The promise of BECCS (Bioenergy with carbon capture and storage) technologies is the ability to harness biomass for energy while capturing and sequestering carbon. With firewood, CO2 can only be stored during the life of the tree, whereas BECCS could trap carbon in geological formations in the long term and therefore produce "negative carbon emissions". This could be a means to hopefully turn the carbon balance of human activities green one day.
However, BECCS remains controversial due to the water and nutrient requirements of biomass production, and the risk of these crops occupying arable land or being established in forests, resulting in carbon losses. It is therefore important to study all aspects of their impact on climate.
Replacing annual plant crops with bioenergy crops is not neutral with respect to the biophysical properties of the earth's surface. Feedback from the United States seems to show clear regional cooling effects, while the assessment of such crops remains unclear on a global scale. A number of factors influence the earth's surface energy balance:
- lighter colors favor the reflection of solar radiation (albedo);
- water evaporated from the ground and "transpired" by plants (evapotranspiration) has a cooling effect on the lower layers of the atmosphere;
- a change in land cover affects the aerodynamic resistance to heat transfer towards the atmosphere.
All of these changes impact climate and alter the air temperature on local and regional scales.
Using a coupled atmosphere-land model, an international team simulated the biophysical effects of BECCS technologies on the basis of four types of lignocellulosic bioenergy crops:
- eucalyptus;
- poplar and willow;
- Miscanthus (or elephant grass, which can reach a height of 4 meters);
- switchgrass (or perennial millet, which can reach a height of 1.30 meters).
Their results show that in a scenario where BECCS bioenergy crops occupy 3.8% of the world's land surface – comparable to the values selected for socio-economic scenarios using BECCS to contain warming below 2°C – the global average air temperature decreases by 0.03 to 0.08°C, but with strong regional contrasts.
Woody bioenergy crops induce stronger cooling effects than herbaceous crops due to their higher evapotranspiration rate and lower aerodynamic resistance.
At the continental scale, changes in air temperature are not proportional to the area cultivated. Sensitivity tests show that the cooling effect is robust for eucalyptus, while it is less certain for switchgrass.
This study shows the importance of the biophysical effects of bioenergy crops, which must be considered when evaluating the climate mitigation capacity of BECCS technologies.