1.3.2.5. Spatial and Temporal Integration
Measurements of NEP are demanding of resources, therefore still are not numerous.
They also are not always representative for the whole growing season, and they
are never representative for the whole life cycle of longer lived plants. Few
observational series extend over more than 5 years. Within decades, a forest
site that was chosen for measuring NEP will likely enter a different phase or
be subjected to natural disturbances such as fire, pests, and wind-throw or
management operations such as thinning and felling. Although measurements made
from satellites may suggest that NPP has been increasing over the past decades
in some regions and measurements of NEP may seem to indicate that many forests
are significant carbon sinks, when the effects of disturbances are included
the sequestration of carbon into these forests may be significantly less or
even, in some areas, close to zero. Importantly, the disturbances themselves
may be influenced by climate, and a change in the disturbance regime may turn
a positive NBP into a negative one-or vice versa. For example, measurements
of NEP in Canadian boreal forests suggest that many of these forests are carbon
sinks (see Section 1.3.2.3), but estimates of NBP from
measurement of changes in disturbance regimes and consideration of forest stand
dynamics indicate that NBP may have declined significantly over the past 3 decades
and that these forests, over large areas, are close to being carbon neutral
(Walker et al., 1999). With the limited empirical data available, however,
it is difficult to derive accurate local estimates of NBP for regions, biomes,
countries, or continents from spatial and temporal integration of the constituent
processes and disturbances (Schulze et al., 1999; Houghton et al.,
2000). Our restricted ability to build estimates of NBP from its components
at the present time defines a gap in our knowledge and the need to use other
methodologies.
If the values in the preceding sections are considered representative for the
major forest biomes-tropical, temperate, and boreal forests, respectively-and
for the total area that they cover (approximately 40 x 106 km2), total NEP for
these systems would be about 10 Gt C yr-1. However, global NBP, derived as the
difference between the output resulting from fossil fuel burning, on the one
hand, and the increase of atmospheric concentrations and net ocean uptake, on
the other, is currently a little less than about 1 Gt C yr-1 (see Table
1-2). If the effects of land-use change are excluded from this estimate
of NBP, the estimate of global NBP increases to about 2-3 Gt C yr-1-about five
times less than the total NEP (see Section 1.2.1.4). Thus,
NEP values as reported so far are clearly not representative of the large-scale,
long-term storage of carbon. This fact emphasizes the importance of viewing
the activities defined in Articles 3.3 and 3.4 of the Kyoto Protocol in a large-scale
and long-term perspective.
Recent attempts to determine the large-scale distribution of terrestrial sources
and sinks (i.e., NBP) indirectly, on the basis of the observed spatial variation
of atmospheric carbon dioxide concentrations, have interpreted low values as
indications of net uptake and high values as the presence of net sources (Fan
et al., 1998; Bousquet et al., 1999; Rayner et al., 2000).
In light of the relatively small magnitude of the regional NBP, this determination
requires accurate knowledge about both the spatial distribution of carbon dioxide
concentrations and its horizontal and vertical transport as a result of air
motions. The inverse modeling analyses of Rayner et al. (2000) and Bousquet
et al. (1999) agree reasonably closely in indicating a net terrestrial
carbon sink in Siberia and comparable, smaller net sinks in North America and
Europe, with small net sources in South America and Africa.
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