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| Titel |
Increased diffuse radiation fraction does not significantly accelerate plant growth |
| VerfasserIn |
Alon Angert, Nir Krakauer |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032683
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| Zusammenfassung |
| A recent modelling study (Mercado et al., 2009) claims that increased numbers
of scattering aerosols are responsible for a substantial fraction of the terrestrial
carbon sink in recent decades because higher diffuse light fraction enhances plant net
primary production (NPP). Here we show that observations of atmospheric CO2
seasonal cycle and tree ring data indicate that the relation between diffuse light and
NPP is actually quite weak on annual timescales. The inconsistency of these data
with the modelling results may arise because the relationships used to quantify the
enhancement of NPP were calibrated with eddy covariance measurements of hourly
carbon uptake. The effect of diffuse-light fraction on carbon uptake could depend on
timescale, since this effect varies rapidly as sun angle and cloudiness change, and since
plants can respond dynamically over various timescales to change in incoming
radiation.
Volcanic eruptions, such as the eruption of Mount Pinatubo in 1991, provide the best
available tests for the effect of an annual-scale increase in the diffuse light fraction. Following
the Pinatubo Eruption, in 1992 and 1993, a sharp decrease in the atmospheric CO2 growth
rate was observed. This could have resulted from enhanced plant carbon uptake. Mercado et
al. (2009) argue that largely as a result of the (volcanic aerosol driven) increase in diffuse
light fraction, NPP was elevated in 1992, particularly between 25Ë N-45Ë N where annual
NPP was modelled to be ~0.8 PgC (~10%) above average. In a previous study
(Angert et al., 2004) a biogeochemical model (CASA) linked to an atmospheric
tracer model (MATCH), was used to show that a diffuse-radiation driven increase in
NPP in the extratropics will enhance carbon uptake mostly in summer, leading
to a lower CO2 seasonal minimum. Here we use a “toy model” to show that this
conclusion is general and model-independent. The model shows that an enhanced
sink of 0.8 PgC, similar to that modelled by Mercado et al. (2009), will result in a
measurable decrease (~0.6ppm) in the seasonal CO2 minimum. This holds regardless of
whether the sink is the result of 1) An increase in NPP, or 2) The combined effect of a
temperature-driven decrease in heterotrophic respiration (Rh) and no change in NPP.
This is since both NPP and Rh peak in summer. By contrast, observations from the
NOAA global CO2 monitoring network show the opposite change in the seasonal
minimum in 1992 and 1993 (~0.2ppm increase) both at Mauna Loa, and in the Marine
Boundary Layer mean (>20Ë N), which is hard to reconcile with increased NPP in
northern summer. Another indicator of annual NPP is tree wood increment. Previous
work (Krakauer et al., 2003) showed that the average response in tree ring series
after past Pinatubo-size volcanic eruptions implied lower NPP north of 45Ë N,
presumably as a result of shorter growing season and lower total irradiance induced by
scattering aerosols, and no significant change in NPP at lower latitudes. Here we show
that In 1992, after the Pinatubo eruption, ring width in the 25Ë N-45Ë N band was
99.3±2.9% of average (n=351 sites), similar to the average of 100.4±2.2% over
past eruptions (n=15 eruptions) (Uncertainty is given as 2 SE.). These results are
also inconsistent with substantial NPP enhancement, although a limitation of the
tree-ring approach is that available measurements do not uniformly sample the
latitude band. The combined evidence of tree rings and the CO2 seasonal cycle shows
that the enhancement of NPP by scattering aerosols on annual timescales is weak.
This result suggests that reducing aerosols through stricter pollution controls may
strengthen the land carbon sink, while geo-engineering schemes which aim to mitigate
global warming by spreading scattering aerosols in the stratosphere may weaken it. |
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