think climate. think change.
Prepared by Will Steffen and Pep Canadell for the Australian Greenhouse Office, April 2005
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Climate change is no longer a matter of interest just for the scientific community. It is now mainstream in policy formation across many sectors of modern society. In Australia, policy interests relating to effects of elevated carbon dioxide (CO2) on plant productivity are based on the fact that these effects will flow directly - one way or another - into the economies of Australia’s agricultural and forest industries, and thus will impact on the economic and social well being of all Australia’s rural communities.
The purpose of this report is to assess our current understanding of the likely effects of increasing atmospheric CO2 on plant growth in Australia under a changing climate. It aims to:
The report is based on reviews of the scientific literature, interviews with Australian scientists, and a facilitated workshop involving experts from wide ranging disciplines.
There is strong consensus that at the leaf level elevated CO2 increases the instantaneous rate of photosynthesis in woody plants and in some grasses, and it decreases the amount of water lost per unit carbon assimilated. Under most conditions and for most plants used in controlled environment experiments, these effects translate at the individual plant level to a positive growth response and an increase in water-use efficiency, that is, to an increase in carbon assimilated per unit of water transpired.
Scaling the effects of elevated CO2 even from the leaf level to whole plant levels presents some dif. culties in interpretation, however, due to the large number of ways that plants can allocate the additional photosynthate produced in the leaves. These difficulties arise primarily from the various phenological phases that plants go through during their life cycles, and the range of environmental and physiological constraints that they experience. In addition, essentially all of the experimental methodologies employed to date use a sudden step-wise increase in the concentration of CO2 - often to a doubling of ambient. Thus, based on research to date there can be no conclusions drawn as to the capacity of plants or systems to adapt (or respond) to a gradual increase in CO2 as occurring in situ.
Scaling from plants to ecosystems or production systems, and from short to long timescales is done by system-level experimentation and modelling. Given the importance of moisture constraints for Australian terrestrial systems, the degree to which elevated CO2 may in. uence water use ef. ciency is of paramount importance. There is agreement that different water balance processes operate at different scales from the leaf to the plant to the ecosystem. Hence any attempt to extrapolate the effects of elevated CO2 on plant water use efficiencies from micro-level studies to macro-level understanding must be undertaken with extreme care.
In scaling from individual plants to whole ecosystems, there is also strong consensus that differential growth responses among individual plants to elevated CO2 will lead over time to change in species composition of the ecosystem - although the effects on ecosystem dynamics clearly remains unresolved. In addition, most elevated CO2 experiments run for five years or less, and thus may not capture longer term effects, especially acclimation phenoma, longer term nutrient dynamics, and changes of turnover in carbon pools. It is also accepted that experiments run in small chambers or FACE (Free-Air Carbon Dioxide Enrichment) plots (1 m2 to 30 m2) behave as ‘islands’ of elevated CO2 surrounded by ambient CO2, which does not allow for full atmospheric feedbacks or interactions with herbivores or pollinators.
The intersection of the effects of elevated CO2 with climate change is especially important given the overriding importance of weather and climate extremes for the strength of Australian plant-based industries. Although CO2 effects become important only at longer timescales (decadal to century) compared with the effects of climate extremes, the interaction between CO2 and climate effects may become important over shorter timescales if increased water use efficiencies are expressed at systems levels.
Models are critical tools to translate experimental finndings and observations of plant and ecosystem responses into more generalised understandings. Most models developed to examine the effects of elevated CO2 contain modules, usually based on empirical relationships. These modules simulate other aspects of ecosystem physiology that are important in determining the effects of elevated CO2 on biomass or yield. These include nitrogen cycle dynamics (inclusion of phosphorus dynamics is less common), allocation of carbohydrate to various plant organs, decomposition of soil carbon, plant phenological effects and, increasingly, management options.
Challenges and controversy may arise when models that have been developed primarily as research tools are later adapted for management or policy studies. There is a long-standing unresolved debate within the scienti.c community as to whether this is an appropriate approach or not. The fundamental issue lies around the treatment of uncertainty. Modellers are, of course, aware of the limitations, and carefully note that process modules within models must be tested rigorously, but nonetheless, the confidence levels attached to the parameterisation of various processes are normally treated in an implicit rather than an explicit manner. This debate is critical to understanding the use of models in supporting policy development.
Bearing in mind the constraints on the scientific knowledge base noted above and the lack of elevated CO2 experimentation that has been done under Australian environmental conditions, the effects of changing climate and atmospheric CO2 concentration on Australian plantbased industries can be summarized as follows.
Cropping (wheat) systems: Given the dearth of experimentally based information for Australian conditions, model-based analyses are the only way to estimate impacts of climate change on the Australian wheat industry. A sophisticated model-based assessment that included the effects of both elevated CO2 and changes in climate means and extremes has proposed (i) small increases in mean production, but a signi.cant probability of lowered production; (ii) marked regional differences in production; and (iii) enhanced production if growers respond with appropriate adaptation strategies. Nonetheless, given that the probabilities of positive or negative overall effects are roughly equal, we might well conclude on the basis of risk assessment that there is a serious cause for concern about the future of the current Australian wheat industry under global climate change.
Grazing systems: A detailed model-based study for Queensland of the impacts of doubling CO2, increasing temperature, and varying rainfall suggests that ‘safe’ animal carrying capacity may increase, but major uncertainties remain on the effects of elevated CO2 and climate change on nutritional quality of feed, plantplant competition, both in terms of the composition of herbaceous species and of the woody:grass ratio.
Forestry systems: Compared with cropping and grazing systems, less is known about the effects of elevated CO2 on Australian forests. The limited observational evidence available internationally is inconclusive but suggests that elevated CO2 effects decrease as trees age, so the effects of elevated CO2 on old growth or mature forests will be less than on short-rotation plantation forests, where it is likely that fast-growing saplings and young trees are more likely to respond to elevated CO2 with enhanced net primary production.
The bottom-line messages of this report relate to fundamental questions about the effects of elevated CO2 on Australian plant-based industries.
(i) How robust is the knowledge base on CO2 effects?
The knowledge base on the effects of step-wise increases in atmospheric CO2 on fundamental physiological effects at leaf level appears quite robust. There is increasing uncertainty, however, as the effects of elevated CO2 on growth, yield, and water use are scaled up to monoculture cropping systems (e.g. wheat), perennial pasture/ rangelands systems and short-rotation plantation forests. Uncertainty increases further when the effects are scaled up to mature forests over long timescales.
In addition, little is known about the effects at the system level when other effects of elevated CO2 (e.g. carbon allocation, nutrient interactions, inter-species competition) are considered concurrently. There is a critical lack of relevant experimentation under Australian environmental conditions.
(ii) With what level of confidence can we apply to policy development our current understanding of elevated CO2 effects?
Our con.dence in the reliability of the knowledge base on the effects of elevated CO2 on their own for policy development may be stronger for cropping and grazing systems than for the forestry industry (apart from short-rotation plantations). However, the effects of elevated CO2 cannot be disentangled from the effects of climate change, which bring their own set of considerable uncertainties and gaps in understanding. Thus, when the cumulative and interactive impacts of elevated CO2 and climate change are considered, our con.dence in the reliability of the knowledge base for policy development in all agricultural and forestry systems in Australia is clearly in the ‘low’ category.
Given these uncertainties, a number of key research priorities were agreed.
