Net primary productivity (NPP) is a conceptual breakthrough in improving ecological footprint (EF) analysis. Three common features shared by the two assessment methods are the following: (1) an overlapping of space occupied. EF converts all consumption into land most of which also participates in global carbon cycle and is involved in net primary production; (2) the same basic measurement units adopted in them. EF that uses biological productivity as the basic measurement unit converts physical hectares into global hectares equal to the global average productivity, and NPP is employed as a common indicator of biological productivity. Therefore, both methods measure ecological sustainability directly based on biological productivity in essence; and (3) the complementary to each other for further information. They both emphasize the desirability of reducing the demand for nonrenewable resources. NPP can provide some missing information on analyzing sustainability when combined with EF. On the other hand, EF is able to compensate NPP for the lack of straightforwardness when tracking the maintenance of the biosphere's capacity.
As a matter of fact, more and more studies attempt to include ocean, grassland and cropland's capacity for carbon sequestration into energy ecological footprint (EEF) assessment. However, EEF, the most important and disputed subject of EF, has rarely been applied in combination with NPP as modified methodologies. Of particular concern is how EEF is defined and what it actually measures, exclusion of a majority of land and water and failure to capture their actual capacity for carbon sequestration. Moreover, it illustrates the hypothetical land appropriation of energy consumption, which is inevitably in conflict with the actual appropriated land for biological production in one aggregation of EFs calculation.
In this paper, a modified method for assessing EEF is developed based on global NPP, thereby primarily focusing on the interaction between CO₂ emissions and land carbon sequestration. The results demonstrate the followings: (1) ocean, forest and grassland are the principal spatial components of EEF equivalents that measure EEF per unit mass of energy, accounting for 28.32%, 27.25% and 21.77% of the total, respectively; (2) the EEF factors that represent the calorific values per area of land appropriation for each group of energy decrease from 9.76×10⁷ to 68 GJ/hm² corresponding to electricity, gaseous energy, liquid energy and solid energy; and (3) all the energy sources would produce less ecological impact when using the modified method compared with the traditional method, as a result of considering the contribution of carbon sequestration from all types of land and water on the globe.
It is indicated as follows: (1) the EEF based on NPP reflects the world-average land appropriation of energy consumption; (2) linking biological productivity and carbon sequestration rate through NPP is able to reduce the disparity in spatial composition between EEF and the biological EF and hereby enhances their logic consistency; (3) changing the basis for EEF calculation from forest's carbon sequestration rate to NPP, is a useful tool for assessing the world-average land appropriation of energy consumption; and (4) the modified method can be used to improve the validity and accuracy of the calculation results and make more meaningful assessment of energy consumption for sustainable development.