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 (τc) to surface light and plant canopy CO2 uptake by comparing satellite retrievals of τc 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 τc, while diffuse PAR increased until an average τc of 6.8 and decreased with larger τc. When diffuse PAR increased with τc, 7–24% of variation in diffuse PAR was explained by τc. Light‐use efficiency (LUE) in this range increased 0.001–0.002 per unit increase in τc. Although τc 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.

Aerosol optical depth thresholds as a tool to assess diffuse radiation fertilization of the land carbon uptake in China

“…The appearance of thin cloud may enhance the net ecosystem productivity (NEP) of trees by 7-11 % Misson et al, 2005), while thick cloud reduces carbon uptake due to large irradiance attenuation (Rocha et al, 2004;Cheng et al, 2016). Aerosol light scattering in clear sky may increase the NEP of trees by 8-29 % (Misson et al, 2005;Cirino et al, 2014), but decreases the carbon uptake of grassland (Niyogi et al, 2004). Previous vegetation modeling results are generally consistent with observations (Table 1).…”

“…Our estimates of GPP sensitivity to DF and AOD are reasonable compared with previous studies (Table 1) as summarized below: (1) the maximum enhancement of GPP is 40 % under clear-sky conditions for most PFTs (Gu et al, 2003); (2) the GPP enhancement of C4 plants is the least due to the lowest LUE compared with other PFTs (Still et al, 2009;Kanniah et al, 2012); (3) the aerosol DFE is much stronger under clear-sky than under all-sky conditions (Cohan et al, 2002); (4) both clouds and aerosols exert similar DFE on land carbon uptake (Kanniah et al, 2012;Cirino et al, 2014); and (5) the maximum GPP enhancement appears when DF = 0.4-0.8 (Rocha et al, 2004;Alton, 2008;Zhang et al, 2010). Most results listed in Table 1 are based on NEP; however, we evaluate the sensitivity of GPP because it is the direct carbon metric affected by DFE.…”

“…Previous observation-based studies of cloud and aerosol DFE are summarized in Table 1. Most observational studies have detected DFE using long-term ground-based measurements (Niyogi et al, 2004;Cirino et al, 2014). Currently, direct measurements of DFE on photosynthesis in China are limited (Li et al, 2014).…”

An Evaluation of Semiempirical Models for Partitioning Photosynthetically Active Radiation Into Diffuse and Direct Beam Components

“…Different relative levels of these forms result in different irradiance patterns within plant canopies (Norman & Welles, ) and across heterogeneous terrain (Oliphant et al, , ) (see Table for a list of symbols). For example, global photosynthetically active radiation (PAR G )—a spectral subset of K G —is more efficient for canopy photosynthesis under conditions of diffuse beam PAR (PAR D ) than direct beam PAR (PAR S ) per unit PAR (Alton, ; Cheng et al, , ; Gu et al, , ; Keppel‐Aleks & Washenfelder, ; Mercado et al, ; Oliphant et al, ). Furthermore, the ratio of K D to K G has been found to lower the Bowen ratio (the ratio of sensible to latent heat flux) in a number of ecosystems (Steiner et al, ) and is important for the spatial distribution of net radiation in complex natural and urban terrain (e.g., Lindberg et al, ; Oliphant et al, ).…”

A Novel Diffuse Fraction‐Based Two‐Leaf Light Use Efficiency Model: An Application Quantifying Photosynthetic Seasonality across 20 AmeriFlux Flux Tower Sites

“…This implies that plant canopies act like one big leaf and the canopy absorbs direct and diffuse radiation at the same LUE during photosynthesis. The assumption is contradicted, however, by observations that canopy LUE strongly correlates with the diffuse-light fraction (D f ) (Choudhury, 2000;Farquhar & Roderick, 2003;Kanniah et al, 2013) and that diffuse radiation produces a higher LUE than direct radiation (Cheng et al, 2015(Cheng et al, , 2016. This LUE-D f relationship is probably explained by the fact that shaded leaves are not light saturated while sunlit leaves can be (de Pury & Farquhar,1997;Gu et al, 2002;Knohl & Baldocchi, 2008).…”