Currently the emissions associated with land-use change and agriculture account for 15% of global greenhouse gas emissions. With a growing population (projected to rise from today's 7.3 billion to 11.2 billion by 2100) of hungry mouths to feed, these emissions are expected to be some of the hardest to mitigate. Algae, however, could hold part of the answer.

The green slime that you see growing on a pond is incredible stuff. Out at sea these tiny plants grow ten times faster than the most productive crops on land. Bring algae onto land (by pumping seawater through "fields" of algae-growing tubes) and you can produce ten times more energy or food than a field of wheat the same size. But farming algae also has a cost: pumping seawater around the algal fields uses significant amounts of energy. The question is, do the benefits of algae outweigh the costs?

To examine these tradeoffs, Michael Walsh from Bentley University, US, and colleagues used the Global Change Assessment Model (GCAM). They investigated the greenhouse gas emissions, land-use change and water footprint of three different pathways – using algae to produce biofuel, using algae to produce food and using algae to produce a mix of biofuel and food – and compared this to a scenario without algae farming. For all the pathways, the team assumed that the algae would be grown in salt-water, in non-arable, low-value land such as desert, or scrubland.

"Unlike arable farmland there is a large stock of this low-value land available, with previous studies identifying areas like the coastal region of Texas, US, as being ideal," said Walsh.

The model results suggest that large-scale farming of algae to produce both food and biofuel could bring about significant reductions in greenhouse gas emissions, mostly because of the reduced land demand for food production.

"Algal food holds the potential to open up a significant amount of land to other uses," said Walsh, who published the findings in Environmental Research Letters (ERL). "With that available land, we can more easily deploy negative emissions technologies such as bio-energy carbon capture and storage."

The conversion of 30 to 40 million hectares of desert and scrubland into algal production farms would be enough to replace all corn and oil crop production (currently using 300 to 400 million hectares of agricultural land), according to the model. "This is only an illustrative forecast, but it would be roughly consistent with downsizing a portion of global agriculture from an area the size of India and Pakistan, to the size of Germany," said Walsh.

But the model also shows that farming algae for food alone would not produce a long-term reduction in emissions, due to the high energy use associated with growing the algae. Although the carbon savings associated with avoiding land-use change are large, they are not big enough to compensate for the energy needed for long-term algal farming.

"After the land use change emissions reductions are exhausted, the production of algal food will emit more carbon than the product is offsetting, and could over time turn algal food products into a net emitter, unless a fuel co-product is used to offset emissions elsewhere," said Walsh. What’s more, the model shows that the high nutrient demands of algal farming for nitrogen and phosphorus are unsustainable in the scenario modelled, but recycling of municipal and animal waste streams could help to overcome this.

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