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QUESTIONS AND ANSWERS
The view from on high: atmospheric CO2
inversions in search of the missing sink.
Kevin Gurney, Purdue
Univ.
Q: Why do boreal North America and boreal Asia show
such differences in the way carbon fluxes are correlated with climate variables?A:
This is a key question. The analysis I have performed hasn't really answered
that question but there may be a few possibilities. The first is the possibility
that one region is having a dominant photosynthetic response while the other is
having a dominant respiration response due to the time of the year that climate
anomalies occur in these regions. For example, boreal North America might be
showing anomalous uptake associated with warmer spring temperatures. Boreal Asia
may be showing excess respiration when there are warmer than average fall
temperatures. The timing of the climate anomaly may simply have a differential
impact because of the general attributes of growing season response. There also
may be ecological reasons why vegetation in boreal North America versus boreal
Asia respond differently to the same anomalous climate signals. Because of
greater continentality in Asia, warmer springs (as measured by air temperatures)
may not penetrate the soil column as rapidly as in boreal NA. Hence, a spring
anomaly may have a lesser response.
Q: Why is tropical Asia a source of CO2 in the
atmosphere?
A: Tropical Asia like the other tropical land
regions emits net carbon to the atmosphere because of the well-observed
deforestation that is ongoing in those regions. What is interesting is that the
inverse approach estimates a smaller net emission that studies that attempt to
calculate net emissions from "ground up." Those studies come up with generally
larger emission estimates. Either one or both of the methods contain biases
(always a possibility) or there is a certain amount of net uptake that is being
missed by the bottom up studies and as such, the two methods are actually seeing
different things.Q: There was little
mention of South America. Some research suggests that South American tropical
forests may contain a portion of the terrestrial sink. How do these calculations
deal with this?
A: The inverse calculations do arrive at an
estimate of tropical American net carbon exchange. And, like most studies
(inverse and otherwise), the tropical land regions are sources of carbon to the
atmosphere. Because of the pattern of CO2 concentrations
in the atmosphere, the sink must be over the mid to high northern latitudes.
That is not disputed at this point in time. However, as I point out in the
question above, the source from tropical land may not be as large as
traditionally thought and that may be due to an overestimation of deforestation,
an underestimation of regrowth or a light incremental growth in intact forests
that have not been measured systematically.Q:
Why does the total ocean flux not coincide with the ENSO cycles (as land and
total flux do)?
A: We certainly find lots of correspondence
between tropical Pacific carbon exchange and ENSO but that does not hold up for
other ocean regions. The summation, as a result, shows little correspondence to
ENSO. The teleconnections of ENSO to the land carbon cycle are more coherent
because the land vegetation is responding to the well- characterized changes in
temperature and precipitation that occur worldwide (though often lagged). The
tropical Pacific response is primarily driven by change in the equatorial
upwelling/downwelling patterns, and as such, are a direct, mechanical result of
ENSO. One would not expect those types of mechanisms to "act at a distance"
especially in comparison to the vegetation response which really does act at a
distance because of the T and PPT changes.
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