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I'm here to talk about climate change,

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but I'm not really an environmentalist.

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In fact, I've never really
thought of myself as a nature person.

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I have never gone camping,
never gone hiking,

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never even owned a pet.

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I've lived my whole life in cities,

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actually just one city.

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And while I like to take trips
to visit nature,

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I always thought it was something
that was happening elsewhere,

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far away,

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with all of modern life
a fortress against its forces.

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In other words,

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like just about everybody I knew,

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I lived my life complacent

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and deluded

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about the threat from global warming.

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Which I took to be happening slowly,

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happening at a distance

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and representing only a modest threat
to the way that I lived.

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In each of these ways,

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I was very, very wrong.

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Now most people, if they were telling you
about climate change,

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will tell you a story about the future.

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If I was doing that, I would say,

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"According to the UN,
if we don't change course,

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by the end of the century,

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we're likely to get about four degrees
Celsius of warming."

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That would mean, some scientists believe,

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twice as much war,

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half as much food,

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a global GDP possibly 20 percent smaller
than it would be without climate change.

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That's an impact that's deeper
than the Great Depression,

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and it would be permanent.

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But the impacts are actually happening
a lot faster than 2100.

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By just 2050, it's estimated,

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many of the biggest cities
in South Asia and the Middle East

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will be almost literally
unlivably hot in summer.

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These are cities that today are
home to 10, 12, 15 million people.

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And in just three decades,

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you wouldn't be able
to walk around outside in them

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without risking heatstroke
or possibly death.

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The planet is now
1.1 degrees Celsius warmer

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than it was before industrialization.

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That may not sound like a lot,

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but it actually puts us entirely
outside the window of temperatures

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that enclose all of human history.

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That means that everything
we have ever known as a species,

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the evolution of the human animal,

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the development of agriculture,

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the development
of rudimentary civilization

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and modern civilization
and industrial civilization,

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everything we know about ourselves
as biological creatures,

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as social creatures,
as political creatures,

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all of it is the result
of climate conditions

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we have already left behind.

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It's like we've landed
on an entirely different planet,

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with an entirely different climate.

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And we now have to figure out

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what of the civilization
that we've brought with us

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can endure these new conditions

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and what can't.

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And things will get worse from here.

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Now for a very long time,

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we were told that climate change
was a slow saga.

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It started with the industrial revolution,

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and it had fallen to us

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to clean up the mess
left by our grandparents

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so our grandchildren
wouldn't be dealing with the results.

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It was a story of centuries.

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In fact, half of all of the emissions

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that have ever been produced
from the burning of fossil fuels

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in the entire history of humanity

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have been produced
in just the last 30 years.

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That's since Al Gore published
his first book on warming.

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It's since the UN established
its IPCC climate change body.

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We've done more damage since then

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than in all the centuries,
all the millennia before.

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Now I'm 37 years old,

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which means my life contains
this entire story.

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When I was born,
the planet's climate seemed stable.

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Today,

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we are on the brink of catastrophe.

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The climate crisis
is not the legacy of our ancestors.

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It is the work of a single generation.

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Ours.

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This may all sound like bad news.

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Which it is, really bad news.

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But it also contains, I think,

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some good news,
at least relatively speaking.

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These impacts are terrifyingly large.

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But they are also, I think, exhilarating.

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Because they are ultimately a reflection

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of how much power we have
over the climate.

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If we get to those hellish scenarios,

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it will be because
we have made them happen,

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because we have chosen
to make them happen.

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Which means we can choose
to make other scenarios happen, too.

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Now that may seem too rosy to believe

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and the political obstacles
are in fact enormous.

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But it is a simple fact --

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the main driver of global warming
is human action:

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How much carbon
we put into the atmosphere.

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Our hands are on those levers.

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And we can write the story
of the planet's climate future ourselves.

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Not just can -- but are.

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Since inaction is a kind of action,

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we'll be writing that story ourselves
whether we like it or not.

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This is not just any story,

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all of us holding the future
of the planet in our hands.

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It's the kind of story
we used to recognize only in mythology

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and theology.

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A single generation

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that has brought the future
of humanity into doubt

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now tasked with securing a new future.

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So what would that look like?

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It could mean solar arrays
barnacling the planet,

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really everywhere you looked.

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It could mean if we developed
better technology,

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we wouldn't even need
to deploy them that broadly,

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because it's been estimated
that just a sliver of the Sahara desert

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absorbs enough solar power
to provide all the world's energy needs.

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But we'd probably need
a new electric grid,

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one that doesn't lose
two-thirds of its power to waste heat,

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as is today the case in the US.

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We could use some more
nuclear power, perhaps,

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although it would have to be an entirely
different kind of nuclear power,

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because today's technology
simply isn't cost-competitive

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with renewable energy
whose costs are falling so rapidly.

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We'd need a new kind of plane,

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because I don't think
it's particularly practical

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to ask the entire world
to give up on air travel,

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especially as so much of the global South

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is, for the very first time,
able to afford it.

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We need planes that won't produce carbon.

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We need a new kind of agriculture.

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Because we probably can't ask people
to entirely give up on meat and go vegan,

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it would mean a new way of raising beef.

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Or perhaps an old way,

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since we already know
that traditional pasturing practices

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can turn cattle farms

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from what are called carbon sources,
which produce CO2,

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into carbon sinks, which absorb them.

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If you prefer a techno solution,

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maybe we can grow
some of that mean in the lab.

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Probably, we could also feed
some real cattle seaweed,

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because that cuts their methane emissions
by as much as 95 or 99 percent.

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Probably, we'd have to do
all of these things,

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because as with every aspect
of this puzzle,

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the problem is simply
too vast and complicated

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to solve in any single silver-bullet way.

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And no matter how many
solutions we deploy,

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we probably won't be able
to decarbonize in time.

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That's the terrifying math that we face.

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We won't be able to beat climate change,

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only live with it and limit it.

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And that means we'd probably need

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some amount of what are called
negative emissions,

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which take carbon
out of the atmosphere as well.

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Billions of new trees,
maybe trillions of new trees.

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And whole plantations
of carbon-capture machines.

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Perhaps an industry
twice or four times the size

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of today's oil and gas business

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to undo the damage that was done
by those businesses in past decades.

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We would need a new kind
of infrastructure,

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poured by a different kind of cement,

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because today, if cement were a country,

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it would be the world's
third biggest emitter.

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And China is pouring as much cement
every three years

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as the US poured
in the entire 20th century.

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We would need to build seawalls and levees

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to protect those people
living on the coast,

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many of whom are too poor
to build them today,

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which is why it must mean an end
to a narrowly nationalistic geopolitics

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that allows us to define the suffering
of those living elsewhere in the world

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as insignificant,

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when we even acknowledge it.

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This better future won't be easy.

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But the only obstacles are human ones.

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That may not be much of a comfort,

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if you know what I know
about human brutality and indifference,

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but I promise you,
it is better than the alternative.

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Science isn't stopping us
from taking action,

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and neither is technology.

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We have the tools we need today to begin.

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Of course, we also have the tools we need
to end global poverty,

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epidemic disease

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and the abuse of women as well.

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Which is why more than new tools,
we need a new politics,

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a way of overcoming
all those human obstacles --

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our culture, our economics,

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our status quo bias,

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our disinterest in taking seriously
anything that really scares us.

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Our shortsightedness.

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Our sense of self-interest.

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And the selfishness
of the world's rich and powerful

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who have the least incentive
to change anything.

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Now, they will suffer too,

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but not as much as those with the least,

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who have done the least 
to produce warming

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and have benefited the least

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from the processes that have brought us
to this crisis point

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but will be burdened most
in the decades ahead.

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A new politics

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would make the matter
of managing that burden,

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where it falls and how heavily,

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the top priority of our time.

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No matter what we do,
climate change will transform modern life.

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Some amount of warming
is already baked in and is inevitable,

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which means probably some amount
of additional suffering is, too.

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And even if we take dramatic action

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and avoid some of these
truly terrifying worst-case scenarios,

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it would mean living
on an entirely different planet.

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With a new politics, a new economics,

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a new relationship to technology

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and a new relationship to nature --

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a whole new world.

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But a relatively livable one.

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Relatively prosperous.

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And green.

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Why not choose that one?

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Thank you.

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(Applause)