Elevated CO2 Helps Reduce the Negative Impacts of High Temperatures on Plant Growth
The environmental stress about which people seem to worry most is global warming. They are concerned that the rise in air temperature that is predicted to result from a doubling of the air's CO2 content will be so great that plants will need to migrate either poleward in latitude or upward in altitude in order to remain within the climatic regimes to which they are currently adapted. And because CO2-induced warming is predicted to be so rapid, they also fear that many plants will not be able to migrate fast enough to avoid extinction.
This scenario may sound reasonable, but it is largely contradicted by a wealth of studies. In an analysis of 42 different experiments, for example, Idso and Idso (1994) found that the percentage growth enhancement due to a 300 ppm increase in atmospheric CO2 actually rose with increasing air temperature, climbing from nearly 0% at 10°C to fully 100% at 38°C, as is also confirmed by ongoing research.
Simultaneously, the optimum temperature for plant growth and development has typically been found to rise right along with the air's CO2 content. For a 300-ppm increase in atmospheric CO2, for example, theoretical and observational studies have shown that the optimum temperatures of most C3 plants rise by approximately 5°C for such a CO2 increase. This rise in optimum temperature is even larger than the rise in air temperature predicted to result from the greenhouse effect of such a CO2 increase. Consequently, it is clear that a CO2-induced warming would not adversely affect the vast majority of Earth's plants; for fully 95% of them are of the C3 variety. In addition, the remainder of the planet's species -- which may not experience as large a rise in optimum temperature -- are already adapted to Earth's warmer climates, which are expected to warm much less than the other portions of the globe. Therefore, a CO2-induced warming would not produce a massive poleward or upward migration of plants seeking cooler weather; for the temperatures at which nearly all plants perform at their optimum would rise even higher than the temperatures of their respective environments.
It is also worth noting that the photosynthetic rates in plants for which these evaluations have been experimentally derived are generally found to be nearly twice as great at their CO2-enriched optimum temperatures as they are at their optimum temperatures under ambient CO2 concentrations. Consequently, not only would typically-predicted increases in atmospheric CO2 and global air temperatures not hurt Earth's vegetation, they would probably help it, as subsequent investigations continue to suggest.
At the highest air temperatures encountered by plants, atmospheric CO2 enrichment has been demonstrated to be even more valuable; for it can often mean the difference between their living or dying, as it typically enables plants to maintain positive carbon exchange rates in situations where plants growing under ambient CO2 concentrations exhibit negative rates that ultimately lead to their demise. This life-sustaining function of atmospheric CO2 enrichment may also be partly due to a CO2-induced stabilization of heat-susceptible enzymes that is provided by the increased concentration of sugars generally found in CO2-enriched leaves. Whatever its mode of action, it is a welcome consequence of the ongoing rise in the air's CO2 content.
Box 1: The CO2-Temperature-Growth Interaction
The growth-enhancing effects of elevated CO2 typically increase with rising temperature. This phenomenon is illustrated by the data of Jurik et al. (1984), who exposed bigtooth aspen leaves to atmospheric CO2 concentrations of 325 and 1935 ppm and measured their photosynthetic rates at a number of different temperatures. The figure below reproduces their results and slightly extends the two relationships defined by their data to both warmer and cooler conditions.
At 10°C, elevated CO2 has essentially no effect on net photosynthesis in this particular species, as Idso and Idso (1994) have demonstrated is characteristic of plants in general. At 25°C, however, where the net photosynthetic rate of the leaves exposed to 325 ppm CO2 is maximal, the extra CO2 of this study boosts the net photosynthetic rate of the foliage by nearly 100%; and at 36°C, where the net photosynthetic rate of the leaves exposed to 1935 ppm CO2 is maximal, the extra CO2 boosts the net photosynthetic rate of the foliage by 450%. In addition, it is readily seen that the extra CO2 increases the optimum temperature for net photosynthesis in this species by about 11°C: from 25°C in air of 325 ppm CO2 to 36°C in air of 1935 ppm CO2.
In viewing the warm-temperature projections of the two relationships, it can also be seen that the transition from positive to negative net photosynthesis - which denotes a change from life-sustaining to life-depleting conditions - likely occurs somewhere in the vicinity of 39°C in air of 325 ppm CO2 but somewhere in the vicinity of 50°C in air of 1935 ppm CO2. Hence, not only was the optimum temperature for the growth of bigtooth aspen greatly increased by the extra CO2 of this experiment, so too was the temperature above which life cannot be sustained increased, and by about the same amount, i.e., 11°C.
Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.
Jurik, T.W., Weber, J.A. and Gates, D.M. 1984. Short-term effects of CO2 on gas exchanges of leaves of bigtooth aspen (Populus grandidentata) in the field. Plant Physiology 75: 1022-1026.
** For additional peer-reviewed scientific references and an in-depth discussion of the science supporting our position, please visit Climate Change Reconsidered: The Report of the Nongovernmental Planel on Climate Change (www.climatechangereconsidered.org), or CO2 Science (www.co2science.org).