Making Sense of Climate Change Effects: Integrated Assessment Models

Scientists seek to understand the relationships between the causes, effects and possible solutions of climate change using Integrated Assessment Models (IAMs). These digital, quantitative models – often based on physics and chemistry – represent environmental components such as the biosphere, atmosphere, oceans and soils, tracking their evolution over time under various influencing factors, primarily CO₂ emissions). The latest generation of IAMs allows researchers to simulate climate change-driven processes, such as declining crop yields, species loss, and desertification, at both global and national scales. These models are considered ‘integrated’ as they account for the spatial and temporal interactions between human activity and natural systems⁴, providing a holistic perspective on climate change. IAMs enable the construction of different scenarios, allowing scientists to compare the effects of potential solutions - such as mitigation strategies (e.g., reforestation to remove CO₂ from the atmosphere) and adaptation strategies (e.g. building dams to prevent coastal flooding) - against the status quo. As a result, they serve as valuable tools for climate assessment.  

For IAM outcomes to be effectively applied in climate mitigation and/or adaptation strategies, they must capture the cause-effect relationships of environmental processes, their spatial impacts and the role of different stakeholders (e.g., oil companies, governments, farmers, and citizens). These models must be grounded in strong scientific evidence while also acknowledging inherent uncertainties in their setup. Naturally, the broader the time horizon, the higher the uncertainty. One major source of uncertainty⁵ is the ability of the biosphere and soils to absorb and cycle CO₂ (i.e. terrestrial carbon sink), due to the complex nature of the factors influencing it (e.g., light intensity, temperature, nutrients). Therefore, integrated environmental assessments must be structured to ensure that their findings are accessible and comprehensible to policymakers and all relevant stakeholders. Finally, insights from IAMs can provide a critical evidence base for large-scale climate policy design. Below, two state-of-the-art IAMs and their applications are described: 

Greenhouse Gas - Air Pollution Interactions and Synergies (GAINS)

Greenhouse Gas – Air Pollution Interactions and Synergies (GAINS) is one of the most extensively used integrated assessment model, applied at both the global level and to more than 150 areas, including all European Union members⁶. GAINS quantifies the emissions, atmospheric movement of greenhouse gases, particulate matter and various other air pollutants. By analyzing atmospheric dynamics, the model is able to quantify the (i) effects on human health (e.g. reduction in life expectancy, premature mortality), (ii) damage to vegetation (e.g. acidification, eutrophication) and (iii) climate change effects (both short- and long-term)⁶. The model outputs susceptibility maps, illustrating which regions are the most affected, allowing researchers to derive economic consequences of climate impacts, such as external costs. Currently, GAINS is the preferred modeling framework in multiple high-level climate policy assessments, including those conducted under the United Nations Convention on Long-range Transboundary Air Pollution and the EU Thematic Strategy on Air Pollution⁷. 

INITIATOR

In the Netherlands, the 'Nitrogen Crisis' is closely linked to climate change dynamics. Beyond its severe effects on Dutch biodiversity and water quality, agricultural nitrogen emissions – originating from animal manure and fertilizers – eventually contribute to the release of nitrous oxide (N_₂O; laughing gas), a potent greenhouse gas. Despite being in relatively low atmospheric concentrations, nitrous oxide is a greenhouse gas almost 300 times greater than CO₂⁸. To address this issue, Wageningen University has developed INITIATOR, a model designed to assess the fluxes and effects of nitrogen and phosphorus across the Netherlands⁹. This model quantifies agricultural emissions into soils, water and the atmosphere based on livestock numbers, manure production and the use of natural and/or synthetic fertilizers in the country. As a result, INITIATOR can be used to evaluate the effectiveness of nitrogen reduction policies, including restrictions on agricultural expansions, limits on livestock output, and the adoption of low-nitrogen fertilizer/fodder. 

Assessing the effects of climate change is inherently complex, requiring a strong scientific foundation to predict how specific regions may be affected. Integrated assessment models (IAMs) play a crucial role in this process, providing valuable insights for developing effective mitigation and adaptation strategies at the global, national and regional levels. Like all conceptual and mathematical models, IAMs are subject to uncertainties and continuous refinements as theories and methodologies evolve. However, despite these challenges, integrated assessment remains a fundamental approach in shaping effective climate policies. 

References

1. Weaver, D., Moyle, B. D., McLennan, C., & Casali, L. (2023). Taming the wicked problem of climate change with “virtuous challenges”: An integrated management heuristic. Journal of Environmental Management, 347, 119136. https://doi.org/10.1016/j.jenvman.2023.119136
2. Termeer, C., Dewulf, A., & Breeman, G. (2012). Governance of wicked climate adaptation problems. In Climate change management (pp. 27–39). https://doi.org/10.1007/978-3-642-29831-8_3
3. Calvin, K., Dasgupta, D., Krinner, G., Mukherji, A., Thorne, P. W., Trisos, C., Romero, J., Aldunce, P., Barret, K., Blanco, G., Cheung, W. W., Connors, S. L., Denton, F., Diongue-Niang, A., Dodman, D., Garschagen, M., Geden, O., Hayward, B., Jones, C., . . . Ha, M. (2023). IPCC, 2023: Climate Change 2023: Synthesis Report, Summary for Policymakers. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland. (pp. 1–34). https://doi.org/10.59327/ipcc/ar6-9789291691647.001
4. Integrated Assessment Models (IAMs) and Energy-Environment-Economy (E3) models. UNFCCC. (n.d.). https://unfccc.int/topics/mitigation/workstreams/response-measures/modelling-tools-to-assess-the-impact-of-the-implementation-of-response-measures/integrated-assessment-models-iams-and-energy-environment-economy-e3-models#ASF
5. Huntzinger, D. N., Michalak, A. M., Schwalm, C., Ciais, P., King, A. W., Fang, Y., Schaefer, K., Wei, Y., Cook, R. B., Fisher, J. B., Hayes, D., Huang, M., Ito, A., Jain, A. K., Lei, H., Lu, C., Maignan, F., Mao, J., Parazoo, N., . . . Zhao, F. (2017). Uncertainty in the response of terrestrial carbon sink to environmental drivers undermines carbon-climate feedback predictions. Scientific Reports, 7(1). https://doi.org/10.1038/s41598-017-03818-2
6. The GAINS optimization module: Identifying cost-effective measures for improving air quality and short-term climate forcing. (2013). In www.jstor.org. International Institute for Applied Systems Analysis. Retrieved March 10, 2025, from https://www.jstor.org/stable/resrep15788.4
7. Greenhouse Gas and Air Pollution Interactions and Synergies (GAINS). (2024, September 30). IIASA - International Institute for Applied Systems Analysis. https://iiasa.ac.at/models-tools-data/gains
8. Jones, M. W., Peters, G. P., Gasser, T., Andrew, R. M., Schwingshackl, C., Gütschow, J., Houghton, R. A., Friedlingstein, P., Pongratz, J., & Quéré, C. L. (2023). National contributions to climate change due to historical emissions of carbon dioxide, methane, and nitrous oxide since 1850. Scientific Data, 10(1). https://doi.org/10.1038/s41597-023-02041-1
9. INITIATOR. (n.d.). WUR. https://www.wur.nl/en/research-results/chair-groups/research-funded-by-the-ministry-of-lvvn/soorten-onderzoek/kennisonline/initiator-1.htm