The space needed for biomass plantations would reduce the land available for farming

Planting trees ‘not viable alternative’ to cutting emissions

Growing plants to capture carbon dioxide is not a viable way to curb climate change if humans fail to cut fossil fuel emissions, a new study shows.

For this strategy to work, the plantations would need so much land that they would eliminate most natural ecosystems or dramatically reduce food production.

However, the research team – including Professor Tim Lenton from the University of Exeter – said growing biomass soon in well-selected places with increased irrigation or fertilisation could support climate policies, alongside emission cuts.

This combination could help prevent warming of 2°C above pre-industrial levels, which would cause devastating environmental consequences.

“Our work shows that carbon removal via the biosphere cannot be used as a late-regret option to tackle climate change,” said Professor Lenton. “Instead we have to act now using all possible measures.

“Reducing fossil fuel use is a precondition for stabilizing the climate, but we also need to make use of a range of options from reforestation on degraded land to low-till agriculture and efficient irrigation systems to limiting food waste.”

Plants suck carbon dioxide (CO2) out of the atmosphere to build their roots, stems and leaves.

The study’s authors said this could be combined with high-tech carbon storage mechanisms – but the effects would be limited.

“If we continue burning coal and oil the way we do today and regret our inaction later, the amounts of greenhouse gas we would need to take out of the atmosphere in order to stabilise the climate would be too huge to manage,” said lead author Lena Boysen, of the Potsdam Institute for Climate Impact Research in Germany.

“Even if we were able to use productive plants such as poplar trees or switchgrass and store 50% of the carbon contained in their biomass, in the business-as-usual scenario of continued unconstrained fossil fuel use, the sheer size of the plantations for staying at or below 2°C of warming would cause devastating environmental consequences.”

The scientists calculate that the hypothetically required plantations would in fact replace natural ecosystems around the world almost completely.

If CO2 emissions were moderately reduced in line with current national pledges under the Paris Climate Agreement, biomass plantations implemented by mid-century to extract remaining excess CO2 from the air still would have to be enormous.

In this scenario, they would replace natural ecosystems on fertile land the size of more than one third of all forests we have today on our planet.

Alternatively, more than a quarter of land used for agriculture at present would have to be converted into biomass plantations – putting at risk global food security.

Only ambitious emissions reductions and advancements in land management techniques between 2005-2100 could possibly avoid fierce competition for land.

But even in this scenario of aggressive climate stabilisation policy, only high inputs of water, fertilisers and a globally applied high-tech carbon-storage machinery that captures more than 75% of extracted CO2 could likely limit warming to around 2°C by 2100.

To this end, technologies minimizing carbon emissions from cultivation, harvest, transport and conversion of biomass and, especially, long-term carbon capture and storage would need to improve worldwide.

The paper is published in Earth’s Future, a journal of the American Geophysical Union.

Date: 24 May 2017

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