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

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 ((c)) to surface light and plant canopy CO2 uptake by comparing satellite retrievals of (c) to ground-based measurements of diffuse and total photosynthetically active radiation (PAR; 400-700nm) and gross primary production (GPP) in forests and croplands. Overall, total PAR decreased with (c), while diffuse PAR increased until an average (c) of 6.8 and decreased with larger (c). When diffuse PAR increased with (c), 7-24% of variation in diffuse PAR was explained by (c). Light-use efficiency (LUE) in this range increased 0.001-0.002 per unit increase in (c). Although (c) explained 10-20% of the variation in LUE, there was no significant relationship between (c) 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.