Allen, L.H., Kimball, B.A.,
Bunce, J.A., Yoshimoto, M.,
Harazono, Y., Baker, J.T.,
Boote, K.J. and White, J.W.
2020
Fluctuations of CO2
in Free-Air CO2
Enrichment
( FACE )
depress plant
photosynthesis,
growth, and yield.
Agricultural and Forest
Meteorology 284: 107899.
FACE
( free-air CO2 enrichment )
studies are not as realistic
of future growing conditions
as the IPCC and others claim.
A correction factor of
about x1.5 must be applied
to their projections of
future biomass and
yield responses of plants
and crops, because FACE
studies will understate
the actual effect of CO2
enrichment in real life.
OTC
( open-top chambers )
studies yield plant
CO2-induced growth
significantly higher
than in FACE studies.
The benefits of
atmospheric CO2
enrichment on plant
growth is nearly always
positive, but likely to be
understated when FACE
technology is used.
Allen et al. (2020)
report that the
CO2 fluctuations
in FACE experimental
systems were found to be
"more than 10-fold greater
than in nature,"
adding that
"exposures to elevated CO2
in FACE are not representative
of exposure to atmospheric
elevated CO2 (with natural
fluctuations) projected
to occur in the future."
According to Allen et al.,
"oscillations
and fluctuations
of elevated CO2
decrease observed
leaf photosynthetic
rates compared to
steady levels
of elevated CO2."
Those reductions
in photosynthesis
reduce the CO2
fertilization gains
to the extent that
the scientists say:
"prior conclusions
of the response
of plants
to elevated CO2
under FACE
conditions
may need to be
reassessed
by applying an
adjustment factor
(about 1.5x)
for photosynthesis,
biomass, and yield.
for conclusions
reached in experimental
and modeling studies."
Early CO2 enrichment
studies were typically
performed in
laboratories,
under controlled-
environment settings.
Then scientists
shifted their work
on elevated CO2
to open-top chambers
( OTCs )
located in outdoor
environments,
where the plants
could be exposed
to the elements
under mostly natural
growing conditions.
The next (current)
generation of studies
is FACE experiments,
constructed without walls.
Typically,
several horizontal
or vertical pipes
are placed around
an experimental
plot area that ranges
from 1 to 30 meters
in diameter.
CO2 enriched air
is emitted from the pipes,
and the CO2 concentration
is kept at the desired level,
via a system of
strategically placed
CO2 sensors.
They feed CO2 data
back to a computer,
that adjusts
the flow of CO2
from the pipes,
trying to counter
CO2 level variations
caused by changes
in wind speed
and direction.