By Tim Radford / Climate News Network


    The sheer size of the plantations required, even for productive plants such as poplar trees, would have devastating environmental consequences. (Alan Levine / Flickr)

Humans cannot simply plant their way out of trouble: trees cannot absorb the ever-increasing quantities of carbon dioxide in the atmosphere.

If the world’s nations really do intend to contain global warming to within 2°C, there is no alternative to drastic cuts in greenhouse gas emissions, according to a new study.

The tree could be regarded as low-technology carbon removal machinery and, in theory, carefully managed plantations could soak up the carbon released from fossil fuel combustion. But the sheer scale of such plantations would have devastating environmental costs, scientists say.

“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,” says Lena Boysen from the Potsdam Institute for Climate Impact Research, who led the study, published in Earth’s Future journal.

If the forests were planted on productive land, then humans would lose the soils urgently needed to nourish a population of 9bn. If the trees were planted on less productive terrain, the necessary costs in water and nitrogen-based fertiliser would be devastating. Either way, natural ecosystems would be irreparably damaged.

And then the trees grown to absorb carbon would have to be stored deep underground, to prevent the carbon returning to the atmosphere to accelerate global warming rather than limit it.

“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,” Boysen says.

The world’s great forests are part of the climate machinery, and more than 195 nations agreed in Paris in 2015 to take steps to contain climate change, both by managing the way they used land and by switching from fossil fuels to renewable sources of energy.

So carbon storage in the form of woodland is one component of a complex problem. Boysen and colleagues report that they looked at a number of scenarios to see whether, even in theory, massive investment in tree planting could remove sufficient quantities of carbon from the atmosphere.

One scenario required 6.9 billion hectares of plantation, fed by 570 million tons of nitrogen each year, and even the smallest theoretical plantation would have extended over a billion hectares and consumed 96 million tons of nitrogen fertiliser every year. One billion hectares is 10 million square kilometres: an area bigger than the whole of Canada.

Another of the considered options would have involved covering up to a quarter of the world’s agricultural land with unproductive biomass – tree trunks that would have to be buried – while another would have meant the loss of natural ecosystems equivalent to one-third of all the world’s present forested land.

“As scientists we are looking at all possible futures, not just the positive ones,” says Wolfgang Lucht, another of the Potsdam authors.

“What happens in the worst case, a widespread disruption and failure of mitigation policies? Would plants allow us to still stabilise climate in emergency mode? The answer is: no. There is no alternative for successful mitigation. In that scenario, plants can potentially play a limited but important role, if managed well.”

Climate scientists have been making an inventory of the world’s forests: they have counted more than 1 trillion trees and numbered the known species.

They have calculated the relative value as carbon “sinks” of trees old and young, and even re-examined the world’s drylands to discover a vast and so-far-uncounted forest the size of the European Union, scattered over the arid plains. But while forest conservation is a vital part of the climate challenge, deliberate plantation might be counterproductive.

The authors make the point that one option for biomass plantation would consume 10% to 25% of the world’s agricultural land, at the cost of 43% to 73% of the world’s annual calorie production.

They supposed a world that abandoned all meat and dairy products, and “planted” all pasture: even that “would not result in substantial climate benefits”. That is because although grazing lands are greater than cropland, they are also less productive. The authors come back, again and again, to the need to reduce fossil fuel use.

“In the climate drama currently unfolding on that big stage we call Earth, CO2 removal is not the hero who finally saves the day after everything else has failed.

“It is rather a supporting actor that has to come into play right from the beginning, while the major part is up to the mitigation protagonist,” says Hans Joachim Schellnhuber, director of the Potsdam Institute.

“So this is a positive message. We know what to do – rapidly ending fossil fuel use, complemented by a great variety of CO2 removal techniques. We know when to do it – now. And if we do it, we find it is still possible to avoid the bulk of climate risks by limiting temperature rise to below 2°C.”

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