Effect of carbon dioxide on plants

E. Linacre


The increase of atmospheric CO2 concentration will have many effects on crops and other plants, including the following (1) -

  1. Gaseous ‘fertilising’ of crops by doubling CO2 to 600 ppm is expected to enhance productivity by at least 10 - 15 %, and even more in drier regions. In natural ecosystems a larger biomass and a more rapid vegetation cycling will result from an increase in atmospheric CO2 concentration, ceteris paribus.
  2. Water-use efficiency will improve since partially closed stomata on the crop’s leaves will still take in the usual amount of CO2 for plant growth, but the throttling of the stomata lessens the loss of water vapour from within the leaves (2). Thus, either a given amount of growth will require less rain, or a given rainfall may be expected to yield more crop; which option is chosen by a plant will vary with rainfall patterns and plant species. On the other hand, less evaporation from the stomata implies less evaporative cooling, which means higher leaf temperatures during the daytime. This may have a slight enhancing effect on global warming, in warm, wet land environments, during the daytime.
  3. The efficiency of photosynthesis depends on the CO2 vapour pressure gradient across the stomata, so an improvement will occur in the use of the available solar energy. In other words, slightly greater crop yields can be expected even when water is not the limiting factor.
  4. A higher CO2 vapour pressure would also increase the mineral use efficiency, in particular that of potassium and calcium. In many soils, in particular in the tropics, mineral deficiency is the main growth-limiting factor.
  5. The global warming, and reduced daily temperature range, resulting from increased greenhouse gas concentrations is likely to enhance the direct effect of CO2 on plant growth, especially in mid- and high-latitude regions.
  6. In a CO2-enriched atmosphere plants appear to be more susceptible to frost damage (3). At least this is the case for snow gums, a kind of eucalypt that is particularly tolerant of frost. Whereas 34% of leaves are damaged by frost in normal circumstances, this rises to 68% if the CO2 is doubled. The incidence of frost may be reduced by global warming, but not eliminated.
  7. The ratio of carbon to nitrogen (and phosphorous) in plant tissue should increase, and this will reduce the concentration of nitrogen-rich compounds such as proteins. In particular, this would reduce the protein content of wheat grain, however adequate soil fertilisation can compensate for this effect.
  8. In most regions of a double CO2 world, there would be more leaf litter in forests. Litter production would be larger because of the enhanced plant growth, and then its decomposition would be inhibited by the increased C/N ratio of the leaf tissue, though accelerated by extra warmth if there is adequate moisture. A thicker layer of humus would result if the litter production temporally exceeds its decay. This would reduce erosion but increase the threat of serious forest fires.
  9. Global warming will lead to increased evaporation in some areas, causing drier soils and hence less growth, and, in other regions, higher rainfalls which may outweigh the evaporation increase. The extra warmth so far is apparently chiefly in the form of higher nocturnal minimum temperatures. This would protect some fruit crops, especially seed fruits, from spring frost damage, yet it would inhibit the ‘setting’ of some stone fruits. In the case of rice, higher temperatures reduce yields by inducing sterility of the spikelets, a matter of increasing concern worldwide (4).


In view of the variety and complexity of the consequences of doubling the atmospheric CO2 concentration, the overall effect is impossible to predict. If the changes are slow enough, most negative effects can be offset by the introduction of new crop varieties or shifting the growing regions.

The change in vegetation patterns in response to a changing climate has, in turn, an effect on the atmospheric CO2 concentration (5). The atmospheric CO2 concentration is increasing primarily due to the burning of fossil fuels, yet a global increase in biomass during this build-up reduces the rate of CO2 increase. For instance, a young forest-plantation area of about 1 Mha in Australia absorbed about 25 megatonnes CO2 annually in the 1990’s (1). In Australia it would be necessary to establish about 0.4 Mha of new forest each year in areas suitable to grow tall timber, to offset the 560 megatonnes CO2 emitted annually in Australia. This includes 440 Mt/a of burnt fossil fuels and 120 Mt/a due to land clearing and agriculture (6). Managed forests in Australia subtract only about 22 Mt/a at this time. The Australian biosphere as a whole is provisionally reckoned to take up approximately 350 Mt/a of CO2 from the atmosphere, i.e. almost enough to compensate the emissions by burning fossil fuels.

Experiments on sour orange show 2.8 times more growth as a result of doubling the CO2 over five years. The greatest enhancement of growth of tree seedlings in New England (US) occurred when doubled CO2 was accompanied by low light levels and added nutrients (7). The effects of temperature, CO2 and UV-B on plan growth were found to be highly specific to the species. Insects may also be affected by CO2 levels, e.g. an agricultural pest moth. Their CO2-sensitive organs for detecting food may be overwhelmed by current CO2 concentrations (8).

Computer simulations of spring wheat in Holland showed that a higher global temperature by itself is likely to decrease yields, but there may be an increase when the warming is combined with more CO2. The increase becomes substantial in dry years, because of the enhanced water-use efficiency. The global effect of warming is expected to cause a small decrease of cereal yields overall, chiefly affecting developing nations.



  1. Noble, I. , R. Gifford and G. Farquhar, 1997: What do we know about impacts of action or no action? A perspective from the biological sciences. Paper to a National Academies Forum, ‘The Challenge for Australia on Global Climate Change’, 29-30 April, Canberra, 8pp.
  2. Koch, G.W. and H.A. Mooney, 1996: Carbon Dioxide and Terrestrial Ecosystems (Academic Press) 443pp.
  3. Matthews, R.B., M.J. Kopff, D. Bachelet and H.H. v. Laar (eds), 1995: Modelling the Impact of Climate Change on Rice Production in Asia. (CAB International, Wallingford, UK) 289pp.
  4. Anon 1998. Frost finding pours cold water on plant greenhouse theory. ANU Reporter (July) 29, p3. Available from the Australian National University.
  5. Barson, M.M. and R.M. Gifford, 1990: Carbon dioxide sinks; the potential of tree planting in Australia. In Swaine 1990, p 433-43 (5).
  6. Swaine, D.J. (ed.), 1990: Greenhouse and Energy (Commonwealth Scientific and Industrial Research Organisation, Australia).
  7. Hengeveld, H. and P. Kertland 1995: An assessment of new developments relevant to the science of climate change. Climate Change Newsletter (Australian Bureau of Resource Sciences, Canberra), 7, August, 24p.
  8. Stange, G. and C. Wong 1993: Moth response to climate. Nature, 365, 699.