Using satellite-derived optical thickness to assess the influence of clouds on terrestrial carbon uptake

Abstract

Clouds scatter direct solar radiation, generating diffuse radiation and altering the ratio of direct to diffuse light. If diffuse light increases plant canopy CO₂ uptake, clouds may indirectly influence climate by altering the terrestrial carbon cycle. However, past research primarily uses proxies or qualitative categories of clouds to connect the effect of diffuse light on CO₂ uptake to sky conditions. We mechanistically link and quantify effects of cloud optical thickness (τ_c) to surface light and plant canopy CO₂ uptake by comparing satellite retrievals of τ_c to ground-based measurements of diffuse and total photosynthetically active radiation (PAR; 400–700 nm) and gross primary production (GPP) in forests and croplands. Overall, total PAR decreased with τ_c, while diffuse PAR increased until an average τc of 6.8 and decreased with larger τ_c. When diffuse PAR increased with τ_c, 7–24% of variation in diffuse PAR was explained by τ_c. Light-use efficiency (LUE) in this range increased 0.001–0.002 per unit increase in τ_c. Although τ_c explained 10–20% of the variation in LUE, there was no significant relationship between τc and GPP (p>0.05) when diffuse PAR increased. We conclude that diffuse PAR increases under a narrow range of optically thin clouds and the dominant effect of clouds is to reduce total plant-available PAR. This decrease in total PAR offsets the increase in LUE under increasing diffuse PAR, providing evidence that changes within this range of low cloud optical thickness are unlikely to alter the magnitude of terrestrial CO₂ fluxes.

Citation

Cheng, S.J.; Steiner, A.L.; Hollinger, D.Y.; Bohrer, G.; Nadelhoffer, K.J. 2016. Using satellite-derived optical thickness to assess the influence of clouds on terrestrial carbon uptake. Journal of Geophysical Research: Biogeosciences. Journal of Geophysical Research: Biogeosciences. 121(7): 1747-1761.