Florian Humpenöder and his colleagues from the Potsdam Institute for Climate Impact Research found that land-based mitigation strategies such as these are sensitive to the different levels of greenhouse gas taxes. They also discovered that afforestation could turn human land use from a net source of carbon into a net sink by the middle of the century.

"Our calculations show that afforestation is a very cost-effective form of mitigation, requiring incentives of only around $6 per tonne of carbon dioxide whereas bioenergy CCS would need much higher incentives to make it economically feasible, at around $165 per tonne of carbon dioxide," Humpenöder told environmentalresearchweb. "This is understandable as bioenergy CCS needs a lot more infrastructure – the crop needs to be grown, harvested, transported, converted to energy and then the carbon dioxide needs to be captured and injected."

However, the researchers’ models also showed that afforestation needs five times more land than bioenergy CCS in order to achieve a similar level of mitigation. "With bioenergy CCS, the same land area is used many times over whereas for afforestation, the land is used just once," said Humpenöder. "To realise the projected mitigation potential of afforestation, current global forest area would need to increase by 60–65%."

Humpenöder admits that this is a vast amount of land and that such a large change is unlikely to be achieved before the end of the century, especially if afforestation is used with another form of land-based mitigation such as bioenergy CCS. Recent research has shown that such tremendous shocks to the land system could, for example, have impacts on food security.

The team used the global land-use optimization model MAgPIE to analyse three scenarios: afforestation, bioenergy CCS and a combination of both.

The researchers concluded that the near-term implementation of afforestation as a climate-change mitigation strategy could increase the likelihood of keeping global warming below 2° above pre-industrial levels, while bioenergy CCS could still contribute to climate change mitigation in the second half of the century if economically, institutionally and technically feasible.

These conclusions are based on the proviso that yields for bioenergy increase considerably and that yields for food crops continue to increase by up to 1.38% per year in order to compensate for the reduction in available agricultural land.

"In contrast to afforestation, which is just planting trees, bioenergy CCS technology is still in development and there are still many hurdles to overcome before it becomes economically and technically feasible," said Humpenöder. "One of these hurdles is social acceptance of CCS. Recently, Vattenfall, one of Europe’s largest energy producers, had to close down an experimental CCS site in Germany because of protests."

The infancy of bioenergy CCS makes it difficult to estimate its impact on the future, but the study found that bioenergy CCS could contribute to climate change mitigation in a cost-efficient way even if only the carbon removal part is valued.

"But for any of our projections to become reality, we first need a global agreement on prices for carbon emission which includes the land-use sector," said Humpenöder. "And this is still far from becoming a reality."

Related links

• Investigating afforestation and bioenergy CCS as climate change mitigation strategies, Florian Humpenöder et al 2014 Environ. Res. Lett. 9 064029
• ERL

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