This page was last modified:

Plants and elevated CO₂: A potential climate change opportunity?

The question of how global warming will affect plants is a source of much debate in the scientific community. It is argued that when climate change is examined in the context of primary production it’s impact will be positive. As CO₂ levels in the atmosphere increase, plant growth rates increase, which has a flow-on effect on the food chain with plants providing more food for animals and humans. Research has also shown that increasing CO₂ levels relieves drought stress in corn, leaves lose less water as CO₂ increases, enabling plants to grow under drier conditions  (Soon et al., 1999).

The problem with these statements are that they are implying that the only effect of global warming is accelerated CO₂, and while this in isolation may be beneficial, other factors are involved in the process of climate change, including altered temperature and rainfall. There is also the possibility of plants acclimatising to the higher level of CO₂ present in the atmosphere, along with a potential benefit to pests, causing increased damage to crops and a reduction in yield.

Research indicating that elevated CO₂ will only benefit plants, have mainly been carried out in the lab where nutrients e.g. nitrogen are plentiful. This will only occur in situ if the soils used to grow the crops are extremely fertile, which in most areas is not the case. Vital elements in nature needed for plant growth are often present in limited quantities, and this needs to be taken into account when predicting the effect of elevated CO₂ on plants. Once CO₂ is no longer the limiting nutrient, another will take its place e.g. phosphorus or nitrogen. A study by Lieffering et al. (2004) suggests that more carbon dioxide could reduce nutritional crop value. So although agricultural production in some areas will increase, with crops producing a higher yield under elevated CO₂ conditions. There becomes a trade-off between quantity and quality; a decline in nutritional value is due to the production of more seeds, but the seeds produced contain less nitrogen. Under this scenario humans and livestock would have to increase their intake to receive the same benefit, as the food is of poorer quality.

The United Nations Environment Programme (2002) recognises the possible impact climate change will have on the world’s ability to provide food for its expanding population, and that affects on crop production will vary considerably. Where crops are at their maximum heat tolerance, for example in the tropics, heat stress and drier soils may reduce crop yields by up to a third. This is where the majority of the world’s poorest people live. A warming of more then 2.5% could significantly reduce global food supplies, and substantially increase food prices. Meanwhile, longer growing periods and higher levels of rainfall may increase yields in temperate regions, increasing the displacement of crop production and demand. Although elevated CO₂ may benefit valuable crops in certain regions it will also increase the growth rate of weeds and nuisance species. The weeds will also be able to expand their range to higher latitude habitats. The changes in climate will also lead to a pole ward migration of insects and plant diseases, adding to the risk of crop losses. The biggest problem is that the world’s poorest populations face the biggest risks. These are the people who are highly dependent on isolated agricultural systems, which are the most likely to be affected by the changing climate.

A study by Cammell and Knight (1992) examines the effects of climate change on the population dynamics of crop pests. Temperature directly affects survival, reproduction, development and movement of pest species, so in effect determines their distribution and abundance. Warmer conditions in temperate regions are likely to extend potential geographic range of insect pests, also increasing their abundance, and decreasing their development time, allowing for more generations in a season. This may lead to the occurrence of new pest species that were previously restricted by unfavourable conditions, and increase the impact of existing pests. However, in other areas, warmer conditions may adversely effect pest populations. Climate change could create drought stress on the insect’s host plant, and may increase or decrease the plants suitability as a host. In temperate climates, the conditions will tend to favour natural enemies, but it is unsure whether they will be more effective in their control of the pest. In order to assess whether a particular pest will become more widespread, considerable investigation of the pest, host plant, natural enemy system is required, along with consideration how this is likely to alter with changing environmental conditions over time.