ALBEDO, CARBON SEQUESTRATION AND CLIMATE CHANGE MITIGATION: WHAT DO WE REALLY KNOW? Remi Cardinael (CIRAD)
Figure: Long-term experiment of conservation agriculture in Zimbabwe. A change in soil tillage and soil cover impacts SOC, GHGs, but also albedo. © Rémi Cardinael.
On 3-4 December 2020, the French Agricultural Research Center for International Development (CIRAD) and the CLAND Convergence Institute organized, with the support of the Global Research Alliance on Agricultural Greenhouse Gases (GRA) and the 4 per Thousand Initiative, a virtual workshop entitled “Can albedo change offset the climate benefit of carbon sequestrating practices?“. The overall objective was to discuss the relevance of current carbon-centered accounting systems to assess climate change mitigation potentials of land use change and management.
This workshop gathered twelve top keynote speakers to provide the most up-to-date knowledge about the potential offset or enhancement of the climate benefit of carbon sequestrating practices, identify knowledge gaps and propose a way forward for future research projects.
This virtual event gathered more than 300 participants, from 52 countries and 156 different institutes. Most of the participants (89%) were from universities and research centers, 60% were from Europe, 17% from Asia, and 14% from North America.
All presentations were recorded and available in replay here:
The climate benefit of best management practices is often quantified through the change in biochemical effects, i.e., soil organic carbon (SOC) stocks and greenhouse gases (GHGs) emissions. However, biochemical effects do not stand alone. Planting trees, covering the soil, reducing tillage, adding organic amendments (compost, biochar…), etc, also modify biophysical effects, for example, albedo.
Surface albedo is the fraction of incident solar radiation that is reflected back to the atmosphere, measured on a scale from 0 (100% absorption, 0% reflection) to 1 (0% absorption, 100% reflection). A modification in surface albedo affects top-of-atmosphere albedo and thus the amount of solar energy absorbed by Earth.
It is now well documented that afforestation of boreal regions would have no climate benefit given the negative radiative forcing due to reduced atmospheric CO2 concentration by carbon sequestration in trees and soils (biochemical effect) is offset by decreased surface albedo (biophysical effect) leading to warming and a positive radiative forcing.
However, comparing biochemical and biophysical effects is not straightforward. There are differences in the spatial extent of the two forcings. CO2 is well-mixed in Earth’s atmosphere thus imposing a spatially homogeneous forcing while a change in surface albedo is more localized.
A lot of progress has been made to compare these effects, but metrics still need to be improved. The first day of the workshop was mainly focused on methodological aspects related to quantifying and comparing these effects, as well as on techniques to measure albedo from field studies to remote sensing. During the second day, keynote speakers presented different studies comparing biochemical and biophysical effects for different practices such as cover crops, chlorophyll-deficient crops, biochar, bioenergy crops and forest management.
It was concluded that biophysical effects are significant and failing to account for example for surface albedo can result in suboptimal or even counterproductive climate-motivated policies of the land-based sectors.
The climate benefit of biochar is for example largely reduced when albedo is considered. In contrast, cover crops in Central and West Europe as a general rule of thumb have been shown to increase SOC, reduce GHGs, and increase albedo, potentially a win-win-win strategy.
However, the effect of a given practice on climate is highly context-specific and could have undesirable effects depending on soil type and climate. More field data to explore a diversity of pedoclimatic contexts combined with a diversity of land use and management is required.
Biophysical effects are also very important contributing factors to mitigate local temperature extremes and can play a role in adaptation to climate change. At present, the description of management practices is too coarse in climate models and a better coupling these with soil-crop models could improve the assessment of practices on local, regional and global climate.
Albedo is not the only biophysical effect to consider. The change in the energy balance also has implications for the water cycle, especially for evapotranspiration. For example, forests have a lower albedo than crops or grasslands (warming effect), but a much higher evapotranspiration (cooling effect). Other aspects to consider are changes in surface roughness and emission of volatile organic compounds, affecting turbulent fluxes and the water cycle.
The role of biophysical effects such as albedo on climate change, in relation to management practices, has been scarcely studied, especially for non-forested ecosystems such as croplands and grasslands. The relationship between the two is a crucial information gap that must be filled in the coming decade by the scientific community.