The scale of human activities worldwide has grown so great that they are increasingly affecting the regular functioning of the biosphere and critically threatening its equilibrium: during the last few decades, human actions have become the main driver of global environmental change. The design of effective policies to rectify current trends critically depends on our ability to understand the complex interactions between the environment and society. In this context, environmental Integrated Assessment Models (IAMs) constitute a powerful tool integrating multiple disciplines and dimensions to shed light on potential sustainability pathways. In particular, IAMs of Climate Change are considered one of the best tools for the implicit evaluation of planetary and social boundaries. This PhD thesis seeks to develop and apply these tools to investigate the risks and opportunities for potential sustainable pathways, and to gain a better understanding of how a transition to a low carbon economy can be achieved.
The field of IA Modelling of Climate Change is reviewed, assessing its trends over time and providing a thorough classification of existing IAMs on seven axes that illustrates the diversity of modelling approaches and assumptions: policy-evaluation vs. policy-optimization; top-down vs. bottom-up; highly-aggregated vs. higher-resolution; regarding the level of integration of subsystems; deterministic vs. stochastic; equilibrium vs. disequilibrium; and standard vs. biophysical economic models. Recent developments in the field are directed towards increasing interactions within the subsystems represented and expanding models to represent other biosphere processes. However, many uncertainties and some controversy about the usefulness of these models still exist in scientific discussion.
Two modelling frameworks are applied in this thesis: a state–of-the-art model (GCAM) and a new biophysical model (WoLiM). GCAM is a high resolution bottom-up IAM model that has been used in all the Intergovernmental Panel on Climate Change reports to date to analyse mitigation scenarios. WoLiM takes a biophysical approach focusing on basic thermodynamic principles to analyse economic process, inheriting from complex systems that concentrate on dynamic interrelations between elements. The two models not only have different structures but also different aims. This diversity has enabled us to address a variety of issues, and to explore the same issue from different perspectives. Both models are applied to shed light on two current debates in the literature: (1) the implications of the uncertainty of fossil fuel resources availability and geological constraints for climate, energy and socioeconomic pathways; and (2) the implications of different post-Paris Agreement (Dec. 2015) international climate policy regimes in terms of climate change and terrestrial and industrial carbon leakage.
In relation to the availability of fossil fuel resources, the results obtained considering the range of remaining ultimately recoverable resources (RURRs) for fossil fuels in the literature show that potential future constraints on their extraction would not solve the climate change challenge. In terms of temperature increase, the probability of surpassing the 2°C from pre-industrial levels by 2100 reaches almost 90% for baseline scenarios (i.e. scenarios with no additional climate policies). However, the likely depletion of fossil fuels during the 21st century drives the transition to renewable energy sources faster than expected by current models. As a consequence, the cost of
future energy systems increases substantially, which in turn translates into lower mitigation efforts to stabilise the climate by 2100.
The integration of flow limitations from geological constraints with the RURRs for fossil fuels in combination with the simulation of the socioeconomic scenarios considered in Global Environmental Assessments enables us to explore the challenges and opportunities of global energy transition pathways. According to our results, future extraction of fossil resources reaches a plateau at around 500-525 EJ/year of maximum extraction in 2020-2035 and declines thereafter. The total primary energy supply can be stabilised or may even grow in some scenarios to 2050, but the growth trends shown in the past cannot be maintained. This is due to the fact that fossil fuels depletion can only be partially offset by the extraction of unconventional fossil fuels, alternative technologies and efficiency improvements. Transport is the most critical sector due to its current dependency on liquid fuels. Our findings indicate that there is a significant systemic energy-scarcity risk: global demand-driven trends in fuel extraction seen in the past might be unfeasible in the future. Overcoming the potential fall in fossil energy availability would thus require structural changes in the economy combined with social changes. Hence, anticipatory strategies should be implemented urgently.
In relation to the implications of different international climate policy fragmentation scenarios in terms of climate change and terrestrial and industrial carbon leakage, the results show that, even in the most optimistic scenarios where only Russia, the Middle East and Africa do not participate in the international climate regime, coordinated climate action by most countries would only be able to limit the temperature rise to around 2.5 ºC by the end of the century. Moreover, the results show that terrestrial carbon leakage may be the dominant type of leakage by 2050. In fact, afforestation in participating regions occurs at the expense of massive deforestation in non-participating regions, where substantial amounts of food and bioenergy production are shifted. The higher the mitigation target, the greater the increase in the global price of food due to the intensification of land competition driven by the substantial deployment of land-based climate mitigation options such as afforestation or bioenergy crops. In the scenarios considered, food prices increase by between 3 and 7 times by 2100 in relation to the baseline. Under these circumstances, food security and biodiversity conservation might be at risk in some countries. These results are essential to the design of effective climate policies under climate fragmentation scenarios such as the Intended National Determined Contributions after the Paris Agreement.
Finally, since there is a large discrepancy between natural scientists’ understanding of ecological feedbacks and the representation of environmental damages in IAMs, the thesis concludes with some final remarks and suggestions for progress in modelling towards sustainability. In particular, four key features are proposed: (i) adoption of a precautionary principle approach; (ii) emphasis on feedbacks between processes and subsystems; (iii) consideration of the relevant planetary and social boundaries; and (iv) enhancement of the credibility and legitimacy of models by improving their documentation and transparency. In fact, to properly assess the risks of environmental unsustainability, IAMs should be able to simulate potential “disaster” scenarios of human societies, especially in baseline scenarios. Thus, the main goal of sustainability analyses should be to detect the conditions required to reach equilibrium.