98
6 Benchmarking the Carbon, Water and Land Footprints …
(PRB 2009), we adopt the mean value of 3.1 t CO 2 -eq./yr for both planetary and
national carbon boundary per capita.
6.2.4.2 Quantifying National Water and Land Boundaries
on a Resource Availability Basis
The distribution of worldwide water and land resources is geographically heterogeneous, with implications for local or regional environment in most cases. This calls
for the need to take spatial variations in resource scarcity into account (Aubauer 2011;
Hoekstra 2009). In theory, nation-specific environmental boundaries for resource use
can be quantified either through the aggregation of local or regional scarcity thresholds, if present, or through the overall estimate of resource availability within the
national borders. The first approach, which likely leads to more accurate results,
is constrained by the lack of data on local resource depletion with evidence of
threshold behaviors and of knowledge on the cumulative effects of multiple regions
(i.e., it is unclear whether the national environmental boundary simply amounts to
the sum, maximum or minimum of regional environmental boundaries). By contrast,
the second approach under current conditions has been found preferable to measure
national water and land boundaries (Cole et al. 2014; Nykvist et al. 2013).
It has been demonstrated that 90% of the green water availability throughout the
world is required to maintain the operation of critical ecosystem services irrespective
of human actions (Bogardi et al. 2013; Rockström et al. 1999), and that a reference
value for the green water availability is not yet available (Hoekstra and Wiedmann
2014). All this allows the blue water availability to be an approximation to the national
water boundary, defined as annually renewable water supply minus environmental
flow requirements for ecological health within the border of a country (Hoekstra et al.
2012). Consideration of these environmental flows is necessary to prevent disastrous
impacts associated with water scarcity (Gerten et al. 2013). Following Rockström
et al. who proceed with planetary water boundary on the explicit understanding
that undesirable or even disastrous consequences may trigger if the ratio of water
withdrawal to the renewable supply surpasses 40%, this chapter defines the water
boundary for a country as 40% of the total renewable water resources (RWRs)—the
sum of the internal and external water resources of that country. Data for the RWRs
of individual countries are obtained from the Food and Agriculture Organization’s
(FAO 2012) database AQUASTAT. In theory, the sum of the water boundaries (not
per capita) for all nations is likely to be equal to the total PB (not per capita). However,
there are some reasons that explain why the two numbers would not be identical,
such as the variation in data sources, and the double-counting of the external water
resources in both the upstream country and downstream country.
In the original version of the PBF, land boundary was assessed by the criterion that
a maximum of 15% of Earth surface is allowed to convert to cropland (Rockström
et al. 2009). This brings the risk of underestimating the role of crop production in
human survival. In practice, converting land for farming would, on the contrary,
promote a great deal of welfare and therefore deserving of positive evaluation (Bass
6 Benchmarking the Carbon, Water and Land Footprints …
(PRB 2009), we adopt the mean value of 3.1 t CO 2 -eq./yr for both planetary and
national carbon boundary per capita.
6.2.4.2 Quantifying National Water and Land Boundaries
on a Resource Availability Basis
The distribution of worldwide water and land resources is geographically heterogeneous, with implications for local or regional environment in most cases. This calls
for the need to take spatial variations in resource scarcity into account (Aubauer 2011;
Hoekstra 2009). In theory, nation-specific environmental boundaries for resource use
can be quantified either through the aggregation of local or regional scarcity thresholds, if present, or through the overall estimate of resource availability within the
national borders. The first approach, which likely leads to more accurate results,
is constrained by the lack of data on local resource depletion with evidence of
threshold behaviors and of knowledge on the cumulative effects of multiple regions
(i.e., it is unclear whether the national environmental boundary simply amounts to
the sum, maximum or minimum of regional environmental boundaries). By contrast,
the second approach under current conditions has been found preferable to measure
national water and land boundaries (Cole et al. 2014; Nykvist et al. 2013).
It has been demonstrated that 90% of the green water availability throughout the
world is required to maintain the operation of critical ecosystem services irrespective
of human actions (Bogardi et al. 2013; Rockström et al. 1999), and that a reference
value for the green water availability is not yet available (Hoekstra and Wiedmann
2014). All this allows the blue water availability to be an approximation to the national
water boundary, defined as annually renewable water supply minus environmental
flow requirements for ecological health within the border of a country (Hoekstra et al.
2012). Consideration of these environmental flows is necessary to prevent disastrous
impacts associated with water scarcity (Gerten et al. 2013). Following Rockström
et al. who proceed with planetary water boundary on the explicit understanding
that undesirable or even disastrous consequences may trigger if the ratio of water
withdrawal to the renewable supply surpasses 40%, this chapter defines the water
boundary for a country as 40% of the total renewable water resources (RWRs)—the
sum of the internal and external water resources of that country. Data for the RWRs
of individual countries are obtained from the Food and Agriculture Organization’s
(FAO 2012) database AQUASTAT. In theory, the sum of the water boundaries (not
per capita) for all nations is likely to be equal to the total PB (not per capita). However,
there are some reasons that explain why the two numbers would not be identical,
such as the variation in data sources, and the double-counting of the external water
resources in both the upstream country and downstream country.
In the original version of the PBF, land boundary was assessed by the criterion that
a maximum of 15% of Earth surface is allowed to convert to cropland (Rockström
et al. 2009). This brings the risk of underestimating the role of crop production in
human survival. In practice, converting land for farming would, on the contrary,
promote a great deal of welfare and therefore deserving of positive evaluation (Bass