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In the US, nearly twice as much carbon dioxide could be removed from the atmosphere by 2100 through planting forests than through growing bioenergy crops and capturing the CO₂ emitted when the crops are burned. What鈥檚 more, growing forests would lead to fewer water shortages and less water pollution, according to a modelling study.

Many scenarios for limiting warming to 1.5掳C or 2掳C that are used in climate models assume that vast amounts of carbon can be removed from the atmosphere by growing bioenergy crops and capturing the CO₂emitted when they are burned to generate electricity or heat.

This concept, known as bioenergy with carbon capture and storage (BECCS), was . It isn鈥檛 being done at scale anywhere in the world.

at the National University of Singapore and her colleagues used a computer model to compare two scenarios. In one, the US relies on non-food-based bioenergy crops, such as switchgrass and miscanthus, to remove CO₂. Nearly 2 million square kilometres 鈥� equivalent to almost 90 per cent of cropland in the US in 2015 鈥� are planted by 2100.

In the other, the focus is on growing forests, with only 500,000 square kilometres devoted to energy crops.

The model suggests the bioenergy expansion scenario would lock away 70 per cent less carbon than the reforestation scenario unless very optimistic assumptions are made about bioenergy yields, carbon capture efficiency and so on.

Around 130 million people would be affected by water shortages in this scenario, compared with 40 million for afforestation. Water quality would also be lower in the bioenergy scenario because of fertiliser run-off.

The model takes into account factors such as forest fires releasing carbon and more land being cleared for farmland to compensate for the land used for bioenergy crops. It doesn鈥檛 include the substantial greenhouse gas emissions from fertiliser manufacture and use.

鈥淲e anticipate that, when taking these into account, it will further diminish the benefits of BECCS,鈥� says Cheng. While the study looked only at the contiguous US, the findings probably apply globally, the team thinks.

Forests would be better for biodiversity, too. Though the team didn鈥檛 specifically mention this in the paper, Cheng says reforestation would lead to a higher diversity of plant species that can support a greater wildlife variety.

Previous studies have also concluded BECCS would , and could have undesirable consequences such as accelerating habitat loss. It would also compete for land with food crops.

Some bioenergy crops can be grown on degraded land unsuitable for agriculture, says Cheng. 鈥淏ut while the population is rising and the land resource is finite, it may lead to conflicts with food production.鈥�

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