Reflections on Water-Food-Ecosystem Nexus after recent reassessment of Planetary Boundaries

The planetary boundaries are the nine environmental indices (climate change, biosphere integrity [biodiversity], land-system change [land use], freshwater use, biogeochemical flows, ocean acidification, atmospheric aerosol loading, stratospheric ozone depletion, novel entities), which can help determine how close to or already out of safety limits for humans and ecosystems as a result of human acitivities. It first became famous by a paper published in the journal Nature in 2009 (Rockström et al., 2009), and was updated by a Science paper in 2015 (Steffen et al., 2015) reflecting six years of scientific development and environmental changes. By 2015, 4 out of 9 indices exceeded the global safety limits (climate change, biosphere integrity, land-system change, biogeochemical flow).

However, in 2022, two important papers were published one after another that even two (wholly or partially) of the remaining four indices exceeded the global safety limits. Persson et al. (2022) were the first to assess the level of ‘increase in novel entities’, which previously had an index due to insufficient data. The authors believe that the production and release of new substances (chemical pollutants, plastics, etc.), which were not present in the geological era, are increasing at a tremendous rate in quantity and types, exceeding society's ability to evaluate and track safety, so that the index has increased. It was judged that the global danger limit was breached.

And the paper of Wang-Erlandsson et al. (2022), published at the end of April, evaluates that even the level of freshwater use, which until now was considered to be no problem, has exceeded the safe range. The use of blue water (rivers, lakes, reservoirs, & renewable groundwater stores), which has been the standard for evaluating the level of freshwater use so far, is 2,600 km³ year⁻¹, which is within the allowable range (4,600 km³ year⁻¹). However, blue water is not the only fresh water. Much more fresh water is green water (terrestrial precipitation, evaporation, & soil moisture). According to the 6th Assessment Report of the IPCC Working Group II, currently 19,000 km³ of soil moisture is used annually by agriculture and forestry in the world (IPCC, 2022). It is more than 7 times that of blue water.

However, the sustainability of green water is completely different from that of blue water. Wang-Erlandsson et al. (2002) selected root-zone soil moisture as a representative indicator of green water. Now (i.es., the 1850–2014 average compared to the mid-Holocene; the 1900–2014 average compared to pre-industrial times), the mean value of the root-zone soil moisture was outside the range of 5~95% of the soil moisture content compared to the mid-Holocene (first 500 years since about 6,000 ago) and pre-industrial times (from 1850 to 1899). That is, they have become significantly wetter or drier than the stable range of soil moisture that has historically supported terrestrial ecosystems. In particular, compared to pre-industrial levels, the average soil moisture deviated completely from the 5–95% range since 1980s for wetter areas and since 1920s for drier areas. The following figure is a comprehensive chart newly drawn by the Stockholm Resilience Center (SRC) that reflects the latest evaluation results.

https://bit.ly/3s7SgQp

The IPCC expects these changes to worsen in the future. Current trends (similar to the SSP2-4.5 scenario in which the world implements current climate policies as shown in the following table) suggest a ‘2°C warming relative to pre-industrial levels’ (or 1.8–2.5°C range) likely to materialize in the 2041–2060s.). Then, the population at risk of drought damage due to lack of soil moisture increases by 370% (30–790%), and the duration of soil moisture drought becomes 2-3 times longer than now (IPCC, 2022).

The SRC summarized the findings of Wang-Erlandsson et al. (2022) and explained that green water is particularly closely linked with land use, biodiversity and climate change. Although it is still a rather unfamiliar concept, it can be expected that the crisis of green water will have a negative impact on the future of food (e.g. agriculture based on land use) and ecosystem (inextricably linked with biodiversity). It raises an urgent research topic for researchers studying the Water-Food-Ecosystem Nexus.

References

IPCC. (2021). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. (In Press).

IPCC. (2022). Climate Change 2022: Impacts, Adaptation and Vulnerability. Working Group II contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. (In Press).

Persson, L., Carney Almroth, B. M., Collins, C. D., Cornell, S., de Wit, C. A., Diamond, M. L., Fantke, P., Hassellöv, M., MacLeod, M., Ryberg, M. W., Søgaard Jørgensen, P., Villarrubia-Gómez, P., Wang, Z., & Hauschild, M. Z. (2022). Outside the Safe Operating Space of the Planetary Boundary for Novel Entities. Environmental Science & Technology, 56(3), 1510–1521.

Rockström, J., Steffen, W., Noone, K., Persson, Å., Chapin, F. S., Lambin, E. F., Lenton, T. M., Scheffer, M., Folke, C., Schellnhuber, H. J., Nykvist, B., de Wit, C. A., Hughes, T., van der Leeuw, S., Rodhe, H., Sörlin, S., Snyder, P. K., Costanza, R., Svedin, U., Falkenmark, M., Karlberg, L., Corell, R. W., Fabry, V. J., Hansen, J., Walker, B., Liverman, D., Richardson, K., Crutzen, P., & Foley, J. A. (2009). A safe operating space for humanity. Nature, 461(7263), 472–475.

Steffen, W., Richardson, K., Rockström, J., Cornell, S. E., Fetzer, I., Bennett, E. M., Biggs, R., Carpenter, S. R., de Vries, W., de Wit, C. A., Folke, C., Gerten, D., Heinke, J., Mace, G. M., Persson, L. M., Ramanathan, V., Reyers, B., & Sörlin, S. (2015). Planetary boundaries: Guiding human development on a changing planet. Science, 347(6223), 1259855.

Wang-Erlandsson, L., Tobian, A., van der Ent, R. J., Fetzer, I., te Wierik, S., Porkka, M., Staal, A., Jaramillo, F., Dahlmann, H., Singh, C., Greve, P., Gerten, D., Keys, P. W., Gleeson, T., Cornell, S. E., Steffen, W., Bai, X., & Rockström, J. (2022). A planetary boundary for green water. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-022-00287-8