacity to deal with our problems

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is much greater than we imagine.

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Thank you very much.

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(Applause)

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This talk sparked
a lot of controversy at TED2017,

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and we encourage you
to look at discussions online

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to see other points of view.

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[A provocation from Danny Hillis:]