Primary production and global environmental change
Professor Iain Colin Prentice: AXA Chair in Biosphere and Climate Impacts, Imperial College London and Dr Han Wang: Maquarie University
Plant scientists and ecologists have been thrown into some confusion by the need to predict how global environmental change might impact on primary production – the process by which photosynthesis uses the Sun’s energy to transform carbon dioxide (CO2), water and minerals into plant life. A number of complex models have been built for this purpose. Unfortunately they give quite different answers and cannot all be right.
Professor Colin Prentice’s AXA Chair team are developing new modelling approaches that are simpler, transparent, based on fundamental science and informed by observations to the greatest possible extent. This is possible partly because we live in a far more data-rich world than was imagined two decades ago, when the current ‘state of the art’ models were first developed.
Results from computer models of crops and the climate
The model results illustrated in the figure below represent possible future potential primary production. The top left panel represents annual potential primary production today. The rest of the maps in the top two rows show the effect of a 4oC temperature increase on potential primary production in the absence of any change in atmospheric CO2 concentration. This is based on seven different computational models of future climate; they take into account the 4oC temperature increase, and also incorporate the expected changes in precipitation and cloud cover caused by that temperature rise.
The bottom two rows show what is expected to happen when a CO2 increase to 750 ppm is also taken into account. We show these strongly warmed, high-CO2 patterns for illustration but note that the patterns of change would be similar, only reduced in amplitude, if CO2 and climate change were stabilized at lower levels. However, 750 ppm is in fact lower than the CO2 levels by the end of the century in the so-called RCP8.5 emissions scenario, which is sometimes considered to be the closest to a “business as usual” scenario of those considered in the latest IPCC working group one report. Similarly, a 4oC rise is projected to be as likely as not by the end of the century for the RCP8.5 scenario, so it is not inconceivable that we could see the consequences on primary production depicted in Figure 1 by 2100.
The key messages according to this model are very simple:
- Warming generally increases potential primary production in cold climates and reduces it in warm climates;
- Higher CO2 increases potential primary production everywhere, relative to the situation with fixed CO2.
We emphasize once again that these maps refer to potential primary production. There are at least two reasons why this level of production will not be achieved in reality without major efforts. First, many regions (especially in the least developed countries in the tropics) achieve agricultural yields far below potential primary production today. Even if the CO2 effect is big enough to counteract the negative effect of warming (as this model suggests) the effect would not be realized without a step-change improvement in the access to fertilizers by farmers in poor countries. Secondly, existing crop varieties have been bred for recent conditions, not for a warmer world with high CO2, so major plant breeding efforts would be needed. Moreover, regions suitable for a given crop today may become unsuitable in a greatly changed climate while new suitable regions may open up.
The impacts of changing temperature, rainfall and atmospheric CO2
The environmental changes that most influence plant growth are changes in temperature, precipitation and atmospheric CO2. Impacts on plants can be measured by looking at global rates of primary production. This is often expressed as potential primary production; the amount of primary production achieved by plants that are suited to the local environment. The parts of crops that we eat (the agricultural yield) is always a fraction of primary production. So any impact on primary production, whether positive or negative, is expected to be reflected in agricultural yield.
Warming tends to increase potential primary production in regions with cold winters by extending the growing season, but can reduce potential primary production in warmer climates because the affinity of the key carbon-fixing enzyme (Rubisco) declines with temperature, and because photosynthesis is impaired generally at the highest temperatures experienced by leaves.
Whereas almost all of the world is warming and is expected to continue warming during the coming decades, precipitation patterns are more complex. Global total precipitation is expected to increase slightly, but some regions where precipitation is already more limited, such as those with Mediterranean climates, are already suffering from declining rainfall. Increased rainfall can increase primary productivity in places where water availability is currently a limiting factor, but very high rainfall tends to reduce the fertility of soils, by erosion and loss of nutrients through the ground, and can damage crops. In dry climates, further drying reduces primary production. Dry, sunny climates can be highly productive but only in so far as external water supplies and infrastructure allow irrigation. It should also be noted that increased amounts of CO2 improve the water use efficiency of plants, and so a given amount of water (whether from rain, or from irrigation) goes further at higher CO2.
The one environmental factor that consistently increases plant growth is an increase in the atmospheric concentration of CO2. This may seem paradoxical, since rising CO2 concentration is the biggest single cause of contemporary climate change. The fertilising effect of CO2 however does mitigate some of the negative effects of the warming or drying caused by increasing CO2. How large or sustainable this capacity will prove to be is subject to dispute, however. No assessment of current literature could come to a well-founded conclusion based on the available, often contradictory evidence.