that will provide the context for this week,

four words that will come to define

this century.

Here they are:

The Earth is full.

It's full of us, it's full of our stuff,

full of our waste, full of our demands.

Yes, we are a brilliant and creative species,

but we've created a little too much stuff --

so much that our economy is now bigger

than its host, our planet.

This is not a philosophical statement,

this is just science

based in physics,

chemistry and biology.

There are many science-based analyses of this,

but they all draw the same conclusion --

that we're living beyond our means.

The eminent scientists of the Global Footprint Network, for example,

calculate that we need about 1.5 Earths

to sustain this economy.

In other words,

to keep operating at our current level,

we need 50 percent more Earth than we've got.

In financial terms,

this would be like always spending 50 percent more than you earn,

going further into debt every year.

But of course, you can't borrow natural resources,

so we're burning through our capital,

or stealing from the future.

So when I say full, I mean really full --

well past any margin for error,

well past any dispute

about methodology.

What this means is our economy is unsustainable.

I'm not saying it's not nice or pleasant

or that it's bad for polar bears or forests,

though it certainly is.

What I'm saying

is our approach is simply unsustainable.

In other words, thanks to those pesky laws of physics,

when things aren't sustainable, they stop.

But that's not possible, you might think.

We can't stop economic growth.

Because that's what will stop: economic growth.

It will stop because of the end of trade resources.

It will stop because of the growing demand of us

on all the resources, all the capacity,

all the systems of the Earth,

which is now having economic damage.

When we think about economic growth stopping,

we go, "That's not possible,"

because economic growth is so essential to our society

that is is rarely questioned.

Although growth has certainly delivered many benefits,

it is an idea so essential

that we tend not to understand

the possibility of it not being around.

Even though it has delivered many benefits,

it is based on a crazy idea --

the crazy idea being

that we can have infinite growth

on a finite planet.

And I'm here to tell you the emperor has no clothes.

That the crazy idea is just that,

it is crazy,

and with the Earth full, it's game over.

Come on, you're thinking.

That's not possible.

Technology is amazing. People are innovative.

There are so many ways we can improve the way we do things.

We can surely sort this out.

That's all true.

Well, it's mostly true.

We are certainly amazing,

and we regularly solve complex problems

with amazing creativity.

So if our problem

was to get the human economy down

from 150 percent to 100 percent of the Earth's capacity,

we could do that.

The problem is we're just warming up

this growth engine.

We plan to take this highly-stressed economy

and make it twice as big

and then make it four times as big --

not in some distant future,

but in less than 40 years,

in the life time of most of you.

China plans to be there in just 20 years.

The only problem with this plan

is that it's not possible.

In response, some people argue,

but we need growth, we need it to solve poverty.

We need it to develop technology.

We need it to keep social stability.

I find this argument fascinating,

as though we can kind of bend the rules of physics

to suit our needs.

It's like the Earth doesn't care what we need.

Mother nature doesn't negotiate;

she just sets rules and describes consequences.

And these are not esoteric limits.

This is about food and water, soil and climate,

the basic practical and economic foundations

of our lives.

So the idea that we can smoothly transition

to a highly-efficient,

solar-powered, knowledge-based economy

transformed by science and technology

so that nine billion people

can live in 2050

a life of abundance and digital downloads

is a delusion.

It's not that it's not possible to feed, clothe and house us all

and have us live decent lives.

It certainly is.

But the idea that we can gently grow there

with a few minor hiccups

is just wrong,

and it's dangerously wrong,

because it means we're not getting ready

for what's really going to happen.

See what happens when you operate a system

past its limits

and then keep on going

at an ever-accelerating rate

is that the system stops working and breaks down.

And that's what will happen to us.

Many of you will be thinking,

but surely we can still stop this.

If it's that bad, we'll react.

Let's just think through that idea.

Now we've had

50 years of warnings.

We've had science proving

the urgency of change.

We've had economic analysis pointing out

that, not only can we afford it,

it's cheaper to act early.

And yet, the reality is

we've done pretty much nothing to change course.

We're not even slowing down.

Last year on climate, for example,

we had the highest global emissions ever.

The story on food, on water, on soil, on climate

is all much the same.

I actually don't say this in despair.

I've done my grieving about the loss.

I accept where we are.

It is sad,

but it is what it is.

But it is also time

that we ended our denial

and recognized

that we're not acting, we're not close to acting

and we're not going to act

until this crisis hits the economy.

And that's why the end of growth

is the central issue

and the event that we need to get ready for.

So when does this transition begin?

When does this breakdown begin?

In my view, it is well underway.

I know most people don't see it that way.

We tend to look at the world,

not as the integrated system that it is,

but as a series of individual issues.

We see the Occupy protests,

we see spiraling debt crises,

we see growing inequality,

we see money's influence on politics,

we see resource constraint, food and oil prices.

But we see, mistakenly, each of these issues

as individual problems to be solved.

In fact, it's the system

in the painful process of breaking down --

our system, of debt-fueled economic growth,

of ineffective democracy,

of overloading planet Earth,

is eating itself alive.

I could give you countless studies

and evidence to prove this,

but I won't because, if you want to see it,

that evidence is all around you.

I want to talk to you about fear.

I want to do so because, in my view,

the most important issue we face

is how we respond

to this question.

The crisis is now inevitable.

This issue is, how will we react?

Of course, we can't know what will happen.

The future is inherently uncertain.

But let's just think through what the science is telling us

is likely to happen.

Imagine our economy

when the carbon bubble bursts,

when the financial markets recognize

that, to have any hope

of preventing the climate spiraling out of control,

the oil and coal industries are finished.

Imagine China, India and Pakistan going to war

as climate impacts

generate conflict over food and water.

Imagine the Middle East without oil income,

but with collapsing governments.

Imagine our highly-tuned, just-in-time food industry

and our highly-stressed agricultural system failing

and supermarket shelves emptying.

Imagine 30 percent unemployment in America

as the global economy is gripped

by fear and uncertainty.

Now imagine what that means for you,

your family, your friends,

your personal financial security.

Imagine what it means

for your personal security

as a heavily armed civilian population

gets angrier and angrier

about why this was allowed to happen.

Imagine what you'll tell your children

when they ask you,

"So, in 2012, Mom and Dad,

what was it like

when you'd had the hottest decade on record

for the third decade in a row,

when every scientific body in the world was saying

you've got a major problem,

when the oceans were acidifying,

when oil and food prices were spiking,

when they were rioting in the streets of London

and occupying Wall Street?

When the system was so clearly breaking down, Mom and Dad,

what did you do, what were you thinking?"

So how do you feel

when the lights go out

on the global economy in your mind,

when your assumptions about the future

fade away

and something very different emerges?

Just take a moment

and take a breath

and think, what do you feel

at this point?

Perhaps denial.

Perhaps anger.

Maybe fear.

Of course, we can't know what's going to happen

and we have to live with uncertainty.

But when we think about the kind of possibilities I paint,

we should feel a bit of fear.

We are in danger, all of us,

and we've evolved to respond to danger with fear

to motivate a powerful response,

to help us bravely face a threat.

But this time it's not a tiger at the cave mouth.

You can't see the danger at your door.

But if you look,

you can see it at the door of your civilization.

That's why we need to feel our response now while the lights are still on,

because if we wait until the crisis takes hold,

we may panic and hide.

If we feel it now and think it through,

we will realize we have nothing to fear