The cultivation of bio-energy crops, such as sugarcane is a potentially lucrative activity. But sugarcane is a thirsty crop, and the infrastructure required to refine it is expensive. These issues raise serious questions about sustainability and profitability. Nowhere is this more true than in Africa.
Assessing the environmental and economic feasibility of sugarcane cultivation is especially urgent in Ghana. In 2008 Brazil’s agriculture research agency, EMBRAPA, opened an office in Accra with the intention of helping Ghana to build up its nascent ethanol industry. In 2010 Brazil made a $300m investment in exporting Brazilian sugar-cane cultivation and refinement technology to Ghana.
There is no doubt that growing sugarcane in Ghana will require irrigation – and a dam has been proposed to provide this (see Figure 1 for the location of the River Daka cultivation – the subject of our case study). But how much irrigation is required? Will these requirements increase, in the future, as the climate changes?
We used a new computer model to investigate sugarcane cultivation both now, and for a simplified future scenario, in which temperature and CO2 increase, but rainfall and other climate variables are held constant. Importantly, our model is based on the land-surface component of the Met Office climate model. This means that it is capable of representing both the growth of plants in the present day, and their response to climate change.
The results were surprising (Figure 2). We found that in the present day, sugarcane can be grown in Ghana – with an irrigation requirement of 3-4mm/day (in agreement with pilot field studies, carried out by our Ghanaian collaborators). When temperatures were warmed by 4 degrees Celsius, as expected, the higher evaporative demand increased irrigation requirements. However, when CO2 concentration was doubled, the irrigation requirements returned to present day levels.
To understand why this is, we need to think about the way that plants respond to climate change. Plants’ response to elevated CO2 largely depends on their mechanism of photosynthesis. Most plants fall into one of two categories: those that follow the C3 photosynthetic pathway and those (such as sugarcane) that follow the C4 pathway. In C3 plants, carbon is fixed by the action of the enzyme Rubisco. The rate at which this reaction happens is enhanced by higher ambient CO2/O2 ratios. There is therefore a theoretical mechanism for greater biomass production under scenarios of higher atmospheric concentration of CO2.
Sugarcane assimilates carbon through the C4 photosynthetic pathway. Unlike C3 plants, C4 plants concentrate CO2 from the atmosphere in sheath (outer) cells via a biochemical pathway that does not have a strong dependence on atmospheric CO2 levels. There is therefore no obvious mechanism for a direct link between elevated CO2 and enhanced photosynthesis. Despite this, there is abundant observational evidence that production of biomass by C4 plants is increased by rising concentrations of CO2 – especially under water-stressed conditions. This is likely to be primarily because of the impact of CO2 concentrations on transpiration and hence on water use efficiency (WUE).
Specifically, plants take in CO2 through microscopic openings on their leaves called stomata, but whenever stomata are open, plants lose water. However, when CO2 concentrations are raised, fewer stomata need to be open in order to maintain optimal CO2 levels within the plant, which leads to a reduction in the rate of transpiration (see Figure 3). For given meteorological conditions, the plant will therefore lose less water to the atmosphere – and hence have greater WUE. Hence, over the course of a growing season, the plant will be less stressed and accumulate more biomass.
It follows from the previous discussion, that under scenarios of climate change, in which both temperature and CO2 are higher than the present day, there are competing effects on C4 plants. Specifically, higher temperatures increase soil and canopy evaporation, potentially reducing the soil moisture available to plants, while higher CO2 levels reduce transpiration rates.
There remain uncertainties about how these competing factors interact to determine the response of C4 vegetation to climate change. Our results tell us about the specific case of sugarcane cultivation in Ghana, and raise interesting questions about the more general interplay between CO2, temperature and water stress.
Returning to our Ghana case study… Of course looking at irrigation requirements cannot, in itself, tell us whether or not growing sugarcane is sustainable. Ongoing work within our project is looking at the wider economic context and the hydrological impacts of the damming and irrigation. Preliminary results suggest that, properly managed, the dam could provide sufficient irrigation water for sugarcane. Furthermore, the dam would reduce the risk of flash floods, and provide a more perennial water supply to the local population.
Bio-energy crop cultivation is a controversial issue – especially in Africa. However, in this case, our modeling results, coupled with local field studies, suggest that sugarcane cultivation is environmentally feasible, and could bring much needed investment into this desperately poor region.
To find out more:
Paper on this topic: Emily Black, Pier Luigi Vidale, Anne Verhoef, Santiago Vianna Cuadra, Tom Osborne and Catherine Van den Hoof (2012) Cultivating C4 crops in a changing climate: sugarcane in Ghana Environ. Res. Lett. 7 044027
News article on Environmental Research Web: Can Ghana match Brazilian sugarcane yields?