By Tim Radford

LONDON, 2 May, 2017 – The next decade will be critical in containing global warming to the limits the world has set itself, European researchers warn. And at least one of the targets stipulated in the 2015 Paris agreement may be unrealistic, according to a second team of European researchers.

Researchers in Austria report in Nature Communications that they took a long hard look at what must happen to keep global warming to “well below” an average global rise of 2°C and if possible 1.5°C above pre-industrial levels.

They deployed a mathematical simulation – in effect a computer model – to measure the carbon emitted in fossil fuel consumption, and the carbon taken up by natural mechanisms such as forests and oceans, and the impact of the ways humans use the land around them.

Unattainable climate target?

And the conclusion is uncompromising. “This study shows that the combined energy and land-use system should deliver zero-net anthropogenic emissions well before 2040 in order to assure the attainability of a 1.5°C target by 2100,” says Michael Obersteiner, one of the authors, and ecosystem services and management programme director of the International Institute for Applied Systems Analysis, based in Laxenburg, Austria.

That means that by 2100, fossil fuel consumption would have to be reduced to less than a quarter of the world’s energy supply. Right now, it makes up 95%. At the same time, humans must stop clearing forests and restore them. Once achieved, this would mean a 42% drop in cumulative emissions by the century’s end – compared to the notorious “business as usual” scenario.

But to make this happen would require a global economy in which wind, solar and bio-energy output increase by 5% a year, and carbon emissions peak by 2022.

“Scientists have repeatedly warned that
the Paris target will be difficult to achieve
without dramatic international collaboration”

Unless humans find some way of actively taking carbon dioxide out of the atmosphere, that would deliver a final temperature rise of 2.5°C, well above the Paris target. If the peak comes at the end of the century, that commits the world to a 3.5°C rise.

Scientists have repeatedly warned that the Paris target will be difficult to achieve without dramatic international collaboration. A second study, in Environmental Science and Technology, shows just how difficult this could be.

Researchers from the Netherlands, the US and the UK chose one simple proposal agreed in Paris: that by managing farmland and forests differently, humans try to increase the soil uptake of carbon by the seemingly trifling level of 0.4% per year.

That alone, in theory, could make a huge difference to the global carbon budget.

But more carbon in the soil would require more nitrogen. To match the 0.4% target, soils would have to absorb 1,200 million metric tons of carbon a year.

To make this possible, the soils would also have to be dressed with an extra 100 million tons of nitrogen. This is the equivalent of 75% of current global fertiliser production. And this would have to be applied in those places where it would be most effective: those places, for instance, where the soil carbon levels are low but nutrients are easily available.

National engagement

These are likely to be overgrazed grasslands or intensively cropped soils. So it is not enough to have a global goal: national authorities must also have a very clear idea of where to set about achieving it.

“Generally speaking, more carbon is good for almost any soil,” says Jan Willem van Groenigen, a soil scientist at Wageningen University, who led the study.

“If we could combine that with slowing climate change, that would be a double win. The problem is the numbers don’t add up.” – Climate News Network