A study on long-term environmental impacts with global scale simulation considering the interaction between the natural environment and economic activity.
-Aiming to achieve a global scale inclusive sustainability^*1-

The University of Tokyo (Headquarters: Bunkyo-ku, Tokyo; President: Teruo Fujii; hereinafter referred to as "UTokyo"^*2) and Nippon Telegraph and Telephone Corporation (Headquarters: Chiyoda-ku, Tokyo; President and CEO: Akira Shimada; hereinafter referred to as "NTT") will reproduce the interaction between the natural environment and economic activities on the global water cycle, to achieve a global simulation environment that enables the study of various policies to achieve inclusive sustainability. By reproducing the interactions between the natural environment and economic activities related to the global water cycle, NTT aims to create a global-scale simulation environment that enables the study of various policies to achieve inclusive sustainability.
　As the first step, under the joint research agreement "Joint Research on the Model of Atmospheric and Land Surface Economic Cycle of Water Use," a global-scale simulation system was established to reproduce how land use patterns (forest, grassland, farmland, etc.) change over time as economic activities change, thereby affecting water flow and environmental impacts. This has enabled observing environmental load (water stress⁺¹) while considering the mutual effect on the environment and the economy over a long time^*3.
　In this research, we have applied the Integrated Land Simulator (ILS)^*4 developed by the Yoshimura Laboratory of the Institute of Industrial Science at UTokyo (hereinafter referred to as "the Yoshimura Lab.") to reproduce surface water circulation processes, and GCAM^*5, an integrated assessment model (IAM) for economic activities developed by the Joint Global Change Research Institute (JGCRI). These simulators in different fields are linked through coupled simulation technology^*6 developed by NTT.

1．Background

The global-scale climate change represented by global warming is currently affecting not only the natural environment but also human activities in various ways, such as a continuous sea level rise due to collapsing glaciers, increases in localized heavy rainfall, and flood damage due to the intensification of flooding. Given this situation, the United Nations Intergovernmental Panel on Climate Change (IPCC) regularly prepares and publishes reports based on papers and other literature written by scientists around the world to provide scientific knowledge for governments' climate change policies^*7.
　The Yoshimura Lab. of UTokyo is researching global surface water cycle processes and climate change. As part of a next-generation Earth system model for predicting future climate, The Yoshimura Lab. has developed an integrated land simulator (ILS) that can simulate the complex behavior of water and energy associated with evaporation, infiltration, runoff to rivers, etc. on the surface of the Earth.
　NTT is working to develop large-scale computational processing technologies so that many customers can use various information and communication technology services with peace of mind. NTT is also working to "derive inclusive equilibrium solutions for the Earth and its social and economic systems" based on the vision that to solve problems related to the environment and people's lives on the Earth, we need to view the entire Earth as a cyclical system in an inclusive manner, instead of viewing individual problems as partial solutions (See the "2020 press announcement^*8 made on November 23, 2020" In it, NTT announces the new R&D projects of Digital Twin Computing --- Four grand challenges towards a new future world creating harmonized relationship between the Earth, society, and people").
　To this end, the Yoshimura Lab. of UTokyo and NTT share the vision that the entire planet must be viewed inclusively as a natural and social circulation system, and they have launched this joint research project to reproduce the mutual effect on the natural environment and economic activities, focusing on the water cycle, which is indispensable for all life and human economic activities on the Earth.
　Until now, various experts in the fields of climate/weather, economics, and society have been engaged in research on models that reproduce complex events and the development of forecasting simulation technology. Although the Earth's environment is autonomous, environmental problems occur when human economic and social activities negatively impact the Earth's environment, and the resulting altered environment is unfavorable to humans. This complex system of chain reactions can be viewed on a global scale as an inclusive cyclical system that includes the environment, economy, and society. However, simulations that reproduce each complex event are computationally intensive, and combining and executing them while reproducing their mutual effects is difficult.
　Therefore, the objective of this joint research was to view the Earth as a natural and social circulation system, to couple models from different fields, to develop a method that ensures the necessary simulation accuracy while using a simple calculation mechanism, and to capture the interaction between the natural environment and economic activities.

2. Summary of Research Results

To view the Earth as a cyclical system, the results obtained from simulations of one model are often used as a constraint for the other in the conventional coupling of models from different fields, and it limits the ability to reproduce the complex interactions between the environment and economy. To overcome this limitation, NTT developed a coupled simulation technology that enables simulators developed independently in different fields to run calculations simultaneously to interoperate in a time-series manner.

Fig1 Diagram of the Coupled Simulation between the Environment Simulator and the Economic Activity Simulator

As the first step of this joint research, we hypothesized that changes in land use patterns due to changes in economic activity would affect the hydrological cycle, which would affect economic activity. To test this hypothesis, we have developed an ILS and a Global Change Analysis Model (GCAM) that can be linked using coupled simulation technology to consider annual land use patterns due to changes in economic activity and to calculate water stress at a finer temporal and spatial resolution (Fig. 1). GCAM is an integrated assessment model (IAM) that is also used to create socioeconomic scenarios (SSP) ⁺².
　The two simulators differ in their representation of global land use in terms of spatial resolution, temporal resolution, and land use parameters. They cannot be used in conjunction with each other. We have developed a conversion technology of land use patterns to eliminate these differences to achieve coupling.
　Using this coupled simulator, we compared the amount of water runoff from land calculated by the ILS with the amount of water consumption calculated by the GCAM. We observed changes in environmental load (water stress) related to water due to changes in economic activities over a long period of time, showing that large environmental loads can occur at times and places that could not be predicted in the past.
　In this experiment, we used one of the five socioeconomic scenarios (SSP126). We worked to implement a system that enables two different simulators to interoperate and perform calculations on a global scale. As an example, we found for the Niger River basin, where many areas are projected to change from grassland to cultivated land between 2020 and 2044, that although water stress tends to increase due to climate change, the increase in water stress can be mitigated to a small extent by changing to cultivated land. (Published in the May 2023 issue of Seisan Kenkyu^*3).

Role of both groups

Role of Yoshimura Lab., UTokyo

Role of NTT

3. Future Development

Two major challenges exist in reproducing the interactive effects of the natural environment and economic activities and considering various policies. One is that the number of simulation computations in the environmental field is excessive, so the number needs to be reduced (lightened) and coupled with other simulations. The other is that current scientific calculations are based on scenarios of changes in socioeconomic factors (population, economic growth, rate of technological development, etc.) (future socioeconomic scenarios) based on various assumptions, making simulations based on other flexible socioeconomic scenarios difficult. Yoshimura Lab., UTokyo, and NTT will continue their joint research efforts to improve the accuracy of global-scale simulations and to increase the number of trials by reducing the number of calculations, thereby improving usability. Furthermore, the combination of new models will enable a more detailed scientific analysis of the various issues discussed by the IPCC, thereby contributing to society's continued sustainable economic development on a scientific basis.

References

^*1Maruyoshi, M., Muto, Y., and Tokunaga, D.: Coupled Simulation Technology for Visualizing Environmental and Economic Social Cycles, NTT Technical Review, Vol. 20, No. 3, pp. 31-35, Mar. 2022. https://doi.org/10.53829/ntr202203fa5

^*2Press release from The University of Tokyo on this matter (Japanese only)
https://www.iis.u-tokyo.ac.jp/ja/news/4220/

^*3Fushio, K., Muto, Y., Fukuda, T., Tsukada, Y., Nitta, T., Yoshikane, T., Yamazaki, D., Yoshimura, K., and Maruyoshi, M.: Water stress assessment by coupled simulation of Integrated Land Simulator (ILS) and Integrated Assessment Model (IAM), Seisan Kenkyu, Vol.75, Issue2, 2023, (Japanese only)
https://doi.org/10.11188/seisankenkyu.75.135

^*4Nitta, T., Arakawa, T., Hatono, M. et al. Development of Integrated Land Simulator. Prog Earth Planet Sci 7, 68 (2020). https://doi.org/10.1186/s40645-020-00383-7

^*5Calvin, K., Patel, P., Clarke, L., Asrar, G., Bond-Lamberty, B., Cui, R. Y., Di Vittorio, A., Dorheim, K., Edmonds, J., Hartin, C., Hejazi, M., Horowitz, R., Iyer, G., Kyle, P., Kim, S., Link, R., McJeon, H., Smith, S. J., Snyder, A., Waldhoff, S., and Wise, M.: GCAM v5.1: representing the linkages between energy, water, land, climate, and economic systems, Geosci. Model Dev., 12, 677-698,
https://doi.org/10.5194/gmd-12-677-2019, 2019.

^*6Fukuda, T., and Maruyoshi, M.: Research and Development of Co-simulation Technology for Attaining Inclusive Sustainability, NTT Technical Review, Vol. 21, No. 4, pp. 38-42, Apr. 2023. https://doi.org/10.53829/ntr202304fa5

^*7AR6 Synthesis Report: Climate Change 2023 https://www.ipcc.ch/report/ar6/syr/

^*8NTT announces new R&D projects of Digital Twin Computing --- Four grand challenges towards a new future world creating harmonized relationship between the Earth, society, and people, NTT Press Release November 13, 2020
https://group.ntt/en/newsrelease/2020/11/13/201113c.html

Glossary

⁺¹Water Stress
An indicator to evaluate the degree of water stress related to water demand. In this joint research, it was calculated as "water consumption/runoff (water availability)" on a monthly and annual basis, and water stress was judged to have occurred when it exceeded 20%.

⁺²Shared Socio-economic Pathways (SSPs)
The " Shared Socio-economic Pathways " are a five-way scenario of how socioeconomic factors (population, economic growth, education, urbanization, rate of technological development, etc.) will change over the next 100 years.