Extracting and transforming energy requires land.
Wind farms, coal mines, biofuel plantations--
how can we evaluate and compare the diverse land use
footprints of energy technologies?
One simple metric is power density, the rate at which energy
is extracted per unit of land.
One common measure of power density is watts of primary energy
per square meter of land surface.
Here is a rough comparison of power densities of various energy
technologies.
First note that this graph has a log scale.
A plantation that produces 10 tons of dry biomass per hectare per year
has a power density of half a watt per square meter,
while a coal mine that sprawls over 25 square kilometers
and produces, say, 30 million tons of coal per year
has a power density of 1,000 watts per square meter.
This is a factor of 2000 times more than the biofuel plantation.
Second note that the uncertainties in the numbers are huge,
and that different studies disagree.
Quantifying land use impacts is inherently more ambiguous
than quantifying air emissions, toxics, or greenhouse gases.
Power density tells you nothing about how long the activity can go on for,
about the total size of the energy resource.
The biofuel plantation can in principle keep
producing forever, whereas the coal mine has to keep growing
or moving to keep accessing new coal.
So if you look over a long enough time horizon,
it would take less land to produce energy for biofuels then coal.
But that time horizon is very long.
Finally, note that the power density of renewable sources
is generally much less than the power density
of fossil fuels or nuclear power.
This means that if humanity moves from fossil fuels to renewables
in order to protect the climate, there will
be an increase in the energy systems' land footprint.
We need to think about the environmental and social impacts
of disturbing more land.
I say disturbing, because land is never used up.
Rather, it is disturbed or impacted by various human activities.
This is what we mean when we say land use.
How does land use matter?
Land is the ultimate finite resource.
Land use plays a central role in mediating
many of the social and environmental impacts of energy extraction.
And if other human activities like agriculture or urbanization
impacts on water, endangered species, and societies
are all linked to land use.
To see what the numbers mean, it's helpful to think
about what fraction of land would be required
to run a future energy system using various energy technologies.
Suppose the world will use 30 terawatts of energy late this century.
And suppose it will be powered entirely by biofuels with a power
density of 1 watt per square meter.
This would require 30x10^12 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%
of the total land surface of the Earth, more
than we now use for cropland-- a huge environmental impact.
Solar power has much higher power densities.
The typical power density of current solar power
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%
of the earth's surface.
Wind power is a more complicated story.
It looks like the power density of very large wind farms
is limited to about one watt per square meter.
However, the wind turbines and associated access roads and power lines
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,
then the power density of wind farms is roughly the same as biofuels.
But if you count only land directly occupied,
then wind farms have a power density 100 times larger.
There's no right answer, because there's no unambiguous metric
of what it means to occupy land.
The configuration of land use, particularly
whether the footprint is diffuse or concentrated,
plays a large role in determining impacts.
Of concentrated development that occupies a single square block,
may have less impact than a diffuse activity
that uses the same total footprint spread over lots of little footprints.
Wind farms and natural gas fields, for example,
are diffuse, while coal mines and biomass plantations are concentrated.
Determining what you think about the land use footprint of dispersed energy
production like natural gas or wind power
depends on how you think about the impacts on the land
adjacent to the wellhead or turbine.
Energy analysts often ignore land footprint
because they're hard to quantify, and because human judgment and values
enter in figuring out what to count as footprint.
One of the reasons I personally take nuclear power
seriously is that it has a very small land footprint.
And I happen to put a lot of value on leaving more land undisturbed.
You may have different views.
What I hope I have done in this lecture is convince you
that we must consider land footprint seriously
in thinking about the future energy systems that
are going to produce energy at the scale necessary to eliminate carbon dioxide
emissions and power a high energy, late 21st century society.