Should we create a solar shade to cool the earth?

[A provocation from Danny Hillis:]

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

What if there was a way
to build a thermostat

that allowed you to turn down
the temperature of the earth

anytime you wanted?

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

everybody would be very excited about it,

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

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

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

And I think part of it

is because there are some real
misunderstandings about it.

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

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

and clear up some
of the misunderstandings.

So, the basic idea of solar geoengineering

is that we can cool things down

just by reflecting
a little bit more sunlight

back into space.

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

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

Everybody knows it's cooler under a cloud.

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

as the transparent air around it.

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

can cause a cloud to form.

We make artificial clouds all the time.

These are contrails,
which are artificial water clouds

that are made by the passing
of a jet engine.

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

By accident.

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

(Laughter)

But we are already doing this quite a lot.

This is a NASA picture of shipping lanes.

Passing ships actually cause
clouds to form,

and this is a big enough effect

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

So we already are doing solar engineering.

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

People have looked at everything,

from building giant parasols
out into space

to fizzing bubble waters in the ocean.

And some of these are actually
very plausible ideas.

One that was published recently
by David Keith at Harvard

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

where it reflects off sunlight.

And that's a really neat idea,

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

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

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

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

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

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

And that turns out to be
very easy to calculate.

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

(Laughter)

(Applause)

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

and it comes out with the same answer,

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

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

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

So what does that look like?

I can't visualize 10 teragrams per year.

So I asked the Cambridge
Fire Department and Taylor Milsal

to lend me a hand.

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

And that is how much

you would have to pump
into the stratosphere

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

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

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

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

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

into every Olympic
swimming pool full of rain.

It's almost nothing.

So why don't people like this idea?

Why isn't it taken more seriously?

And there are some
very good reasons for that.

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

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

who I respect a lot,

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

And the reason is that they're concerned

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

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

And I do worry about that.

I think it's actually a serious problem.

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

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

I certainly don't.

I love this planet, I really do.

And I don't want to mess with it.

But we're already changing our atmosphere,

we're already messing with it.

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

to mitigate that impact.

And we need to do research to do that.

We need to understand
the science behind that.

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

which is kind of, "fear versus hope,"

or "creativity versus caution."

And of course, we need both of those.

So there aren't any silver bullets.

This is certainly not a silver bullet.

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

that informs both
our creativity and our caution.

So I am an optimist
about our future selves,

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

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

is much greater than we imagine.

Thank you very much.

(Applause)

This talk sparked
a lot of controversy at TED2017,

and we encourage you
to look at discussions online

to see other points of view.