Complex Systems are an interdisciplinary subject of research and are used as the theoretical framework in a wide range of experimental fields such as economic and medical disciplines, as well as in mathematical and computer science studies.

A complex system is composed by a large number of elements that interact with other.

The microscopic interaction among the elements gives rise to a global behavior of the system that can be very different, and at times even counterintuitive, with respect to the evolution rules of the single parts.

The simulated system does not necessarily reproduce the real one in all the details. Indeed, it is rather a synthetic representation of the real world that portraits a subset of the real observables that are supposed to be relevant for the description of the process under investigation.

That is why in complex-systems research projects the most important task is to identify and select the relevant variables and the structural foundations of the system being simulated.

This involves transitions in many sectors – the Green Growth pilot of CoeGSS considers the case of sustainable mobility. It investigates the diffusion of electric vehicles in the global car fleet to gain an understanding of mechanisms that may foster or hinder a sustainability transition and to explore potential evolutions of the global car fleet with their implications for inclusive green growth.

The transport sector is responsible for around one quarter of Europe’s greenhouse gas (GHG) emissions, contributing to climate change. Emissions from road vehicles also contribute to high concentrations of air pollutants in many of Europe’s cities. Further, road transport is the main source of environmental noise pollution in Europe, harming human health and well-being. (ref: European Envinronment Agency – 2017).

To assess possible socio-technical transition pathways towards sustainable mobility, the Mobility Transition Model (MoTMo) has been developed. It is applied to Germany as an example country. MoTMo is an agent-based model representing individuals and households that choose from a list of mobility modes (such as conventional cars, electric vehicles, or public transport or car sharing) based on opinions about these and on the priorities they assign to convenience, emissions, costs, and being innovative. Opinions evolve according to experience and influence from others in a social contact network. A synthetic population of 10 Mio individuals with mobility profiles, that statistically matches the population of Germany for relevant features, has been developed as a starting point for model simulations.

In a first phase, technological and social innovations (here, electric vehicles and car sharing) are generally used by very small numbers of people. Also, innovations often develop in niches (e.g., spatial or social groups) first. Therefore, several technical challenges are encountered in modelling the diffusion of innovations: a large number of agents and a high resolution of each model simulation run are needed to statistically capture small proportions of the population and to represent niches. Many simulation runs and thorough analysis of their output data are then needed to explore potential evolutions of the system. This required a parallelized MoTMo and large computational resources.

Figure 1 shows the spatial distribution of electric vehicles in the Niedersachsen, Bremen, and Hamburg area obtained from simulations. The four maps depict the development over time that highlights how electric mobility first develops within cities and expand to the surrounding rural areas in later stages. Dominant focal points are the cities of Hamburg, Bremen and Braunschweig, whereas in Hannover, the very dense population favours the use of other (public) mobility.

The work is carried out in the context of enhancing GSS modelling through High Performance Computing (HPC) and Data Analytics (HPDA), by enabling the use of high-resolution data sets, by allowing models to grow in complexity and grow towards global scales, and by facilitating deeper analysis of larger sets of output data from model simulation runs.

The Centre of Excellence for Global Systems Science provides help on all of the above: generating synthetic populations, generating interaction networks between agents in a synthetic population, technical support for parallelizing an agent-based model and the computing infrastructure for running it, as well as data analytics and visualization of results.

Authors:
Steffen Fuerst – steffen.fuerst@globalclimateforum.org
Andreas Geiges – andreas.geiges@globalclimateforum.org
Mario Scovazzi – mario.scovazzi@csp.it
Sarah Wolf – sara.wolf@globalclimateforum.org

Targeting real world challenges
Optimizing city development choices is an essential challenge for the future, driven by the growth of world population increasingly living in cities and the opportunities offered by technological innovations.

Scientific challenges
A global system science and complex systems approach, proves essential to conceptualize in a clearer way and predict more realistically the complexity of city evolution. It is essential in solving future challenges by seeing the both whole picture and details, and not only a limited view.
Cities are complexly defined by the interaction of processes as different as real estate, transportation, economy, society, and politics, which need to interact in the model. These processes occur at different scales, going from individual transport and real-estate choices, to institutionally defined public transport offer adaptability, and from hourly traffic congestion to long term city development. These processes can prove non-linear and difficult to predict.

Synergy GSS-HPC with the city pilot
By its global scope, involving many processes of different nature and at different scales, including the difficult-to-predict human free will, global system science has high specific computing needs, which might benefit from the HPC resources. It concerns particularly data pre-processing and post-processing and model simulation: refining one simulation or running many simulations.
We’ll present the results of the city pilot and put into light this multi-fold synergy corresponding to three steps of GSS modelling. Firstly, model design, by finding the appropriate agent and data granularity. Secondly, model scenario exploration . Thirdly, seeking further results analysis and insights over simulation results’ post-processing.

The city pilot
The global urbanization pilot aims at studying the two-way relation between transport infrastructure decisions and price mechanisms, particularly concerning real estate.
We chose Paris as use case. The simulations are based on open data, such as the average real-estate pricing per district in Paris (Figure 1).

Here after figures showing the principles of the pilot.

Studying the city pilot
Finding the appropriate data and agent granularity.
Firstly, we find the appropriate data and agent granularity. We calculate the difference between results with different initial data granularity, while varying agent granularity and a key parameter (the transport scenario).

More precisely, figure 3 shows 12 sets of results: every map represents the difference of real estate prices between simulations initialized either with fine-grained (interpolated), and coarse-grained (district) real-estate prices; red is a negative difference, blue is a positive difference, and the intensity indicates the value (pale close to 0). If the basic difference is calculated following the data granularity, we additionally vary the agent granularity (i.e. how many humans every agent represents) (lines) and a key parameter, the initial number of car commuters (columns). We seek to find out whether the difference changes with them, i.e. whether the impact of data granularity varies following the case.
We can see that the difference value varies significantly not only following the agent granularity, but also following the value of a key parameter (defining the initial transport mode choice) in a non-trivial way. This calls to get further insights we calculated. We finally choose the finest data and agent granularity.

Model scenario assessment
Once the data and agent granularity decided upon, we secondly explore the model.

Studying the influence of pollution and real-estate
Increasing the level of ecological awareness, by concerning every commuter, allows to change transport mode choices and decrease therefore significantly the level of pollution. Public transport adaptability plays also a positive, yet less important, since to targeting unequally areas following their present level of public transport.

Furthermore, when studying more precisely the influence real-estate price mechanisms, we can see that prices influenced mainly by pollution lead to higher percentages of commuters choosing public transport than prices influenced mainly by public transport offer; the scenario where prices are influenced both by the public transport offer and pollution leads to intermediate values.

Finding further insights on simulation results
Exploring the parameter space requires thirdly to assess precisely the simulation results, while clarifying their implications. This study illustrates the possible benefit of HPC for GSS by facilitating the post-processing of data to find new insights. In the following figure we show the time evolution of spatial heterogeneity of green commuters.

Calculating this spatial heterogeneity shows how green commuters vary spatially. Homogeneity would mean green commuters percentages are similar everywhere. Typical heterogeneity would be sharp differences in green commuter percentages between adjacent spatial cells.
Here there is spatial heterogeneity, since living in a place with good public transport offer (which shows some spatial heterogeneity) increases the probability of becoming a green commuter.
We further observe how it evolves with time. The heterogeneity first increases with new green commuters reflecting the heterogeneity of the initial public transport offer. However, the increase in the demand leads to develop public transport offer, which becomes spatially more homogeneous, leading to new green commuters, the map of which becomes then more homogeneous too. In this study spatial heterogeneity is high since living in a place with good public transport offer (which shows some spatial heterogeneity) increases the probability of becoming green. Furthermore, the heterogeneity observed increases with the size of the neighbourhood considered (depth horizontal axis), showing that heterogeneity don’t even out at larger scale.

Conclusion
We study a city model, while investigating different possible forms of synergy between GSS and HPC.
We have studied the influence of parameters on transport mode choice and consequently on pollution, from individual ecological awareness to institutional public transport offer, and its adaptability to demand. Pollution impacts itself real-estate pricing, depending also on public transport offer.
To summarize, even if their specific influence varies, the more observation and adaptation mechanisms (commuters with ecological awareness, real-estate influenced by pollution, public transport adaptability) are present in the model, and the easier it will resiliently respond to high levels of pollution to help reduce it.
Furthermore, we have shown, with city pilot examples, various possibilities of synergy between GSS and HPC.
The meeting between GSS and HPC opens to new challenges and possibilities.

Authors:

A complex system is composed by a large number of elements that interact between each other.

The microscopic interaction among the elements gives rise to a global behavior of the system that can be very different, and at times even counterintuitive, with respect to the evolution rules of the single parts.

Leveraging on the complex systems approach, the ISI Foundation has developed a model that reproduces the adoption of and quit from the smoking habit in a population.

The model falls within the field of Global System Science (GSS) since it describes a large scale social system (in this case a whole country) to simulate its evolution in time.

Global Systems Science is a new area of research that is a generalization of Systems Science and that has an impact on several disciplines. It requires a realistic and accurate modelling of the systems under investigation and large computational capacity to simulate their evolution.

Examples of Global Systems Science are the study of climate changes, financial stability and the description of new technologies adoption.

The simulations performed at the ISI Foundation enable the study of the process of adopting and giving up the smoking habit in Great Britain and have been performed using a High Performance Computing (HPC) infrastructure. In particular, the HPC supercomputers called Hazelhen at the HLRS in Stuttgart (Germany) and the Eagle at the University of Poznan (Poland) have been used to run the simulations.

The system description relies on an agent-based-model (ABM), i.e., each person in the population is represented as a computer variable and the behavior of each single element / agent in the system is simulated accordingly to the proposed model.

With this type of modelling approach, it is possible to reproduce the macroscopic evolution of the system by simulating the microscopic evolution of every single element.

The ABM model is a modelling framework used in game theory, in computational sociology and in complex systems.

The model accounts for socio-economic indicators that measure the possibility to access tobacco products, thus simulating in a realistic way the behavior of the population of the country. In fact, the consumption of tobacco depends on several diverse factors such as social condition, age, economic factors etc. [ref. Psicol Soc (Bologna). Author manuscript; available in PMC 2014 Aprs 22. Italian.Published in final edited form as: Psicol Soc (Bologna). 2012; 2012(1): 7.30. Published online 2012. doi: 10.1482/36754 and others].

The developed ABM model and the behavior of the population reproduced by the simulations have been validated against data of smoking prevalence in Great Britain from 1976 to 2016 (data from the National Health Service and the Office for National Statistics), showing a very good agreement between the empirical data and the simulations’ results.

The validation procedure against the statistical data has been carried out performing hundreds of simulations, with thousands of variables, on the HPC infrastructure.

After the validation phase, the resulting model closely reproduces the historical series of data, thus providing a prediction of the evolution of the smoking habit in Great Britain for the next years. Moreover, the model is able to forecast the smoking prevalence in a country based on the expected effectiveness of different policies regulating the tobacco consumption put in place by the government. Specifically, the smoking prevalence in Great Britain will have a decreasing trend in the next years. This trend can be either speeded up or slowed down by government interventions on the tobacco price or accessibility.

Authors:
Enrico Ubaldi — ISI Foundation, Torino
Mario Scovazzi — CSP Innovazione nelle ICT, Torino