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[A provocation from Danny Hillis:]

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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's 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 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

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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 lines.

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So passing ships
actually cause clouds to from.

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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
from building giant parasols,

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on to space, 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 in 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 in the 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, you know, 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
bio department and Taylor Milsal

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to lend me a hand.

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And 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
that 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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And 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

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it makes sense for us to look for ways
to mitigate that impact.

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And that 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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you know, 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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and 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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Chris Anderson: Danny, you know we do have
a real-life expert on clouds with us.

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Do you mind if I invite her up onstage?

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Kate, do you mind
if I invite you back onstage?

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We can have a conversation about this.

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(Applause)

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So, Kate, what did you make
of Danny's talk?

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Kate Marvel: Denny, you seem so nice
and I hope we can be friends

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and you terrify me.

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(Laughter)

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I am very worried
about geoingeneering solutions

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for many different reasons.

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I feel like if I went to the doctor
and the doctor said, "You have a fever.

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I know exactly why you have a fever.

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We're not going to treat that,
we're just going to give you ibuprofen,

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and also your nose is going to fall off."

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I would be really upset
and I would want a new doctor.

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And I feel like dealing
with climate change

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by reducing the amount of sunlight we get

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is really problematic.

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Because there are physical reasons
to be frightened by that.

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For example, it's not going to do
anything about ocean acidification.

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And I care very much
about the health of the oceans.

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But I think -- I'm a physicist,
but I'm also a human being and a citizen.

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And I'm frightened as a citizen.

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Because, I feel like, when I want
to do an experiment,

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I have to go to my university

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and if there are human subjects,
I have to obtain informed consent

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from every single human subject.

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And I feel like what
you are proposing to do

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is involving all of us
as your human subjects

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and I'm not sure I consent to that.

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(Applause)

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Danny Hillis: I sympathize
with all of that.

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And I think it would be great
if we lived in a world

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where people had to get their consent
before messing with it,

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but you are already
in the middle of an experiment.

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KM: And I'm not happy
about that one, either.

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DH: Right, I'm not, either.

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People are already messing
with the atmosphere.

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And we don't really understand
the effects of it.

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So, we are geoingeneering.

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And so what I'm arguing for
is we should understand it,

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we should do the research
to understand it.

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I'm not claiming it's secure,
but it may be...

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It may be something that we need

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to help us cool down
while we fix the problem.

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And I don't think we can
throw away any options now.

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I think we really need
to get the knowledge.

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If we're going to do geoingeneering --
which we are going to do,

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even if it's a bad idea --

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we need to understand it
and we need to do a better job of it.

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CA: OK, this is the start of
an incredibly important conversation.

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Thank you, both of you,
we're going to continue this.

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KM: I hope we can still be friends.

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CA: Thank you, thanks Denny.

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(Applause)