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Using satellite‐derived optical thickness to assess the influence of clouds on terrestrial carbon uptake
Author(s) -
Cheng S. J.,
Steiner A. L.,
Hollinger D. Y.,
Bohrer G.,
Nadelhoffer K. J.
Publication year - 2016
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1002/2016jg003365
Subject(s) - environmental science , satellite , canopy , atmospheric sciences , radiation , photosynthetically active radiation , range (aeronautics) , carbon fibers , remote sensing , materials science , chemistry , optics , physics , photosynthesis , geology , biology , astronomy , botany , biochemistry , composite number , composite material
Clouds scatter direct solar radiation, generating diffuse radiation and altering the ratio of direct to diffuse light. If diffuse light increases plant canopy CO 2 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 CO 2 uptake to sky conditions. We mechanistically link and quantify effects of cloud optical thickness ( τ c ) to surface light and plant canopy CO 2 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 τ c 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 τ 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 CO 2 fluxes.