Your Carbon Footprint
Philip H. Crowley, Department of Biology, University of Kentucky
Release of CO2 from fossil fuels into the atmosphere is the main cause of climate change. The
implications of these changes— such as global shifts in agriculturally productive locations and water
access, increases in climate variability, rising sea levels—are enormous, and lifestyle changes are
inevitable. This raises the question of how much each of us contributes to the problem, YOU in
particular. One way to ask this question is “How big is your carbon footprint”?
To address this, we need to do some bookkeeping on CO2. When the atmosphere is the source of CO2,
then uptake by some process (= influx) that ultimately leads to re-release (= efflux) doesn’t change the
balance sheet of atmospheric CO2. Photosynthesis is generally considered a balanced process of this
type, because the CO2 used to make organic compounds by plants, animals, and microbes is ultimately
re-released through decomposition processes. Another example is the influx of CO2 into solution in
oceans, lakes, and rivers, from which almost all soon returns to the atmosphere by outgassing.
There is, however, a major net flow of CO2 into the atmosphere that originates as fossil fuels. Coal, oil,
and natural gas sequester carbon in mineral deposits; but when these fuels are mined and burned, all of
the resulting CO2 is efflux, contributing directly to the atmospheric concentration. So another way to
pose the question about your carbon footprint is to ask how much CO2 are you responsible for releasing
into the atmosphere that didn’t originate in the (recent) atmosphere? (Discuss why it doesn’t “count”
that this CO2 may have come from the atmosphere 200,000,000 years ago.) It turns out that there are
many ways in which you make these contributions, and most of them have to be determined as your
proportion of some source to which you make part of a total group contribution. See the large diagram
at left on the next page.
Some of your annual CO2 contributions come from major home utilities and appliances. Most of these
may be electrical, in which case you need to know how many kilowatt-hours is your proportion of your
housemates’ usage, how much coal or natural gas it takes in your area to generate a kilowatt-hour of
electricity, and how much CO2 is released into the atmosphere when that much fuel is burned. Then you
multiply that by the number of hours in a year. If your heat comes entirely or partly from fuel oil or
liquid propane, then you must determine the equivalent numbers for these sources at your rate of use.
CO2 contributions to the atmosphere from your travel are based on CO2 release from the burning of
refined petroleum per mile of travel, times the number of miles you travel per year, discounted to take
account of others who share these CO2 travel costs with you. “Other stuff”, also suitably pro-rated,
includes public utilities like street lights, pumping of the domestic water supply, and trucks that pick up
garbage; recreation incorporates running the lights and machines at the gym you frequent, lights and
grounds-keeping at public parks, and lights and sound at the stadium; workplace usage is computers,
lights, heat, and heavy machinery; and “items purchased” are included, because these are often powerexpensive to manufacture and may travel long energy-devouring distances to get to you.
ANNUAL CO2
CONTRIBUTION
PER PERSON
AIR
CONDITIONING
HEATING
HOME
UTILITIES AND
APPLIANCES
ATMOSPHERIC
CO2
CONCENTRATION
OTHER HOME
ELECTRICAL
USE
POPULATION
SIZE
CAR
AIRPLANE
TRAVEL
TRAIN
BUS
PUBLIC
UTILITIES
RECREATION
OTHER
STUFF
WORKPLACE
ITEMS
PURCHASED
YOUR ANNUAL
CO2 RELEASE
YOUR ANNUAL
CONTRIBUTION TO
ATMOSPHERIC CO2
Putting all of these components together on an annual basis determines your annual CO2 release into
the atmosphere. Since 48% of this is eventually sequestered in the deep oceans through the death of
plankton, your contribution to the yearly build-up of CO2 is the other 52% of your annual release
amount. In case your contribution seems like a modest number to you, keep in mind that the USA has a
current population size of slightly over 316 million (in early 2014). Suppose your number is typical of the
American population (at least roughly); then what happens when you multiply it by 316,000,000? The
US population is currently growing at about 0.7% per year. Suppose you factor that in and look at the
increase in atmospheric CO2 over the next 50 years from US contributions alone. There are about 7
billion people on the planet at present, and fortunately the average earthling contributes only about 1/3
of the US amount. [Note: the current estimate of annual world-wide release of CO2 from fossil fuels is
23.8 x 1015 g, of which 12.4 x 1015 g winds up in the atmosphere.] Suppose (optimistically) that this
proportion of 1/3 stays constant over the next 50 years. How much extra CO2 would be contributed
from the whole world population if world population growth rate stays constant at the current 1.17%
per year?
Ideally, this can all be approached by having students go online for the numbers to make these
calculations, sharing them so that each person can make appropriate adjustments, and using Simulink
and Stateflow to organize the data and conduct the 50-year simulations. Note that translating CO2
efflux rates into CO2 concentrations requires taking account of the total current mass of CO2 in the
atmosphere (2.79 x 1018 g) and current concentration of atmospheric CO2 (400 PPM). Students should
collect some scientific estimates of what the major effects of particular higher values of atmospheric
CO2 might be expected to do. The results should lead to discussions about the magnitude of the
problem, and how to address it.