So, I'm a climate scientist,
and if this room is representative of the country we live in,
that means about 60 percent of you, so maybe from about there over,
don't strongly trust me for information on the causes of climate change.
Now, I promise to tell the truth tonight,
but just to humor that demographic,
I've started this talk with a falsehood.
[The Paris Climate Accord is a product of the recognition
that climate change is a global problem ...]
This statement was not made by President Obama.
It was made by President Reagan,
and it wasn't about climate change and the Paris Climate Accord.
It was actually about the Montreal Protocol
and stratospheric ozone depletion.
Now, I'm sure that many of you aren't familiar with this environmental problem,
but you should be,
because it's a rare environmental success story.
And it's worth revisiting,
because sometimes, we need to examine the world we've avoided
in order to find guidance for the choices we make today.
So let's go back to the 1970s,
when some questionable choices were made:
first of all -- hoo -- hairstyles. (Laughs)
Second of all, objectively terrible quantities of hairspray,
and third, CFCs, chlorofluorocarbons,
man-made chemicals that were used as propellant in aerosol spray cans.
And see, it turns out these CFCs were a problem
because they were destroying the ozone layer.
Now I'm sure most of you have heard of the ozone layer,
but why does it matter?
Well, quite simply, the ozone layer is earth's sunscreen,
and it's really fragile.
If you could take all of the ozone,
which is mostly about 10 to 20 miles up above our heads,
and compress it down to the surface of the earth,
it would form a thin shell only about two pennies thick,
about an eighth of an inch.
And that thin shell does an amazing amount of work, though.
It filters out more than 90 percent of the harmful UV radiation
coming from the sun.
And while I'm sure many of you enjoy that suntan that you get
from the remaining 10 percent, it causes a lot of problems:
cataracts,
damage to crops,
damage to immune systems
and also skin cancer.
It's not an exaggeration
to say that a threat to the ozone layer is a threat to human safety.
And actually, ironically, it was human safety
that motivated the invention of CFCs in the first place.
You see, in the early days of refrigeration,
refrigerators used toxic and flammable chemicals
like propane and ammonia.
For good reason, the refrigeration industry
wanted a safe alternative,
and they found that in 1928,
when a scientist named Thomas Midgley
synthesized the first commercially viable CFCs.
And in fact, Midgley famously inhaled CFCs and blew out a candle
to demonstrate, at a scientific conference,
that they were safe and nonflammable.
And in fact, as a scientist, I can tell you there is no way
you could get away with that kind of antic today.
I mean, wow.
But really, at the time,
CFCs were a really remarkable invention.
They allowed what we now know as modern-day refrigeration
and air-conditioning and other things.
So it wasn't actually until over 40 years later, in the 1970s,
when scientists realized that CFCs would break down high in the atmosphere
and damage the ozone layer.
And this finding really set off a lot of public concern.
It led, ultimately, to the banning of CFC usage in aerosol spray cans
in the US and a few other countries in 1978.
Now, the story doesn't end there,
because CFCs were used in much more than just spray cans.
In 1985, scientists discovered the Antarctic ozone hole,
and this was a truly alarming discovery.
Scientists did not expect this at all.
Before the Antarctic ozone hole,
scientists expected maybe a five or 10 percent reduction
in ozone over a century.
But what they found over the course of less than a decade
was that more than a third of the ozone had simply vanished,
over an area larger than the size of the US.
And although we now know that CFCs are the root cause of this ozone hole,
at the time, the science was far from settled.
Yet despite this uncertainty,
the crisis helped spur nations to act.
So that quote that I started this talk with,
about the Montreal Protocol, from President Reagan --
that was his signing statement when he signed the Montreal Protocol
after its unanimous ratification by the US Senate.
And this is something that's truly worth celebrating.
In fact, yesterday was the 30th anniversary of the Montreal Protocol.
(Applause)
Because of the protocol,
ozone-depleting substances are now declining in our atmosphere,
and we're starting to see the first signs of healing in the ozone layer.
And furthermore, because many of those ozone-depleting substances
are also very potent greenhouse gases,
the Montreal Protocol has actually delayed global warming
by more than a decade.
That's just wonderful.
But I think it's worth asking the question,
as we face our current environmental crisis, global warming,
what lessons can we learn from Montreal?
Are there any? I think there are.
First, we don't need absolute certainty to act.
When Montreal was signed,
we were less certain then of the risks from CFCs
than we are now of the risks from greenhouse gas emissions.
A common tactic that people who oppose climate action use
is to completely ignore risk and focus only on uncertainty.
But so what about uncertainty?
We make decisions in the face of uncertainty all the time,
literally all the time.
You know, I'll bet those of you who drove here tonight,
you probably wore your seat belt.
And so ask yourself,
did you wear your seat belt because someone told you
with a hundred percent [certainty]
that you would get in a car crash on the way here?
Probably not.
So that's the first lesson.
Risk management and decision making always have uncertainty.
Ignoring risk and focusing only on uncertainty is a distraction.
In other words, inaction is an action.
Second, it takes a village to raise a healthy environment.
The Montreal Protocol wasn't just put together by industry and governments
or environmental advocacy groups and scientists.
It was put together by all of them.
They all had a seat at the table,
and they all played an important role in the solution.
And I think in this regard,
we're actually seeing some encouraging signs today.
We see not just environmental groups concerned about climate change
but also civic and religious groups,
the military and businesses.
So wherever you find yourself on that spectrum,
we need you at the table,
because if we're going to solve global warming,
it's going to take actions at all levels,
from the individual to the international
and everything in between.
Third lesson:
don't let the perfect be the enemy of the good.
While Montreal has become the brake pedal for stopping ozone depletion,
at its beginning, it was more just like a tap on the brakes.
It was actually the later amendments to the protocol
that really marked the decision to hit the brakes on ozone depletion.
So to those who despair
that the Paris Climate Accord didn't go far enough
or that your limited actions on their own won't solve global warming,
I say don't let the perfect be the enemy of the good.
And finally, I think it helps us to contemplate the world we've avoided.
Indeed, the world we have avoided by enacting the Montreal Protocol
is one of catastrophic changes
to our environment and to human well-being.
By the 2030s, we'll be avoiding millions of new skin cancer cases per year
with a number that would only grow.
If I'm lucky, I'll live long enough to see the end of this animation
and to see the ozone hole restored to its natural state.
So as we write the story for earth's climate future
for this century and beyond,
we need to ask ourselves, what will our actions be
so that someone can stand on this stage
in 30 or 50 or a hundred years
to celebrate the world that they've avoided.
Thank you.
(Applause)