United States
Environmental Protection
Agency
Office of Exploratory
Research
Washington DC 20460
Research and Development
EPA-600/S6-8 3-001 Sept. 1983
&EPA Project Summary
The Direct Biological Effects of
Increased Atmospheric Carbon
Dioxide Levels
Marilyn K. Bland, Howard C. Bailey, and Michael J. Lipsett
This report assesses the likely bio-
logical nonclimatic, direct effects of
carbon dioxide (CO2) on terrestrial and
aquatic ecosystems and on human
health. It summarizes the current liter-
ature on the direct effects of rising CO2
levels on the biosphere and identifies
technical information needs about
direct biological effects of rising CO2
levels. The environmental and human
health implications of these effects
were evaluated in the context of four
scenarios describing possible future
carbon dioxide levels to the year 2100.
The report discusses six categories
of CO2 control options: (1) increasing
the rate of carbon fixation or mass
transfer, (2) changing the fuel use
strategy, (3) changing energy use pat-
terns, (4) using emission source con-
trols, (5) using institutional controls to
adapt society, and (6) implementing
exotic strategies. No single option
emerged as the best approach to con-
trolling CO2. The best approach will
depend upon the specific problems
and resources in a given region. How-
ever, because of the global nature of
the problem, international cooperation
will be required for truly effective
control.
In order to make future policy deci-
sions regarding the problem of increas-
ing CO2 concentrations, we need a
better understanding of the basic bio-
logicial processes involved. Future re-
search should address the direct ef-
fects of increased C02 levels, rather
than the potential effects that may
occur if and when the climate changes.
Because aquatic species exhibit ad-
verse effects at lower concentrations
of CO2 than other organisms, they
should receive first priority in future
investigations.
This Project Summary was developed
by EPA's Office of Exploratory Re-
search, Washington, DC. to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Background
Carbon dioxide (C02) comprises approx-
imately 0.03% (by volume) of the atmos-
phere and plays a key role in the growth of
living organisms. The term, carbon cycle,
is used to describe the chain by which
carbon, in the form of C02, is assimilated
into plants through photosynthesis and
then returned to the atmosphere through
the process of decay. Animals are part of
the cycle because they release C02 through
respiration, consume plants, and excrete
products that decay and release C02. C02
is also released through combustion.
C02 is used in carbonated beverages,
refrigeration equipment and fire extin-
guishers. It is also used to enrich the
atmosphere in greenhouses, because the
rate of photosynthesis depends upon the
concentration of C02. The mean annual
C02 concentration of the global atmos-
phere has apparently been increasing
since the industrial revolution. Current
levels are approximately 330 parts per
million (ppm), an increase of 50 ppm over
levels associated with the preindustrial
atmosphere. Since systematic measure-
ments were begun in 1958, atmospheric
C02 content has increased 7%.
The major sources of the additional C02
are anthropogenic—largely from man's
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burning of fossil fuels, from deforestation
and agricultural practices, and from burning
nonfossilized wood and peat(seeTable 1).
Bolin (1970) notes that the carbon locked
in coal and oil exceeds the amount of
carbon in all living organisms by a factor of
50. According to the most probable models,
the percentage of the cumulative produc-
tion of C02 that will remain airborne 100
years from now will be between 46% and
80%.
Increased C02 levels are expected to
have far-reaching environmental effects,
because C02 traps heat in the atmosphere
and thereby produces the so-called "green-
house effect". Numerous studies have
reported potential climatic effects from
the carbon dioxide buildup.
This report addresses the nonclimatic,
direct biological effects that are likely to
result from a doubling of the present
atmospheric C02 levels. Potential effects
on terrestrial and aquatic ecosystems, as
well as on human health, are assessed.
The C02 issue is defined and scoped in
terms of public health and environmental
policy implications, and a forecast of future
environmental trends is presented. Major
information gaps and potential areas for
future research are also discussed.
Methods
To identify pertinent information sources
and data, internal files on C02 effects were
reviewed, and computerized literature
searches were performed on 23 data bases.
These findings were then compared with
results of a search on the direct effects of
C02 performed by the Oak Ridge National
Laboratory's Carbon Dioxide Information
Center using their Carbon Cycles and Cli-
mate Data Base. After the key articles were
critically reviewed and analyzed, principal
scientists currently active in investigating
the direct biological effects of enhanced
atmospheric C02 levels were interviewed,
and, in addition, a one-half day seminar on
carbon dioxide control options was con-
ducted.
Terrestrial Ecosystem Effects
At least two schools of thought concern-
ing the effects of increased carbon dioxide
levels on terrestrial ecosystems have been
expressed. One group is optimistic about
the fertilization effect of these enhanced
levels. This school stresses the well-estab-
lished knowledge that carbon dioxide is
essential for plant photosynthesis, and, up
to certain levels, is a vital resource. The
other school is more cautious. They believe
that we do not currently have enough
information to predict the long-term effects
of C02 enrichment. They stress that plant
growth is the result of the interaction of
many processes, any of which may limit
the growth rate, and all of which are
influenced by various environmental fac-
tors. For example, nutrients and light are
commonly limiting factors, particularly in
forest ecosystems. They also point out
that controlling the yield of the economical-
ly important part of a plant is a many-
faceted problem, and increased net photo-
synthesis does not necessarily translate
into increased yield.
The major studies on the response of
vegetables, field crops, and woody species
to increased atmospheric C02 have shown
that for greenhouse-grown vegetables and
flowers, enhanced C02 levels up to 1,000-
1,500 ppm result in increased crop yields,
earlier maturation, and better quality pro-
duce for certain species. At levels greater
than 2,000 ppm, photosynthesis com-
monly decreases and plant injury may
occur. However, some plant species have
been shown to respond positively to short-
duration exposures at much higher levels.
Most field crops have also shown increased
yields, although only a few long-term studies
have been conducted under field condi-
tions. Plants that metabolize C02 by means
of a three-carbon-acid pathway ("C3 plants")
seem better able to utilize higher levels of
C02 than those that fix carbon dioxide by a
four-carbon-acid pathway ("C4 plants").
Experiments on woody species have
shown that enhanced C02 levels up to
3,000 ppm have resulted in increased net
photosynthesis and growth; however,
almost all of the reports have concerned
short-term experiments on seedlings. It is
generally agreed that not enough is known
to assess the effects of increased C02
levels on entire forest ecosystems, in part
because of the complexity of such systems.
For example, field measurements have
shown large diurnal, seasonal, and vertical
natural variations in C02 levels in forests,
with concentrations nearing 600 ppm just
before dawn at the ground level in some
plant communities. Substantial diurnal and
seasonal variations in the normal C02
concentrations have also been measured
in agricultural fields. A few studies have
also indicated that if ambient C02 increases
concurrently with concentrations of phyto-
toxic gases, the C02 could potentially
ameliorate pollutant stress effects on photo-
synthesis.
Thus the evidence does not seem to
support one school of thought over the
others. What is clear is that the further one
moves from the controlled conditions of a
greenhouse to the highly complex environ-
ment of a forest, the more difficult it is to
assess the likely effects of increased C02
levels.
Doubling atmospheric C02 levels is ex-
pected to have little or no direct effects on
fauna. Animals are more likely to be in-
directly affected through their response to
the changes in the plant community that
may result from the increased C02 levels.
Aquatic Ecosystem Effects
The likely effects of increased levels of
atmospheric C02 on aquatic ecosystems
were assessed by analyzing the direct
toxic effects on aquatic life forms, the
potential nutrient effects that might stimu-
late primary productivity, and the possible
effects on water quality. Existing data
indicate that a projected concentration of
0.90 mg/l in equilibrium with an atmos-
pheric concentration of 600 ppm is not
likely to have direct toxic effects on aquatic
organisms. However, some fish species
are apparently sensitive to and may respond
to C02 levels as low as 0.88 mg/l. C02
concentrations as low as 2.1 mg/l may
have adverse effects on hatching success
in certain sensitive fish species. Although
levels as high as 10 mg/l are typically
found in catfish-rearing ponds, long-term
survival of certain fish species is known to
be reduced at 1 2 mg/l. An increase in
atmospheric C02 from 300 to 500 ppm is
likely to result in increased phytoplankton
productivity only in nutrient-rich areas.
These areas may respond with increased
algal blooms.
C02 affects water quality primarily by
contributing to the carbonate buffer system,
which, in turn, affects the pH of the water.
We predict that for C02 levels of 600 ppm,
the change in pH will be slight, approxi-
mately 0.03 pH units in water with an
initial pH in the range 7 to 8. These
changes should be well within the tolerance
ranges of almost all marine organisms.
However, a greater change is expected in
water with lower pH values. The most
sensitive aquatic ecosystems will probably
be freshwater lakes with limited water-
sheds and low buffering capacities. In
addition, cold water systems may be of
greater concern than warm water systems,
because C02 is more soluble at low tem-
peratures.
Human Health Effects
This report concludes that a doubling of
ambient concentrations of C02 to 600
ppm will probably have negligible human
health effects. Endogenous human pro-
duction of C02 generates an alveolar
(deep iung) concentration of about 5% of
the concentration of the atmosphere at sea
level-nearly two orders of magnitude
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Table 1.
Estimated Input of Carbon Dioxide into the Atmosphere from Anthropogenic and
Natural Sources (Adapted from Wong, 1978)
Source
Gross Input
(W>6 g Year-')
Net Input <3!
(10
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chronic exposures to carbon dioxide at
concentrations of 600 to 2,000 ppm.
Marilyn K. Bland, Howard C. Bailey, and Michael J. Lipsett are with SRI
International, Menlo Park, CA 94025.
Morris A. Levin is the EPA Project Officer (see below).
The complete report, entitled "The Direct Biological Effects of Increased
Atmospheric Carbon Dioxide Levels,"(Order No. PB 83-224 360; Cost: $14.50,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Office of Exploratory Research
U.S. Environmental Protection Agency
401 M Street, S. W.
Washington, DC 20460
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