Reflect sunlight or use it to store carbon?

In considerations about land management and global climate, biophysical effects like those of albedo are known to modify biochemical effects of greenhouse gas release or uptake. In particular, the cooling effect of afforestation via creation of carbon sinks has been shown to be partly offset by the low albedo and snow-masking effect of tree canopies.

In this presentation, we give a global overview on the relationship between albedo and CO₂ uptake (net ecosystem productivity NEP and net biome productivity NBP). We focus on a recent study (Graf et al. 2023, https://doi.org/10.1038/s43247-023-00958-4) and the questions:

(i) Do ecosystems sequestering more CO₂ have a lower albedo as a rule?

(ii) How close would such a relation be and how much room does it leave for climate-smart land use?

(iii) Given the different immediacy of albedo and NBP based radiative forcing, are there different mitigation policies to be preferred at different points in time?

To empirically investigate these questions with direct in-situ measurements, we identified 176 FLUXNET stations with sufficient coverage of NEP, incoming and outgoing shortwave radiation and ancillary data. A method to fill gaps in outgoing shortwave radiation and identify snow cover periods was developed and validated against available data and PI-provided snow statistics. 

We found a hyperbola-like decrease in maximum achievable effective (flux-weighted) long-term albedo as NEP increases, and vice versa. Apart from this joint limit, which also applied to non-forest and snow-free sites, the relation scattered strongly, indicating some room for climate-smart land use considering both albedo and carbon sequestration.

A conceptual model based on a paired-site permutation approach showed that maximizing each site’s NEP without considering albedo, leads to albedo-based positive radiative forcing (warming) during the first approximately 20 years, before being offset by an even stronger NBP-based cooling. However, the fact that most sites are currently far below their possible maximum albedo-NEP combination also allows for a balanced scenario in which both parameters are improved simultaneously. It avoids warming on all timescales, but provides less cooling than pure NEP maximization in the long term. We discuss how these timelines would interact with current emission reduction policies, the reasons underlying the relationship and real-world examples of joint NEP and albedo change.