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Ecoregions in CarbonTracker [goto top]
1.   What are ecoregions?
Ecoregions are the actual scale on which CarbonTracker performs its optimization over the land. Ecoregions are meant to represent large expanses of land within a given continent having similar ecosystem types, and are used to divide continents into smaller pieces for analysis. The ecosystem types use in CarbonTracker are derived from the Olson [1992] vegetation classification (Table 1, Figure 1).

We define an ecoregion as an ecosystem type within a given Transcom land region. There are 11 such Transcom land regions (Figure 2), so there are 11*19 = 209 possible ecoregions. However, not all ecosystem types are present in all Transcom regions, and the actual number of land ecoregions ends up being 126.

Note on "Semitundra": this is a potentially misleading shorthand abbreviation for a collection of ecosystems comprising semi-desert, shrubs, steppe, and polar+alpine tundra. The "Semitundra" zones appearing in northern Africa where one expects to find the Sahara desert are not, of course, tundra environments. They are instead semi-desert zones.

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Figure 1. Global distribution of Olson ecosystem types.

Table 1. Ecosystem areas over the two Transcom regions covering North America.
Ecosystem Type North American Boreal North American Temperate
Area (km2) Percentage Area (km2) Percentage
Conifer Forest 2315376 22.9% 1607291 14.0%
Broadleaf Forest - - 269838 2.4%
Mixed Forest 592291 5.9% 930813 8.1%
Grass/Shrub 53082 0.5% 2515582 21.9%
Tropical Forest - - 58401 0.5%
Scrub/Woods - - 416520 3.6%
Semitundra 3396292 33.6% 866468 7.6%
Fields/Woods/Savanna 29243 0.3% 1020939 8.9%
Northern Taiga 1658773 16.4% - -
Forest/Field 61882 0.6% 1243174 10.8%
Wetland 322485 3.2% 66968 0.6%
Deserts - - 21934 0.2%
Shrub/Tree/Suc - - 11339 0.1%
Crops - - 1969912 17.2%
Conifer Snowy/Coastal 41440 0.4% 73437 0.6%
Wooded tundra 360388 3.6% 6643 0.1%
Mangrove - - - -
Non-optimized areas - - - -
Water 1269485 12.6% 384728 3.4%
Total 10100736 100.0% 11463986 100.0%

2.   Why use ecoregions?
A fundamental challenge to atmospheric inversions like CarbonTracker is that there are not enough observations to directly constrain fluxes at all times and in all places. It is therefore necessary to find a way to reduce the number of unknowns being estimated. Strategies to reduce the number of unknowns in problems like this one generally impose information from external sources. In CarbonTracker, we reduce the problem size both by estimating fluxes at the ecoregion scale, and by using a terrestrial biological model to give a first guess flux from the ecoregion. The model is also used to give the spatial and temporal distribution of CO2 flux within a region and week.

2.   Ecosystems within Transcom regions
Each Transcom land region (Figure 2) can contain up to 19 ecoregions.

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Figure 2. The 11 land regions and 11 ocean regions of the Transcom project.

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Figure 3. Ecoregions within the North American Boreal (left) and North American Temperate (right) Transcom regions.

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Figure 4. Ecoregions within the South American Tropical (left) and South American Temperate (right) Transcom regions.

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Figure 5. Ecoregions within the Europe Transcom region.

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Figure 5. Ecoregions within the Northern Africa (left) and Southern Africa (right) Transcom regions.

3.   Further Reading