a stewardship of the Global Commons:
engaging “my neighbor” in the issue of sustainability
members of the Critical Issues Committee, Geological Society of America
ECOLOGICAL FOOTPRINTS AND CARRYING CAPACITY:
MEASURING OUR IMPACT
A. R. Palmer, Institute of Cambrian Studies, Boulder,
Every one of us requires a finite area of Earth’s surface
to support his/her existence. This is our Ecological Footprint (Wackernagel
and Rees, 1996). Its principal components are our food footprint, our
wood products footprint, and our degraded land footprint.
If I eat potatoes, in the course of a year I consume a
measurable quantity of potatoes. There is thus an area of potato production
somewhere in the world that is dedicated solely to me for my annual
consumption of potatoes. Ditto for every other terrestrial food product
I consume. That’s my food footprint. This footprint is not fixed in
size. I can change it by changing my eating habits – beef carries a
bigger footprint than chicken.
My use of printer paper, the packaging of the products
I buy, the magazines and newspapers I read, the wood in my furniture
and my home, and the firewood I consume if I have a fireplace, constitute
my personal wood products footprint. This puts real demands on an area
of the global forest that must be dedicated solely to me. However, this
footprint must also include my share of the wood products in the infrastructure
that supports me. I can change the overall footprint only a bit with
decisions about my personal consumption.
My degraded land
footprint is comprised of the area under my house and driveway. For
others, it maybe a part of the shared area under our apartment buildings
and adjacent parking lots. We also share a part of the land under our
city streets, businesses and public buildings and under the industrial
infrastructure that supports us, as well as a part of the land beneath
our highways, railroads, airports and garbage dumps. I can’t do too
much to change this, which is a reflection of our culture.
It is possible to calculate a semi-quantitative estimate
of our food, wood products and degraded land footprints and thus a measure
of our minimal land-use needs at current levels of consumption. If this
level of "need", when projected to the global population,
exceeds the available land areas of earth, we have a problem. On the
other side of this coin, if we decide on the desirability of a particular
level of consumption, we can get a rough idea of how many of us can
be supported at this level by the land resources at our disposal – i.e.,
the carrying capacity of the land.
In addition to the "accountable" elements of
our footprint cited below, there are other less tangible footprint elements
represented by our use of fossil energy and water. Approximately 50%
of the carbon dioxide we generate burning fossil fuels cannot be accommodated
by existing terrestrial or oceanic sinks. If we had to create new forest
to serve as a carbon dioxide sink, to keep the human contribution to
atmospheric carbon dioxide from increasing, we would need to more than
double the world’s area of forest – an improbable solution. And warming
oceans will hold even less carbon dioxide than they do now. It appears
that the human component of carbon dioxide buildup in the atmosphere
will remain with us until we stop burning fossil fuels. The footprint
effects of water use are more subtle. When the lower reaches of the
Yellow and Colorado rivers, for example, run dry because of upstream
human water use, this seriously impacts downstream ecosystems in ways
that are difficult to measure.
I have calculated
the accountable components of the per-capita Ecological Footprint for
the United States (Palmer, 1999). Our food footprint, using figures
from the U.S. Department of Agriculture and related sources is about
1.5 acres. In simple terms, this is obtained by determining yields in
pounds per acre for each foodstuff. That number is easily converted
to acres per pound. Data on our per-capita consumption of each foodstuff
in pounds is also available. The per-capita area required for each foodstuff
is then calculated by multiplying these two figures. The sum of the
resulting areas is our per-capita food footprint. Similarly, our annual
U. S. per-capita demand on the world’s forest for all wood products
needs is estimated to be 0.04 to 0.05 acres. This sounds trivial, but
that area cannot be reused until it has regrown. On average, this takes
about 40 years. Thus the estimated area of forest that must be dedicated
to each one of us to sustain our present level of wood products consumption
– our wood products footprint -- is about 1.6 acres (40 × 0.04 acres).
Our U. S. per-capita degraded land footprint is estimated to be about
0.4 acre. Therefore, the total ecological footprint for the average
American is a minimum of about 3.5 acres.
Lets put this in perspective. Earth has about 22 billion
acres of ecologically productive land. This is comprised of about 3.3
billion acres of arable and crop land, 8.4 billion acres of pasture
land, and 10.1 billion acres of forest land. Not all of the arable land
is of high quality, and improving agricultural productivity by use of
fertilizers and insecticides, or shifting to monocultural forestry,
affects ecosystems in other, often deleterious, ways. Expansion of land
use in any of those categories can only be done at the expense of one
of the other categories, and development of the land for human structures
of all kinds competes for this same area. Not only that, but we have
to share this land with the other organisms on Earth who might not be
able to tolerate our land use ‘improvement’ measures, or to survive
as a group as environmental fragmentation becomes extensive.
If we maintain our current footprint and the human population
of 2050 (estimated at 9 billion) reaches consumption levels similar
to ours, which is a practical goal for the developing world, humanity
would need 13.5 billion acres of land for food production and 14.4 billion
acres for wood products on a steady-state basis to be sustainable, and
we would have degraded about 3.6 billion acres for human structures.
For humans alone, excluding the needs of other organisms, there is not
that much land available simply by considering these three computable
sorts of personal footprints!
Furthermore, the food footprint calculations cited above
used U.S. yields, which are significantly higher than average global
yields. If global yields were used in those calculations, our food footprints
would be closer to 3 acres. Earth’s carrying capacity for a population
with 3-acre food footprints might be no more than about 4 billion people
(12 billion acres of arable, crop and pasture land ÷ 3). Each year more
of our most productive farmland is buried under human structures, and
both good and marginal farmland becomes unusable due to poor farming
practices, so even the estimate of a sustainable carrying capacity of
4 billion people eating and living as we do may be high.
The simple calculations cited above should raise some
warning flags that humanity already has a problem with the demands we
make on Earth. And we seem to be continuing our present course unabated!
Refinement of footprint and carrying capacity figures should be an ongoing
part of the process of evaluating and monitoring the sustainability
of the human enterprise.
DEMONSTRATION 1. Have
students estimate their annual consumption, in pounds, of various non-meat
food items that they eat most often (beans, corn, potatoes, apples,
etc.) – for meats, see Demonstration 2. On the Web there are data about
U.S. food production where yields for most common agricultural products
can be calculated in pounds/acre (sometimes with a little clever manipulation),
and these figures can then be converted to acres/pound.. Multiplying
the acres/pound figure by the student’s personal annual consumption
in pounds for each foodstuff gives the area of the Earth dedicated to
each individual for consumption of that food item. The Web also has
tables of data on U.S. annual per capita consumption of various foodstuffs
in pounds. The student can then compare her or his own footprint with
the U.S. footprint for the same product and begin a discussion about
whatever differences are found. The relevant websites are: www.usda.mannlib.cornell.edu
DEMONSTRATION 2. Do
a similar exercise to Demonstration 1 but regarding the beef footprint,
using the following data: Each beef animal on average needs 10 acres
of pasture; when the animal goes to a feedlot, it consumes grain equivalent
to 0.4 acre of a grainfield to reach the desired slaughter weight of
1,200 pounds. About half of that weight returns to the supermarket as
the beef that we buy. Thus, 600 pounds of beef at the supermarket had
a footprint of about 10.4 acres. What is the footprint of 1 pound? What
is the per capita level of beef consumption, in pounds, in the U.S.?
What is the student’s annual consumption of beef in pounds? Multiply
the annual consumption in pounds by the footprint for one pound to obtain
a beef footprint. Compare the two beef footprints, and also compare
them with the footprints of the agricultural products from Demonstration
1. Develop a general discussion of ways in which the food footprint
can be used to evaluate the impact that we make on earth just from our
eating habits. How might the food footprint be used to evaluate carrying
DEMONSTRATION 3. Our
footprints directly impact land areas that were balanced parts of the
natural ecosystem prior to the advent of human activities. Have students
consider what is lost or disrupted, versus what is gained, by conversion
of former forests, temperate grasslands (savannah), or semi-arid prairie
to human agricultural use or human habitations. How might we determine
when ecosystem losses outweigh human gains?
Palmer, A. R., 1999, Ecological Footprints: Evaluating Sustainability:
Environmental Geosciences, v.
6, p. 200-204.
and Rees, W., 1996, Our Ecological Footprint: Reducing Human impact
on the earth: Philadelphia,
PA, New Society Publishers, 160 p.
For a current discussion of
Ecological Footprints with most of the important literature citations,
check the March 2000 issue of the journal Ecological Economics (v. 32,
no. 3, pp. 341-394),which can be found in many larger university libraries.