Download Is carbon di,oxide a `good` greenhouse gas?

Is carbon di,oxide a
‘good’ greenhouse
gas?
Effects of increasing carbon dioxide
on ecological systems
Eric D. Fajer and F.A. Bazzaz
Carbon dioxide (CO,) in the atmosphere, a greenhouse gas, may slso provide benefits for mankind: many plants
grow better under increasing CO2 concentrations. Indlvlduals have speculated that agricultural yields will increase up to 30% under future CO2
concentrations, and natural ecosystems will become more lush and resilient. However, Infertile conditions and
compkx ecological interactions often
limit improved plant growth under increasing CO2 atmospheres. Future policies to adapt to a CO,-rich world must
not overstate benefits from these conditions, nor ignore their usefulness for
increasing future agricuttural yields or
restoring degmded habitats.
The authors are with the Department of
Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02136, USA.
We thank the US Department of Energy for
supporting research on the direct effects of
elevated CO* concentrations on vegetation. Comments by S. Bassow of Harvard
University and especially Dr C. Rosenzweig of the Goddard Institute for Space
Studies at Columbia University greatly improved the manuscript. Any remaining
errors are the full responsibility of the authors.
Edltor’s note
This article provides an ecologist’s per+
pective on the impacts and consequences
of CO*-rich environments. It is a response
to Sherwood B. Idso’s Viewpoint article
continued on page 302
As interest in the impacts of anticipated global climate change increases,
policy makers and the public at large are being confronted with a wealth
of information and misinformation
about potential future impacts.
From these inputs, hazy crystal balls have yielded global change
forecasts ranging from the apocalyptic, in which warming temperatures
and disrupted hydrological cycles threaten civilization with coastal
inundation,
agricultural
disruption,
and cataclysmic
species
extinctions,* to the sanguine, in which potential temperature increases
are neutralized by negative feedbacks, and increasing atmospheric
carbon dioxide (CO-J concentrations
fertilize plant growth, thereby
greatly increasing agricultural productivity and the resiliency of natural
habitats.2 Sophisticated policy analysts often view the global climate
change issue as intermediate between these two extreme scenarios,
recognizing that profound negative environmental surprises can ensue
from anthropogenic
atmospheric alterations (ie stratospheric ozone
loss), yet appreciating that the uncertainties of climate models and
possible beneficial impacts of our changing atmosphere may render
substantial expenditures to combat greenhouse gas emissions wasteful,
if not inappropriate.’
How likely are these alternative scenarios of global change? The
probability of the apocalyptic scenario, if it can be reasonably estimated, will help determine how much societies should invest to limit its
occurrence. It may also be unwise to dismiss casually all elements of the
sanguine scenario of a greening of Earth, even if it seems simplistic or
merely too good to be true. Parts of the sanguine scenario may be
realistic and should be incorporated into policies to adapt to changing
climatic, as well as demographic, conditions.
In this paper we examine the ecological underpinnings of the sanguine
scenario. We explore whether CO 2, in contrast to methane, ozone,
chlorofluorocarbons
(CFCs) and nitrous oxide, is a ‘good’ greenhouse
gas, which, in increased atmospheric concentrations,
will lead to a
OQ59-3760~92/040301-10@ 1992 Butterworth-Heinemann Ltd
301
Is carbon dioxide a ‘good’ greenhouse gas?
continued from page 30 1
‘Carbon dioxide and the fate of Earth’,
Global Environmental Change, Vol 1, No
3, June 1991, pp 178-182.
‘Michael Oppenheimer and Robert H.
Boyle, Dead Heat, Basic Books, New York,
1990; Stephen H. Schneider, Global
basil:
Are We Entering the Greenhouse Century?, Sierra Club Books, San
Francisco, CA, 1989.
%herwood 8. Idso, ‘Carbon dioxide and
the fate of Earth’, Global Environmental
Change, Vol 1, No 3, June 1991, pp 178
182.
3For example, ‘Count before you leap’,
fhe Ec~omist, 7 July 1999, pp 21-24.
‘Idso, op tit, Ref 2.
5Fakhri A. Bazzaz, ‘The response of natural ecosystems to the rising global CO:!
levels’, Annual Review of Ecology and
Systematics, Vol 21, 1990, pp 167-196.
H.A. Mooney, B.G. Drake, R.J. Luxmoore,
W.C. Oechel, and L.F. Pitelka, ‘Predicting
ecosystem responses to elevated CO*
concentrations’, ffi&cience, Vol 41, No 2,
1991, pp 96-104; Boyd R. Strain and
Jennifer D. Cure, Direct Effects of Incfeasing Carbon Dioxide on Vegetation, US
Department of Energy, Washington, DC,
1985; Boyd R. Strain, ‘Direct effects of
increasing atmospheric COP on piants and
ecosystems’, Trends in Ecology and
Evo/ution, Vol2, NO 1, 1987, pp 18-21.
6J.T. Houghton, G.J. Jenkins, and J.J.
Ephraums, Climate Change.’ The lPCC
Scientific Assessment,
Cambridge University Press, Cambridge, 1990. (Note that
some General Circulation Models, such as
the Goddard Institute for Space Studies
(GISS) model, expect double equivalent
atmospheric CO;! conditions to cause significant temperature increases in tropical
regions - further, significant alterations in
regional hydrological regimes (ie more frequent droughts and storms) may ensue
even with reduced global warming.) J.
Hansen, 0. Rind, A. DelGenio, A. Lacis, S.
Lebedeff, M. Prather, R. Ruedy, and T.
greenhouse
climate
Karl, ‘Regional
effects’, in John C. Topping, ed, Coping
with Climate Change, Proceedings of the
Second North American Conference on
Preparing for Climate Change, Climate In-
stitute. Washinat~, DC. 1989, RR68-81.
‘Ban&, op cit:Ref 5; Strain anb‘cure, op
cit. Ref 5; Strain, op cit. Ref 5; Mooney et
al, op tit, Ref 5.
*Walter C. Oechel and Boyd R. Strain,
‘Native species responses to increased
atmospheric carbon dioxide concentrations’, in Boyd R. Strain and Jennifer D.
Cure, eds, Direct Effects of Increasing
Carbon Dioxide on V~etation,
US Department of Energy, Washington, DC, 1985, pp
118-154.
‘Houghton et al, op tit, Ref 6.
“Ibid.
“Bazzaz,
op tit, Ref 5; Bruce A. Kimball,
Carbon Dioxide and Agricultural Yield: An
Assemblage
and Analysis of 770 Prior
continued on page 303
302
of Earth and an improvement in the fate of humankind. If CO:!
is unconditionally a good greenhouse gas, then efforts and expenditures
to limit its atmospheric concentration will be counter-productive
to
improving the human condition. However, if the benefits resulting from
increasing COz conditions, such as increased crop yields, are realized
only under limited circumstances, and/or if certain environmental
adverse impacts, such as biological diversity reductions, accompany the
benefits, then we should not categorically promote the desirability of
increasing atmospheric CO2 concentrations.4 Instead, we must incorporate scientific findings of when natural and man-made ecosystems might
benefit from anticipated CO*-rich atmospheres into policies to adapt to
future environments.
Here, we report what recent scientific evidence tells us about how
anticipated COz-rich atmospheres will affect natural and man-made
ecosystems, even in the absence of climate change. This is a legitimate
line of enquiry because theoretical considerations and experimental
evidence show that CO*-rich atmospheres have real and considerably
direct impacts on vegetation, 5 and therefore on its wild, domesticated
and human consumers. Further, only atmospheric CO:! concentrations,
and not global temperatures, may increase significantly within the next
50 years or so, if the thermal capacitance of oceans delays global
warming, if one accepts the low end of the ‘Business as Usual’ estimate
of global warming of the Intergovernmental
Panel on Climate Change
(IPCC) - l.O”C increase every 50 years - or if one examines the effects
on specific global regions, such as the tropics.”
greening
CO2 fertilization and other positive effects
The fundamental biochemical process underlying life on Earth is
photosynthesis: the ability of plants and algae to use light energy to
transform CO2 and water into sugars. All else being equal, augmenting
the resources necessary for photosynthesis, such as through the provision of additional CO2 via fossil fuel burning and forest clearing, should
increase the amount of sugars (food) plants make for themselves during
photosynthesis, thereby increasing their growth.’ Further biochemical
considerations also suggest that increasing atmospheric COz concentrations should ‘fertilize’ plant growth: at higher COz concentration, the
rate of plant photosynthesis increases for most plant species such that
more sugars are produced per unit (quanta) of light absorbed. In some
experiments, photosynthetic efficiency increases 50% for plants grown
in COz-rich envir0nments.s
Not surprisingly, then, copious short-term experiments have demonstrated that individual plants exposed to COz-rich atmospheres (around
650-700 ppm - atmospheric concentration expected around 2075 by
IPCC’s Business as Usual scenario”) usually grow significantly larger
than their counterparts grown under current atmospheric concentrations (ie about 350 ppm).‘* Both wild and agricultural plants show this
response to COz-rich environments: under these conditions, wild plants
tested increased their vegetative growth (ie shoots, leaves, stems, roots)
by an average of 13%, and their reproductive output (ie flowers, seeds,
etc) by an average of 31%, while grain yields increased, on average, by
34%.”
In addition io CO+nduced short-term growth enhancement, called
the ‘CO2 fertilization effect’, other benefits accrue to plants growing in
GLOBAL
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Is carbon dioxide a ‘good’ greenhouse
continued from page 302
Water
Conservation
Observations,
Laboratory Report No 14, 1983, US Water
Conservation
Laboratory,
Phoenix,
AZ,
1983.
“Boyd
R. Strain and Fakhri A. Bazzaz,
‘Terrestrial plant communities’, in Edgar Ft.
Lemon, ed, CO, and Plants: The Response of Plants to Rising Levels of
Atmospheric
Carbon Dioxide, Westview
Press, Boulder, CO, 1983, pp 177-222;
R.J. Luxmoore,
‘CO2 and phytomass’,
Bioscience, Vol 31, No 9, 1981, p 828;
R.B. Thomas, D.D. Richter, H. Ye, P.R.
Heine, and B.R. Strain, ‘Nitrogen dynamics and growth of seedlings of an N-fixing
tree (Gliricida sepium (Jacq.) Walp.) exposed to elevated carbon dioxide’, Oecoloyia, Vol 88, No 3, 1991, pp 415421.
3S.C. Wong, ‘Elevated atmospheric partial pressure of CO2 and plant growth’,
Oecofogia, Vol44, No 1, 1979, pp 68-74;
see also Strain and Cure, op tit, Ref 5;
Strain, op tit, Ref 5; Bazzaz, op tit, Ref 5.
IdPeter M. Ray, The Living P/ant, Sander
College
Publishing,
Philadelphia,
PA,
1972.
15Roger W. Carlson and Fakhri A. Bazzaz,
‘The effects of elevated CO2 concentrations on growth, photosynthesis, transpiration, and water use efficiency of plants’, in
J.J. Singh and A. Deepak, eds, Environmental and Climatic Impact of Coal Utilization, Academic Press, New York, 1989, pp
609-623.
“%arlson and Bazzaz, op tit, Ref 15.
“Roger W. Carlson and Fakhri A. Bazzaz.
‘Photosynthetic
and growth response to
fumigation with SO2 at elevated CO2 for C3
and C4 plants’, Oecotogia, Vol 54, No 1,
1982, pp 5&54.
“W .D . Bowman and B.R. Strain, ‘Interaction between CO2 enrichment and salinity
stress in the Cq halophyte Andropogon
glomeratus (Walter) BSP’, Plant, Cell and
Environment, Vol 10, 1987, pp 267-270.
“L.H. Ziska, B.G. Drake, and S. Chamberlain, ‘Long-term photosynthetic
response
in single leaves of a C3 and Cq salt marsh
species grown at elevated atmospheric
do2 in situ’, Oecofogia, Vol 83, No 4,
1990. DD 469-472:
P.D. Curtis. B.G.
Drake, ‘P.W. Leadley, W. Arp, and D.
Whigham,
‘Growth and senescence
of
plant communities
exposed to elevated
CO2 concentrations
on an estuarine
marsh’, Oecotogia, Vol 78, No 1, 1989. pp
2&26;
Sherwood B. ldso and Bruce A.
Kimball, ‘Effects of atmospheric CO2 enrichment on photosynthesis,
respiration
and growth of sour orange trees’, Plant
Physkgy,
Vol 99, No l,-1992, pp 341343: David T. Tissue and Walter C.
Oe&el, ‘Response of Eriophorum vaginaturn to elevated CO2 and temperature in
the Alaskan tussock tundra’, Ecology, Vol
68, No 2, pp 401410:
Richard J. Norby,
Carla A. Gunderson, Stan D. Wullschleger,
E.G. O’Neil, and Mary K. McCracken, ‘Productivity and compensatory
responses of
yellow-poplar trees in elevated COP’. Nacontinued on page 304
gas?
CO&h
environments.
Plants can use the additional sugars made
under higher CO2 concentrations to feed symbiotic organisms associated with their roots which enhance plant nutrition.‘* The two most
important symbiotic organisms are mycorrhizal fungi (eg many forest
mushrooms), which attach themselves to plant roots and help plants
forage the soil for nutrients, especially phosphorous, and nitrogen-fixing
bacteria, associated with certain plants such as legumes (eg soybeans)
and alders, which convert normally inaccessible nitrogen from the
atmosphere into forms which plants can absorb and use, such as nitrate.
In a CO*-rich atmosphere, plants associated with root symbionts will
have more sugars to feed them and in exchange should receive
additional nutrients, thereby improving their growth.
In addition to producing more sugars, virtually all plants consume less
water per unit leaf area when grown in higher CO2 concentrations.‘”
Normally, in the process of letting CO2 diffuse into the interior of leaves
(where it is taken up as part of the photosynthetic process), water is lost
to the outside air at a rate of 100-400 water molecules to every CO2
molecule taken in.14 Under CO*-rich conditions, this rate of water loss
can be greatly curtailed (eg by 30% for soybeans and 50% for maize”),
thereby reducing the amount of water a plant needs to develop: it has
been suggested that this increased water-use efficiency will enable plants
to expand their ranges into drier areas.16
A CO*-rich atmosphere also promotes a more subtle way to increase
plant growth. The same physiological mechanism which limits water loss
under CO*-rich conditions also protects plants from some pollutant
damage. For example, for many plant species, damage from sulphur
dioxide (SO,), a common emission from coal burning and an important
antagonist causing acid rain, is reduced under increasing CO2 atmospheres because fewer SO2 molecules enter the interior of leaves.” In
addition, CO*-rich environments may help alleviate the negative effects
of other environmental stresses: increasing CO2 atmospheres enables
some plants to overcome salinity stress, common to plants growing near
arid, marine or formerly irrigated habitats.lx
An unresolved question and other ecological complications
A basic and still unresolved question is whether short-term COzinduced increases in photosynthesis and plant growth can be maintained
for plants grown over the long term - an especially relevant query for
the issue of whether forest trees will benefit from a CO*-rich atmosphere. Experimental evidence about the sustainability of CO*-induced
photosynthesis and growth increases has yielded mixed results: marsh
grasses from the nutrient-rich Chesapeake Bay region and sour orange
trees from experimental plots significantly increase their growth over
several growing seasons (at most, five years thus far) whereas the
response of tundra sedges from nutrient-poor northern Alaska and US
deciduous trees (ie oaks, birches, yellow-poplar, and some maples)
from glasshouse experiments declines over time.”
Even if CO,-induced growth responses are sustainable over the long
term, under many circumstances other complexities inherent in ecological systems will dampen potential beneficial aspects of increasing
atmospheric CO2 concentrations. These complexities include the fertility of the local growing environment; the specific identity of the plant (ie
species); the modification of plant growth by the presence of neighbour-
GLOBAL ENVIRONMENTAL CHANGE December 1992
303
Is carbon dioxide a ‘good’ greenhouse gas?
~ntinu~
from page 303
tore, Vol367, i8 May 1992, pp 322-324;
F.A. Bazzaz and S.L. Mao. ‘Enhancement
of growth in high COP environments declines in 2nd.year tree seedlings’, Nature,
1992, in press.
“For
example, Arthur Ft. Zangerl and
Fakhri A. Bazzaz, ‘The response of plants
to elevated CO,: II. Competitive interactions among annual plants under varying
light and nutrients’, Oeco/ogia, Vol 62, No
3, 1984, pp 412-417; William E. Williams,
Keith Garbutt and Fakhri A. Bazzez, ‘The
response of plants to elevated CO,: V.
Performance of an assemblage of serpentine grassland herbs’, ~n~fo~mentat
Experimental Botany, Vol28, No 2, 1988, pp
123-130.
“For a more detailed description of the
biochemical and anatomical differences
between CB and Cq plants, see Fakhri A.
Bazzaz and Eric D. iajer, ‘Plant life in a
CO,-rich world’, ~~enti~c American. Voi
266: No 1, January 1992. pp 68-74.
22Bazzaz, op tit, fief 5; Strain and Cure,
0~ cit. Ref 5: Strain. oo cit. Ref 5: Moonev
et al, op cit. -Ref 5.
’
.
“For example, Fakhri A. Bazzaz and Roger W. Carlson, ‘The response of plants to
elevated CO,: I. Competition among an
assemblage of annuals at two leveis of soil
moisture’, Oeco@ia,
Vol 62, No 2, pp
196-198;
Ed G. Reekie and Fakhri A.
Bazzaz, ‘Competition and patterns of resource use among seedlings of five trooical trees grown $ ambient and elevated
C02’, Oecologia, Vol 79, No 2, 1989, pp
212-222.
24For example, William E. Williams, Keith
Garbutt, Fakhri A. Bauaz, and Peter M.
Vitousek, ‘The response of plants to elevated COP: IV. Two deciduous tree communities’, Oecotoaia, Vol 69, No 3, 1986,
pp 454-459; Williams et al, op tit, Ref 20.
25Lucy St Omer and Steven M. Horvath,
‘Elevated carbon dioxide concentrations
and whole plant senescence’, Ecology, Vot
64, No 5,1983, pp 1311-1314; Keith Garbutt and Fakhri A. Bazzaz, ‘The effects of
elevated COz on plants: Ill. Flower, fruit
and seed production and abortion’, New
Phytologist, Vol 98, 1984, pp 433-446.
=Garbutt and Bazzaz, op cit. Ref 25.
“D E Lincoln, D. Couvet, and N. Sionit,
*Re&&tse
of an insect herbivore to host
plants grown in carbon dioxide enriched
atmospheres’,
Oeco/ogia, Vol 69, No 4,
1986, pp 556-560; Eric D. Fajer, M. Deane
Bowers, and Fakhri A. Bazzez,
‘The
effects of enriched carbon dioxide atmospheres on plant-insect herbivore interactions’, Science, Vot 243, 1989, pp 11981200.
304
ing plants; and the interactions with pollinators and herbivores. For
example, experimental work has demonstrated that plants need a fertile
growing environment to respond positively to increasing CO2 concentrations. In low nutrient habitats, the CO2 fertilization effect is greatly
diminished, if not entirely lost. 20 This might partially explain the results
mentioned above, in which marshland grasses from high nutrient
Chesapeake sites maintained Cot-induced growth enhancement whereas sedges from the infertile Alaskan tundra did not.
Another complicating factor is the fact that not all plant species
increase their growth in a C02-rich atmosphere. Ecologists call this
phenomenon ‘species-specific growth responses’ to a high CO2 environment. One predictable factor influencing whether a plant species will
grow larger in C02-rich conditions is the type of biochemical machinery
a plant uses to capture CO2 for photosynthesis. Plants possessing C3
biochemistry, such as wheat, rice and all trees, usually increase their
growth much more in C02-rich conditions than those possessing C4
biochemistry, such as corn, sugarcane and many dryland grasses.*’ Yet,
for reasons we do not yet understand, even certain C3 plants species
grow much better than others in response to an increased CO2
atmosphere.22
These species-specific growth responses are accentuated when plants
are grown together in competition, as they commonly do in nature.
Plants which are more responsive to CO?-rich environments grow
better, often at the expense of less responsive plant species.2” More
responsive species grow faster and co-opt other resources necessary for
growth (light, water and nutrients), thereby restricting the growth of
species less responsive to increasing CO2 atmospheres. Therefore,
under competitive conditions, the total ‘biomass’ (ie the amount of
living tissue) of plant communities frequently does not increase under
COz-rich ~onditions;2~ rather, only certain plant species grow better,
producing hierarchies which may have different dominant species from
communities grown under current ambient CO2 conditions.
The interactions between plants and their animal pollinators and
herbivores (ie plant eaters) are likely to change under CO*-rich
conditions in ways which might limit the importance of the CO2
fertilization effect. In response to these conditions, the timing of plant
development, called ‘phenology’, often changes. For example, many
annual plant species, those which set seed and die after one growing
season, often produce flowers earlier when grown in C02-rich
conditions.25 Ecologists have speculated that shifting flowering times
might disrupt the synchrony between peak flowering times and peak
pollinator abundance.26 Under circumstances in which the number of
pollinators limits plant seed set, shifting flowering times could severely
reduce the number of seeds produced by certain species. Clearly, one
generation of increased plant growth under C02-rich conditions becomes less relevant for species maintenance if reduced seed numbers limit
the population of the next generation of plants.
In response to CC&-rich atmospheres, plants not only change the
amount and rate at which they grow and flower, but also their quality ie the chemical composition of their tissues. Specifically, plants grown in
increasing CO2 atmospheres have lower concentrations of proteins in
their leaves than those grown under today’s CO2 concentrations.”
From
the perspective of an herbivorous animal, this means that the nutritional
quality of plants grown in C02-rich conditions is lawer. Studies using
GLOBAL
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Is carbon dioxide
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gas?
insect herbivores, such as grasshoppers and caterpillars of moths and
butterflies, have demonstrated that insects eat dramatically more leaves
of plants grown in CO*-rich conditions, presumably to compensate for
their lower protein concentration. 28 For example, in three separate
experiments, a common western grasshopper, Melanoplus differentialis,
ate from 3674% more sagebrush leaves which grew in a C02-rich
atmosphere. 29 Given these remarkable increases in insect consumption
under C02-rich conditions, it seems reasonable to wonder whether
C02-induced increases in plant growth will simply end up in the stomachs
of herbivores.
Lower leaf protein concentrations do more than stimulate herbivores
to eat more. Performance usually declines when insects are reared an
entire lifecycle on low-protein, high C02-grown plants.“’ ‘Fitness’,
measured as the insect’s survival rate, how long it takes to mature, and
its final size and potential fecundity (ie how many offspring it is likely to
produce), often declines for insect herbivores reared on plants grown in
C02-rich conditions. Conceivably, these fitness reductions may result in
smaller populations of certain insect herbivore species - perhaps good
news for some previously consumed plants, but less beneficial for butterfly populations or predatory animals which eat caterpillars and grasshoppers.
Implications
Agriculture in a C02-rich world
Perhaps the best way to examine agricultural productivity under future
atmospheric conditions is to determine when the benefits of a C02-rich
environment might be garnered. Essentially, crops grown at 650-700
ppm can be expected to increase their yields by lO-35%, provided that
plants grow in fertile conditions, water is supplied in adequate amounts,
competition from weeds is minimized, increased consumption by insect
pests is minimized, and unpredictable climatic extremes (eg drought,
hot spells, frosts) are rare. When might these circumstances occur?
As mentioned earlier, scientists do not expect atmospheric concentrations to reach 650-700 ppm until around the year 2075. Because energy
use and population growth will probably grow exponentially,
the
presence of higher atmospheric CO2 concentrations
will be skewed
closer toward 2075 than to the present (ie according to IPCC’s Business
as Usual energy-use scenario, the atmosphere will reach 500 ppm CO2
around 2040 and 600 ppm CO2 around 2060);3’ lower atmospheric CO2
concentrations, even around 500 ppm, often have less dramatic effects
on plant growth.32 In other words, crop productivity increases in
response to a C02-rich environment will not reach lO-35% during most
lifetimes of today’s agricultural scientists and policy analysts (though
even small yield increases will be welcomed). Unfortunately, the global
scientific consensus expects significant climate change (a mean global
temperature increase of 1.2”C) by 2030 or so, when atmospheric CO2
concentrations are expected to be only 450 ppm.33 Conceivably, however, crop breeders and experts in biotechnology can select for strains
which are more responsive to lower atmospheric CO2 concentrations (ie
between 350 and 450 ppm) - perhaps a suitable goal for agricultural
research.
Assuming that the climatic extremes do not negate the CO2 fertilization effect (but see Hansen et al”), we can only assume that crop
GLOBAL ENVIRONMENTAL CHANGE December
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Is carbon dioxide a ‘good’ greenhouse gas?
2*W.L.A. Osbrink. J.T. Trumble, and R.E.
Wagner, ‘Host suitability of Phase&s
lunata for Trichop/usia ni (Lepidoptera:
Nootuidae) in controlled atmospheres’, Environmental Entomology Vol 16, 1967, pp
639-644; R.H. Johnson and D.E. Lincoln.
‘Sagebrush and grasshopper responses to
atmospheric carbon dioxide concentration’, &co/ogia, Vol 64, No 1, 1990, pp
103-l 10; Lincoln et al, op tit, Ref 27; Fajer
et a/, op cif, Ref 27.
=Johnson and Lincoln, op tif, Ref 26.
30For examele. Faier et al. ODcif. Ref 27.
31Houghton’ et’al, bp cif, def’6.
=For example, R. Hunt, D.W. Hand, M.A.
Hannah. and A.M. Neal. ‘Resuonseto CO,
enrichment in 27 herbacedus species’:
Functional Ecology, Vol5, No 3, 1991, pp
410-421.
%Houghton et al, op tit, Ref 6.
34Hansen et al, op cif, Ref 6.
35J.S. Boyer, ‘Plant productivity and environment’, Science, Vol 216, 1992, pp
443446.
%Data from David Pimental, Cornell University, cited in Peter Weber, ‘A place for
pesticides’, World Watch, Vol 5, No 3,
May/June 1992, pp 16-25.
37David T. Patterson, Elizabeth P. Flint,
and Jan L. Beyers, ‘Effects of CO* enrichment on competition between a C4 weed
and a C3 crop’, Weed Science, Vol 32,
1964, pp 369-394.
%Lincoln et al, op cif, Ref 27; Fajer et al, op
tit, Ref 27.
%Data from George P. Georghiou, University of California, Riverside, cited in
Weber, op tit, Ref 36.
40Fajer et al, op tit, Ref 27.
41Peter W. Price, Carl E. Bouton, Paul
Gross, Bruce A. McPheron, John N.
Thompson, and Arthur E. Weis, ‘Interactions among three trophic levels: Influence
of plants on interactions between insect
herbivores and natural enemies’, Annual
Review of Ecology and Systematics, Vol
11, 1960, pp 41-65.
306
productivity will increase on fertile sites, or for farmers with adequate
access to fertilizers (given the other caveats mentioned above and
described in detail below). Low-input farmers, or poorer farmers in the
developing world, are less likely to benefit from a C02-rich atmosphere.
However, for tropical development projects, the incorporation
of
nitrogen-fixing legume trees into agro-forestry projects may be even
more successful in the future: increasing CO2 atmospheres are likely to
lead to greater nitrogen capture by legumes. This nitrogen (as ammonia
or nitrate) will then diffuse into agricultural soils and fertilize them.
Under these circumstances, food crops from agro-forestry plots may
show a CO2 fertilization effect.
Water availability is also a crucial feature to increasing yields in an
increased CO2 atmosphere.35 Although drought tolerance of plants
generally increases under C02-rich conditions, presence or absence of
adequate water supplies will have a much greater influence on crop
yields than will atmospheric CO* concentration. CO*-rich conditions are
likely to increase yields on fertile, dryland sites; however, yield increases from increased water supplies (ie ‘via irrigation) would likely
dwarf yield increases in response to CO*-rich conditions. Conversely,
CO*-rich conditions will not save farmers from the ravages of drought,
although crop yield depressions occasionally might not be as severe.
Assuming ideal atmospheric and soil conditions for CO2 fertilization
of crops, CO*-rich environments will also impact crop pests and weeds,
thereby potentially minimizing farm yield increases under these conditions. For 1986 US farmers, crop competitors (weeds, diseases and
insects) led to over a 35% harvest 10~s.~~How might this change in a
COz-rich world? When a C02-responsive C3 crop, such as soybean, was
grown together with C4 weed johnsongrass, soybean actually outgrew
the weed more under C02-rich conditions.37 Hypothetically, however,
if one grew C4 crop, such as corn, with a C02-responsive C3 weed, we
would anticipate the opposite result - that the weed would outperform
the crop under COz-rich conditions. Under competitive conditions
between crops and weeds in which the physiological dice are not loaded
one way or the other, the expectation would be that the inference of
competing weeds under COz-rich conditions would limit crop yield
enhancement. To benefit from increasing CO2 conditions, then, it may
be necessary to eliminate the competitor weeds, in all likelihood, via
herbicide application. Again, poorer farmers may be unable to afford
needed herbicides to benefit from a C02-rich world.
Insect pests may present even greater problems under future CO;?rich conditions. If wild pest species behave comparably to those studied
experimentally, they are likely to eat dramatically more leaves under
increasing CO2 atmospheres, 38 thereby reducing CO*-induced yield
increases. Insecticide applications may become even more essential to
gain the advantage of a COz-rich environment. Due to the environmental and economic costs of insecticides, and the growing resistance of
insect pests to these chemicals (ie currently there are 500 pest species
resistant to insecticides worldwide”‘), it may be difficult to eliminate
anticipated additional crop loss. However, due to the slower development of insect pests on low-protein, high CO*-grown plants,40 integrated pest management (IPM) techniques may be more effective:
slower development times render insects more susceptible to natural or
introduced predators and parasitoids.41
In summary, it is incorrect to assume automatically that all crops will
GLOBAL ENVIRONMENTAL CHANGE December 1992
Is carbon dioxide a ‘good’ greenhouse gav
increase their yield by l&35% in a CO*-rich atmosphere. Only under
specific conditions of soil fertility, water supply, climate moderation,
and weed and insect pest control can we attain these yield increases.
High input ‘industrialized’ agriculture can probably create many of the
conditions necessary to benefit from a COz-rich atmosphere, although
the economic and environmental costs of developing CO*-responsive
varieties and IPM techniques, applying chemical fertilizers, herbicides
and pesticides, and diverting and consuming water from rivers and
aquifers, have yet to be rigorously assessed.
Further, the assumption that ‘full’ CO2 fertilization will occur concurrently with climate change is unsupportable: we do not yet understand
how higher temperatures,
altered hydrological regimes and increased
atmospheric CO* concentrations interact to affect crop yields. However, it is likely that increased COZ concentrations
will not totally
compensate, let alone overcompensate, for lost crop productivity resulting from other sub-optimal growth conditions. Similar conclusions were
drawn by Rosenzweig in her assessment of future agricultural production in the US Great Plains under CO*-rich conditions.42
Assuming crop productivity does increase by 35% worldwide (in
addition to other assumed crop improvements and innovations), is this
necessarily a big deal? Are the benefits of an increased CO2 atmosphere
worth the other risks of dramatic climatic changes? By 2075, when we
are more likely to garner a crop yield of 35%, the population of the
globe will be between 10 and 11 billion people (given a medium fertility
projection with a replacement-level
fertility of 2.06).43 This is approximately double our current global population
(about 5.4 billion
people). 44 Crop yield increases due to CO2 fertilization would facilitate
feeding more people and, by this logic, would be ‘a big deal’; however,
these potential C02-induced
crop yield increases alone would be
drastically insufficient to feed a burgeoning population.
Natural ecosystems in a C02-rich world
When will natural ecosystems and their component species benefit from
an increasing CO2 atmosphere? Here, we use a simple anthropocentric
viewpoint of natural ecosystem ‘benefits’ because there is not an
equivalent ecological perspective. Thus, ‘benefits’ accrue (to humans)
when natural ecosystems store more carbon (especially if they store
excess carbon added to the atmosphere by humans), maintain a greater
number and/or variety of plant and animal species, and are more
resilient in the face of human-induced disturbances, such as pollution
and climate change.
By this definition, only plant communities from fertile sites, such as
marsh grasslands or habitats on volcanic soils, will have the resources to
‘*Cynthia Rosenzweig, ‘Potential effects respond positively, in terms of growth, to a C02-rich world. Even under
of climate change on agriculturalproduc- these conditions, recent experimental evidence demonstrates that plants
tion in the Great Plains, a simulation are not guaranteed
to increase their growth over the long term.
study’, in J.B. Smith and D.A. Tirpak, eds,
Yellow-poplar
(Liriodendron
tulipifera) seedlings grown for three years
The Potential Effects of Climare Chanae
on the US, US Environmental Protect&
under elevated CO2 conditions were no larger than their ambient
A$ncy, Washington,DC, 1990, pp 3-9-3C02-grown counterparts,
and red oak seedlings could not maintain
higher
relative
growth
rates
under C02-rich conditions, suggesting that
‘%Vorld Resources Institute, World Resources
15X72-93, Oxford University their ability to store more carbon in their tissues than ambient C02Press, Oxford, 1992.
grown plants diminishes as they age.45 Although scientists have specu“Ibid.
lated that increased forest productivity, especially in temperate regions,
45Norby et al, op tit, Ref 19; Bazzaz and
Mao, op cir, FM 19.
has been sequestering and will continue to sequester ‘extra’ carbon
GLOBAL ENVIRONMENTAL CHANGE December 1992
307
Is carbon dioxide a ‘good’ greenhouse
gas?
46Wilfred M Post. Tsung-Hung Peng, William Ft. Emanuel, Anthony W. King, Virginia H. Dale, and Donald L. DeAngelis, ‘The
global carbon cycle’, American Scientist,
Vol78, 1990, pp 31g-326; Pieter Y. Tans,
lnez Y. Fung, and Taro Takahashi. ‘Observational constraints on the global atmospheric COP budget’, Science, Vol 247,
1990. pp 1431-1438.
47H.H. Shugart, AM Y.A. Antonovsky, P.G.
Jan&, and A.P. Sandford, ‘COP, climate
change, and forest ecosystems’, in Bert
Bolin, Bo Ft. Doos, Jill Jager, and Richard
A. Warrick, eds, The Greenhouse Effect,
Climate Change and Ecosystems, Scope
29, John Wiley and Sons, Chichester,
1986, pp 476-521.
92tis
ef al, op tit, Ref 19.
%.P.
Smith, B.R. Strain, and T.D. Sharkey, ‘Effects of COP enrichment on four
Great Basin grasses’, Functional Ecology,
Vol 1, No 2,1987, pp 139-143.
%Jerry M. Melillo, John D. Aber, and John
F. Muratore, ‘Nitrogen and lignin control of
hardwood leaf litter decomposition dynamics’, Ecology, Vol 63, No 3, 1982, pp
621-626.
308
liberated into the atmosphere from anthropogenic sources,46 seedling
responses from simplified experimental conditions warn us that additional carbon storage (via greater plant growth) is not guaranteed in a
future high-CO2 world.
Further, under more complex natural settings, competitive interactions between neighbouring plants are likely to diminish growth improvements in response to enriched CO2 environments.47 At the species
level, the disproportionate response of some plant species to a COz-rich
environment, often at the expense of less responsive species, means that
all species will not benefit from future atmospheric conditions, or that
plant communities in total will not increase their. carbon storage
capacity. The decline of less responsive species, such as many C4 plants,
seems likely in certain plant communities, with rarer less responsive
species becoming endangered, if not extinct over time. The loss of C4
species seems especially likely in low-diversity habitats, such as the salt
marsh grasslands of the Chesapeake Bay.48
Changes in species composition can induce second-order changes in
the functioning of entire ecosystems. For example, when exposed to a
C02-rich environment, Bromu.s rectorurn, a weedy grass, grew much
better than three other grass species which share its rangeland Great
Basin habitat.49 Because Bromus predisposes land to burning, its
potential increase in future Great Basin communities may lead to more
frequent and/or severe wildfires. Increased wildfire frequency would
change which species (both plants and animals) could survive in this
habitat, potentially leading to local extinctions, as well as release more
CO2 and other pollutants into the atmosphere.
Biodiversity reductions, the less benign element of a COz-rich world,
may also result from the decline in the protein concentration of leaves
from high CO*-grown plants. Herbivorous insect performance is greatly
determined by the nutritional quality of its diet; therefore, if a general
lowering of plant nutritional quality occurs in a COz-rich world,
populations of herbivorous insects are likely to decline. Again, currently
rare insect species, as well as those specialist insect herbivores which
depend upon less CO*-responsive plant species for food (as leaves,
pollen or nectar), may become endangered if not extinct over time. The
ramifications of declining herbivorous insects for the plants that they
pollinate or the predators they nourish are unknown but the consequences are potentially damaging for complex ecological interactions and
food webs.
Declining leaf protein (ie nitrogen) concentrations in high CO*-grown
plants may also impact future soil fertility. In a COz-rich environment, if
dead plant matter, called ‘litter’, has reduced protein concentrations in
parallel to its living precursors, it is likely that future soil fertility will
decline. Lack of nitrogen sources constrains the rate at which soil
bacteria and fungi decompose dead material.50 Under CO*-rich conditions, reductions in litter nitrogen concentration will slow down the rate
of decomposition - the rate of nutrient liberation by these microorganisms. More nutrients will therefore remain trapped in litter, and
this will be inacessible to plants. Clearly, if soil fertility declines in a
COz-rich world, growth increases under these conditions will be minimized. However, if temperatures
do increase and soil moisture is
maintained, decomposition rates will increase, potentially negating the
effect of reduced nitrogen in litter. Biologists are actively researching to
ascertain whether effects of higher temperatures increasing decomposi-
GLOBAL
ENVIRONMENTAL
CHANGE
December 1992
Is carbon diaxide a ‘good’ greenhouse gas?
51Gaius R. Shaver, W. Dwight Billings, F.
Stuart Chapin, AMe E. Giblin, Knute J.
Nadelhoffer, Walter C. Oechel, and EB.
Rastetter, ‘Global change and the carbon
balance of arctic ecosystems’, Bioscience,
Vol42, No 6,1992, pp 433-441. (Note that
increased respiration rates from higher
temperatures may lead to increases in soil
fertility as formerly trapped nutrients are
liberated and made available to growing
plants. This increase in soil fertility could
increase plant productivity, thereby increasing an ecosystem’s capacity to
‘scrub’ COP from the atmosphere. Further,
plants sometimes respond to increasing
CO2 atmospheres by lowering their dark
respiration rates (ie the rates at which they
maintain their life processes while liberating CO*). Under these conditions, living
plants could release less COP from their
own metabolic processes while simultaneously increasing their rate of CO2 uptake via photosynthesis. Under this scenario, future ecosystems would have a
greater capacity to scrub CO* from the
atmosphere than they do presently (see
ldso and Kimball, op tit, Ref 19; J.S.
Amthor, ‘Respiration in a future higherCO, world’, Plant Cell and Environment.
Vol-14, No 1, 1991, pp 13-20).
52Bazzaz and Fajer, op cif, Ref 20; Robert
L. Peters and Joan D.S. Darling, ‘The
Greenhouse Effect and nature reserves’,
Bioscience, Vol35, No 11, 1985, pp 70771 7.
-Houghton 81 al, op cit. Ref 6.
“For example, Norman J. Rosenberg,
‘The increasing CO, concentration in the
atmosphere and its implication on agricultural productivity: II. Effects through
CO,-induced climate change’, Climatic
Change, Vol 4, 1962, pp 239254; R.A.
War&k, R.M. Gifford, with M.L. Parry,
‘CO*, climatic change and agriculture’, in
Bert Bolin, Bo R. Does, Jill Jager, and
Richard A. Warrick, eds, The Greenhouse
Effect, Climatic Change, and Ecosystems,
Scope 29, John Wiley and Sons, Chiches-
ter, 1986, pp 395-473.
tion rates, or increasing CO* concentration decreasing these rates, will
predominate in future natural ecosystems.
To summarize, current ecological evidence suggests that increasing
COz atmospheres will induce changes in the components and functioning of natural ecosystems. However, many of these changes may not be
beneficial, let alone benign, as suggested by the sanguine scenario of a
COz-rich world. We will not witness a greening of Earth under these
future atmospheric conditions. Rather, due to the limits imposed by low
soil fertility and water supply, most natural areas will not become more
lush. Hypothesized changes in the frequency of fires or the rates at
which nutrients are liberated could further affect ecosystems in ways
which benefit some species at the expense of others, or conceivably,
reduce total ecosystem productivity.
Only pockets of fertile natural areas will increase their productivity,
potentially at the cost of some elements of plant species diversity, and a
reduction in herbivorous insect diversity and density. The implications
of reduced herbivorous insect numbers for upper trophic level organisms (ie those that eat herbivorous insects, and so on), as well as
reduced pollinator numbers for certain plant species, are unclear
although conceivably destabilizing. Finally, all bets are off on ecosystem
response if rapid climate change occurs. Fertile marsh grasslands cannot
grow more if they are under 30 cm of water from rising seas; temperature increases and especially changing rainfall patterns will compromise
plant responses to increasing CO* atmospheres; and finally, under
warmer conditions, microbial respiration rates and the decay of dead
matter will increase, liberating additional CO2 into the atmosphere;
thereby adding a potential critical positive feedback loop to the global
atmosphere-biosphere
system.51
From the perspective of natural ecosystem response, it is premature
at best, and irresponsible at worst, to assume that CO*-rich environments will benefit natural ecosystems. Further, if COz-induced climate
change is rapid, we expect major disruptions in the functioning of
terrestrial
natural ecosystems,
including possible severe losses of
species.52 Under those circumstances, CO? clearly would not be a ‘good’
greenhouse gas.
Conclusions - evaluating the sanguine scenario
In evaluating the validity or likelihood of the sanguine scenario for
global climate change, we do not feel qualified to dispute its first part:
the range of global warming rates generally accepted by expertss3
However, we feel more than justified in asserting that, in the absence of
dramatic climatic changes, increasing concentrations of atmospheric
CO* will not provide (1) a widespread agricultural panacea; (2) a world
replete with more lush natural habitats; or (3) guaranteed additional
forest growth to absorb carbon added to the atmosphere by fossil fuel
burning. Put simply, according to the best experimental evidence, CO*
is not, unconditionally.
a good greenhouse gas: only under certain
limited circumstances will humans benefit from the fertilizing effect of
increasing CO2 atmospheres on crops and wild plants, including the
condition that climate change does not occur too rapidly.
Further, we fear that recent models and discussions concerning the
agricultural responses to C02-rich conditions, which, for example,
routinely assume lO-30% crop yield increases,54 do not adequately
GLOBAL ENVIRONMENTAL CHANGE December 1992
Is carbon dioxide a ‘good’ greenhouse gas?
consider the ecological circumstances which could severely limit a
COTfertilization effect. Consequently, these provide little guidance as
to how human societies should adapt to these future environments.
Instead, we welcome analyses which outline limitations of their assumptions, and include the careful delineations of when the benefits of a
CO*-rich atmosphere accrue, plus the potential economic and environmental costs (ie fertilizer and pesticide use) necessary to gamer these
-Richard M. Adams, Cynthia Rosenz- benefits.”
Fortunately, when planning to adapt to our future environment, we
weig, Robert M. Pear-t, Joe T. Ritchie,
Bruce A. &Carl, J. David Glyer, R. Bruce can incorporate
the knowledge of when increasing atmospheric CO2
Curry,James W. Jones, KennethJ. Boote.
and L. Hartwell Allen, ‘Global climate concentrations enhances plant growth. Benefits to agriculture will occur
change and US agriculture’, Nature, Vol under the proper conditions, especially if we gear current agricultural
345. 1990. DD 214-223: ML. Parrv. J.H.
development, management and research efforts to this end. For examPorter,and T.R. Carter, ‘~gr~u~ure~~lirnatic change and its implications’,Trends in ple, we recommend breeding or engineering crops to respond positively
to atmospheric concentrations the world is likely to experience within 50
Ecologyand Evolution, Vol5, No 9, 1990,
pp 318-322; for a detailed regional and years, rather than 100 years. In the case of natural habitats, restoration
global treatment of potential world agricufturalproductivityunder climate change of degraded and/or desertified sites may proceed more rapidly under
in which the ~~~ti~
of the CO, ferti- C02-rich conditions, especially if more CO*-responsive
and drought
liiation effect is carefully delineated, see
tolerant species are initially used. These benefits from our anticipated
Cynthia Rosenzweig, Martin Parry, GunC02-rich world will be most welcomed by the 10-11 billion future
ther Fishcer, and Klaus Frohberg,C/&ate
human inhabitants, as well as whatever semblance of wildlife survives
Change and World Food Supply: A Preliminary Report, Environmental Change the intensification
of land use on our planet. That is no excuse for
Unit, University of Oxford, May 1992,
overstating
the
benefits,
nor for belittling the potential risks of CO*-rich
manuscript.
ssldso,op tit, Ref 2.
atmospheres.56
310
GLOBAL ENVIRONMENTAL CHANGE December 1992