Alexander Popp from the Potsdam Institute for Climate Impact Research, Germany, and colleagues linked a global dynamic vegetation and water balance model, a global land- and water-use model, and a global energy-economy-climate model, to investigate the potential contribution of bioenergy to climate-change mitigation, including its costs and trade-offs with food and water security. The findings are published in Environmental Research Letters.

Within the researchers' integrated model, bioenergy competed directly with other energy technology options on the basis of costs, including implicit costs associated with land and water availability. The model was run until the year 2095, under three different scenarios: a reference scenario, where global population grows and food and water demand increase but bioenergy is not required for climate-change mitigation; a climate-change mitigation scenario where climate policies aim to limit emissions such that global mean temperatures rise by no more than 2 °C; and a forest conservation scenario where there is less land available for growing biofuel crops.

Under the climate-change mitigation scenario Popp and colleagues found that bioenergy from specialized grassy and woody crops, such as Miscanthus or poplar, could contribute approximately 100 Exajoules (1018J) in 2055 and up to 300 Exajoules of primary energy in 2095.

It is hard to say what proportion of energy consumption this would be by 2055 or 2095; in 2008 worldwide energy consumption was approximately 475 Exajoules. Simulations have indicated that worldwide energy consumption will be around 1000 Exajoules by 2100, though this figure very much depends on whether climate-change mitigation is applied or not.

However, the climate-change mitigation scenario has one major drawback. "This scenario increases deforestation and therefore leads to additional emissions from land-use change, mainly in the tropics," explained Popp. "But if biomass is converted into secondary energy in combination with carbon capture and storage (CCS), 'negative emissions' could be provided."

When forests are conserved the researchers found that biocrops can still supply a respectable amount of energy – around 70 Exajoules by 2055 and 270 Exajoules by 2095. But forest conservation is likely to be accompanied by significantly increased prices for food and water, and the need for technology to increase agricultural yield. "Increased biomass growth can put enormous pressures on water supplies, especially if it is based on irrigation water," said Popp. "Research on methods that increase agricultural yields would be needed to compensate for land restrictions due to forest conservation."

One way of freeing up land and reducing pressure on the water supply would be for people to eat less meat. And CCS technology – if further developed and proven – could help to alleviate emissions from cutting down forest. But there are no easy answers, and still many significant hurdles to overcome. "Large-scale energy crop production may create conflicts with sustainability aspects, like food and water security or protection of forests for climate-change mitigation and biodiversity conservation," said Popp. It is a bumpy road ahead for bioenergy.