Extracting and transforming energy requires land.
Extracting and transforming energy requires land.

Wind farms, coal mines, biofuel plantations--
Wind farms, coal mines, biofuel plantations--

how can we evaluate and compare the diverse land use
how can we evaluate and compare the diverse land use

footprints of energy technologies?
footprints of energy technologies?

One simple metric is power density, the rate at which energy
One simple metric is power density, the rate at which energy

is extracted per unit of land.
is extracted per unit of land.

One common measure of power density is watts of primary energy
One common measure of power density is watts of primary energy

per square meter of land surface.
per square meter of land surface.

Here is a rough comparison of power densities of various energy
Here is a rough comparison of power densities of various energy

technologies.
technologies.

First note that this graph has a log scale.
First note that this graph has a log scale.

A plantation that produces 10 tons of dry biomass per hectare per year
A plantation that produces 10 tons of dry biomass per hectare per year

has a power density of half a watt per square meter,
has a power density of half a watt per square meter,

while a coal mine that sprawls over 25 square kilometers
while a coal mine that sprawls over 25 square kilometers

and produces, say, 30 million tons of coal per year
and produces, say, 30 million tons of coal per year

has a power density of 1,000 watts per square meter.
has a power density of 1,000 watts per square meter.

This is a factor of 2000 times more than the biofuel plantation.
This is a factor of 2000 times more than the biofuel plantation.

Second note that the uncertainties in the numbers are huge,
Second note that the uncertainties in the numbers are huge,

and that different studies disagree.
and that different studies disagree.

Quantifying land use impacts is inherently more ambiguous
Quantifying land use impacts is inherently more ambiguous

than quantifying air emissions, toxics, or greenhouse gases.
than quantifying air emissions, toxics, or greenhouse gases.

Power density tells you nothing about how long the activity can go on for,
Power density tells you nothing about how long the activity can go on for,

about the total size of the energy resource.
about the total size of the energy resource.

The biofuel plantation can in principle keep
The biofuel plantation can in principle keep

producing forever, whereas the coal mine has to keep growing
producing forever, whereas the coal mine has to keep growing

or moving to keep accessing new coal.
or moving to keep accessing new coal.

So if you look over a long enough time horizon,
So if you look over a long enough time horizon,

it would take less land to produce energy for biofuels then coal.
it would take less land to produce energy for biofuels then coal.

But that time horizon is very long.
But that time horizon is very long.

Finally, note that the power density of renewable sources
Finally, note that the power density of renewable sources

is generally much less than the power density
is generally much less than the power density

of fossil fuels or nuclear power.
of fossil fuels or nuclear power.

This means that if humanity moves from fossil fuels to renewables
This means that if humanity moves from fossil fuels to renewables

in order to protect the climate, there will
in order to protect the climate, there will

be an increase in the energy systems' land footprint.
be an increase in the energy systems' land footprint.

We need to think about the environmental and social impacts
We need to think about the environmental and social impacts

of disturbing more land.
of disturbing more land.

I say disturbing, because land is never used up.
I say disturbing, because land is never used up.

Rather, it is disturbed or impacted by various human activities.
Rather, it is disturbed or impacted by various human activities.

This is what we mean when we say land use.
This is what we mean when we say land use.

How does land use matter?
How does land use matter?

Land is the ultimate finite resource.
Land is the ultimate finite resource.

Land use plays a central role in mediating
Land use plays a central role in mediating

many of the social and environmental impacts of energy extraction.
many of the social and environmental impacts of energy extraction.

And if other human activities like agriculture or urbanization
And if other human activities like agriculture or urbanization

impacts on water, endangered species, and societies
impacts on water, endangered species, and societies

are all linked to land use.
are all linked to land use.

To see what the numbers mean, it's helpful to think
To see what the numbers mean, it's helpful to think

about what fraction of land would be required
about what fraction of land would be required

to run a future energy system using various energy technologies.
to run a future energy system using various energy technologies.

Suppose the world will use 30 terawatts of energy late this century.
Suppose the world will use 30 terawatts of energy late this century.

And suppose it will be powered entirely by biofuels with a power
And suppose it will be powered entirely by biofuels with a power

density of 1 watt per square meter.
density of 1 watt per square meter.

This would require 30x10^12 square meters.
This would require 30x10^12 square meters.

The land area of the planet is only 150x10^12 square meters.
The land area of the planet is only 150x10^12 square meters.

So to run the whole energy system of biofuels will take about 20%
So to run the whole energy system of biofuels will take about 20%

of the total land surface of the Earth, more
of the total land surface of the Earth, more

than we now use for cropland-- a huge environmental impact.
than we now use for cropland-- a huge environmental impact.

Solar power has much higher power densities.
Solar power has much higher power densities.

The typical power density of current solar power
The typical power density of current solar power

systems in reasonably good locations is about 10 watts per square meter.
systems in reasonably good locations is about 10 watts per square meter.

Thus, the total land requirement of an all-solar system would be more like 2%
Thus, the total land requirement of an all-solar system would be more like 2%

of the earth's surface.
of the earth's surface.

Wind power is a more complicated story.
Wind power is a more complicated story.

It looks like the power density of very large wind farms
It looks like the power density of very large wind farms

is limited to about one watt per square meter.
is limited to about one watt per square meter.

However, the wind turbines and associated access roads and power lines
However, the wind turbines and associated access roads and power lines

take up a very small fraction of land inside the wind farm.
take up a very small fraction of land inside the wind farm.

If you count the entire area of the wind farm that is disturbed,
If you count the entire area of the wind farm that is disturbed,

then the power density of wind farms is roughly the same as biofuels.
then the power density of wind farms is roughly the same as biofuels.

But if you count only land directly occupied,
But if you count only land directly occupied,

then wind farms have a power density 100 times larger.
then wind farms have a power density 100 times larger.

There's no right answer, because there's no unambiguous metric
There's no right answer, because there's no unambiguous metric

of what it means to occupy land.
of what it means to occupy land.

The configuration of land use, particularly
The configuration of land use, particularly

whether the footprint is diffuse or concentrated,
whether the footprint is diffuse or concentrated,

plays a large role in determining impacts.
plays a large role in determining impacts.

Of concentrated development that occupies a single square block,
Of concentrated development that occupies a single square block,

may have less impact than a diffuse activity
may have less impact than a diffuse activity

that uses the same total footprint spread over lots of little footprints.
that uses the same total footprint spread over lots of little footprints.

Wind farms and natural gas fields, for example,
Wind farms and natural gas fields, for example,

are diffuse, while coal mines and biomass plantations are concentrated.
are diffuse, while coal mines and biomass plantations are concentrated.

Determining what you think about the land use footprint of dispersed energy
Determining what you think about the land use footprint of dispersed energy

production like natural gas or wind power
production like natural gas or wind power

depends on how you think about the impacts on the land
depends on how you think about the impacts on the land

adjacent to the wellhead or turbine.
adjacent to the wellhead or turbine.

Energy analysts often ignore land footprint
Energy analysts often ignore land footprint

because they're hard to quantify, and because human judgment and values
because they're hard to quantify, and because human judgment and values

enter in figuring out what to count as footprint.
enter in figuring out what to count as footprint.

One of the reasons I personally take nuclear power
One of the reasons I personally take nuclear power

seriously is that it has a very small land footprint.
seriously is that it has a very small land footprint.

And I happen to put a lot of value on leaving more land undisturbed.
And I happen to put a lot of value on leaving more land undisturbed.

You may have different views.
You may have different views.

What I hope I have done in this lecture is convince you
What I hope I have done in this lecture is convince you

that we must consider land footprint seriously
that we must consider land footprint seriously

in thinking about the future energy systems that
in thinking about the future energy systems that

are going to produce energy at the scale necessary to eliminate carbon dioxide
are going to produce energy at the scale necessary to eliminate carbon dioxide

emissions and power a high energy, late 21st century society.
emissions and power a high energy, late 21st century society.