Planetary boundary downscaling for absolute environmental sustainability assessment — Case study of Taiwan

Abstract

• The PB concept has been applied in assessing AES in local level. • The current national data can be easily connected with PB concept. • Downscaled PBs by territorial perspective can be the local targets for AES. • Dynamic concept was introduced for enhancing normalization and weighting. The planetary boundary (PB) concept can be used as a guide for the absolute environmental sustainability (AES) of humanity. However, the downscaling of PB as a guide remains a challenge at the local scale. This study aims to establish absolute environmentally sustainable indicators (AESIs) and a threshold based on five processes of the Earth system, namely, climate change, ocean acidification, biogeochemical flow (phosphorus and nitrogen cycles), land system change, and freshwater use, for local AES assessment. A case study of Taiwan was conducted. This work also introduced the dynamic concept into developing dynamic normalization factors (DNFs) and dynamic weighting factors (DWFs) for AESIs to support decision-making. DNFs were defined as desired targets in the future; they were developed on the basis of the thresholds of AESIs at the global and national levels in a different timeline. DWFs were defined as weights for assessing the distance of a current situation from the desired targets; they were evaluated on the basis of the distance-to-target approach. The territorial perspective was adopted as a downscaling method to develop 11 AESIs and thresholds based on the PB framework for the AES assessment of Taiwan. Results showed that the nationally determined contribution target of the carbon pathway is not ambitious compared with the annual and cumulative boundaries of climate change and ocean acidification. Phosphorus and nitrogen fertilizers are two of the three subcategories in biogeochemical flow with very high risks. Phosphorus flow from a freshwater system to the oceans and forest cover was evaluated as moderate risk. Meanwhile, annual freshwater use was determined as low risk. However, when spatial and temporal factors were considered, annual freshwater use was assessed as high risk during the dry season in Southern and Central Taiwan. DNFs and DWFs are more suitable at the local level than at the global level when applying normalization and weighting to reflect a real situation. This study not only provided a new concept for local policy makers to rethink environmental sustainability, but also explored various AES tools at the local scale to connect the local situation with global issues to support decision-making.