Achieving the 1.5 degree target: The dark side of bioenergy

Using bioenergy crops to remove carbon dioxide from the atmosphere could have more undesirable side effects than reforestation.

This is the result of an American study that was published last week in "Science Advances".

Both approaches are therefore able to store a similar amount of carbon.

However, if energy is generated from biomass and the resulting carbon dioxide is separated and stored, there could be a widespread risk of water shortages.

"The current study provides important arguments that forests should not be cut down in order to grow bioenergy crops instead," says Bernhard Wern, Head of Material Flows at Saarland University of Applied Sciences.

In expert circles, the controversial method is known as "Bioenergy with Carbon Capture & Storage" (BECCS).

It is one of many ideas on how carbon dioxide that has already been emitted can be removed from the atmosphere and stored.

In its special report, the Intergovernmental Panel on Climate Change (IPCC) stated in 2018 that 100 to 1,000 gigatons of carbon dioxide would have to be removed from the atmosphere over the course of the century to achieve the 1.5 degree target.

Not a single 1.5 degree scenario can do without such negative emissions.

In its most recent publication, the IPCC once again emphasizes that in order to achieve the target, residual emissions must be offset, which sectors such as agriculture or aviation will hardly be able to avoid by the middle of the century.

High consumption of land, water and fertilizer

BECCS, says geographer Florian Zabel from the LMU in Munich, is an exciting possibility because energy can be produced as a positive side effect.

"In the current energy crisis, we see that BECCS in Germany and Europe can also be an important strategic opportunity to become less dependent on energy imports," he says.

An advantage that alternatives such as artificially accelerated weathering processes or the renaturation of moors and grasslands do not offer.

However, one should not neglect the fact that the cultivation of energy crops requires space, water and fertilizer and thus causes new conflicts.

In the two-degree scenarios of the Intergovernmental Panel on Climate Change, areas of between 1.4 and 7.5 million square kilometers are planned for BECCS - four to twenty times the size of Germany.

In terms of climate, a warming of 2.4 degrees Celsius is assumed by the year 2100.

The fact that BECCS and afforestation can store similar amounts of carbon requires more efficient combustion and storage of carbon dioxide than is currently possible.

Otherwise, forests could absorb about 70 percent more carbon per area than bioenergy crops.

What is significant, however, is that if the BECCS scenario is implemented in the USA, one fifth of the land area will be affected by water shortages and 130 million people could live in threatened regions by the end of the century.

In addition to the quantity, the quality of water is also at risk, since the cultivation of energy crops is associated with nitrogen leaching and eutrophication.

Doubts about efficiency

Before recommendations for action can be derived from the results, more in-depth research is needed.

Zabel points out that feedback between land and atmosphere was not taken into account in the study and that the impact on biodiversity and food supply was only mentioned in passing.

There is also a question mark over the future efficiency of various carbon sinks.

According to the IPCC scenarios, the American study assumes that a higher CO2 concentration in the atmosphere leads to higher rates of photosynthesis and more stored carbon.

The exact figures are the subject of current discussions: last week a study was published in "Science"

according to which the physiological effects of a higher CO2 concentration could be significantly smaller than previously assumed.

Researchers had studied the effects of fluctuating greenhouse gas concentrations in the past on plant density in the African tropics.

The question of whether more should be invested in reforestation or more in bioenergy is of course also important in Central Europe, says BOKU researcher Helmut Haberl from Vienna.

However, the results from the USA can hardly be transferred to the German-speaking area due to the different framework conditions: "The population density is much higher in Europe, the area potential for both bioenergy and afforestation is correspondingly smaller and the energy use per capita is lower."