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Cloud Effects on Forest Carbon Uptake
Reference
Min, Q. and Wang, S. 2008. Clouds modulate terrestrial carbon uptake in a midlatitude hardwood forest. Geophysical Research Letters 35: 10.1029/2007GL032398.

What was done
The authors examined "interannual variability of temperature, precipitation, and cloud distribution and their effects on carbon uptake by analyzing long term turbulent CO2 exchange and radiation measurements from 1992 to 2004 at a northern hardwood forest (Harvard Forest, 42.5N, 72.2W)." In doing so, they employed "the atmospheric transmittance index (TI) as a measure of cloudiness and aerosol loading, as derived from measured surface shortwave radiation," which "can be viewed as a measure of clouds with the combined effect of both cloud fraction and cloud optical depth," where "a smaller TI is produced by a larger cloud/aerosol optical depth or a greater cloud cover or a combined effect of cloud cover and cloud optical depth in partly cloudy conditions."

What was learned
Min and Wang report there is a tradeoff between reduced total incoming solar radiation and a higher radiation use efficiency under cloudy conditions that "results in a maximum of canopy carbon uptake at intermediate values of TI and reduction of carbon uptake at both ends of the TI distribution." Hence, there are two carbon uptake regimes -- "a cloud enhanced carbon uptake regime and a cloud suppressed carbon uptake regime" -- which differ somewhat for different air vapor pressure deficit regimes, such that "cloud modulation of net primary productivity is more significant in the more humid years than in the less humid years."

What it means
The two researchers conclude that clouds "and their distribution are the most important climatic factors in driving the interannual variability of the terrestrial carbon uptake during the growing season." In further support of this conclusion, they note that several studies "have shown previously that in the past few decades terrestrial primary productivity has increased in the North American midlatitudes," concurrent with an increase in cloud cover there. However, the productivity of the Amazonian rainforest also increased over this period, but cloud cover there decreased. In explaining this dichotomy, they say studies have shown that reduced cloud cover "enhances carbon sequestration in the tropical rainforest," which is just the opposite of what they observed in the northern hardwood forest they studied.

Of course, both of these computed responses could be correct. On the other hand, correlation does not necessarily prove causation; and it is possible that something else -- either by itself or acting in concert with the phenomenon here described -- could have been driving the historical increase in productivity in both the midlatitude and tropical situations, such as the concomitant increase in the air's CO2 concentration. Nevertheless, the findings of Min and Wang certainly seem robust in their own right; and they likely play a part (whether major or minor) in determining the ultimate long-term trajectory of terrestrial carbon uptake, at least in the case of the specific ecosystem they studied, and probably within a wider context as well.

Reviewed 3 September 2008