Integrated assessment models

Integrated assessment models are used to assess the interaction between human societies and their natural environment with the aim of supporting policymaking. Human societies interact with their natural environment in numerous ways. They extract services and resources, shape the natural environment according to their needs and cause impacts by emissions, disturbance, or waste. This can cause severe environmental degradation, including climate change and biodiversity loss. Hence there is a strong need to analyze sustainable and unsustainable strategies for human societies to interact with their natural environment. This is further complicated by strong interactions and feedback among relevant processes and coupling across all scales (local, national, regional and global).

Integrated assessment models (IAMs) capture human-society interactions by describing them as coupled systems on the relevant geographical and intertemporal scales for policymaking. Often these systems involve the interactions between energy-land-economy and climate. For example, the assessment of strategies to limit global warming is conducted by IAMs with global coverage and a time horizon until the end of the 21st century. To understand future changes in the interaction between human society and the Earth system, and assess available management strategies, IAMs use scenario approaches that describe a range of outcomes under different courses of action and assumptions. Such scenarios are often based on narratives describing how society could develop, a set of trends in socio-economic drivers, policies, and technology developments, and in many cases societal goals to be achieved. IAMs translate such assumptions, trends and goals into quantitative projections. They typically include a detailed description of human activity (e.g. energy and land use), direct drivers of environmental change (e.g. emissions, land use and resource use), resulting impacts (e.g. climate change and consequences for crops) and response options. The output of IAMs is a detailed description of human-earth system coupling and development over the 21st century.

The most common use of IAMs is in the field of climate change. Here, the field has evolved from global aggregate models focused on cost-benefit analysis to detailed process models used to generate emissions scenarios and to coupled model frameworks for impact analyses. The objective of IAMs of climate change is to project alternative climate change futures with and without various types of climate change policies in place to provide policymakers at all levels of government and industry an idea of the stakes involved in deciding whether or not to implement various policies.

The literature on IAMs is vast and continues to grow rapidly. It is spread across many disciplines, with publications appearing in a wide range of journals, including those that focus on earth sciences, biological sciences, environmental engineering, economics, sociology, technological change, and other related fields. In the literature, there is some confusion about what IAMs are and for which purpose they have been developed. To this end, it is important to distinguish between different characteristics of IAMs, including their topic, type of model, optimization versus simulation, and global versus national scope:

• Topic
  While by far most IAMs are used for climate change, similar models have also been developed for air pollution, water use and other topics.
• Type of model
  There are two basic types: detailed process (DP) IAMs and benefit–cost (BC) IAMs. DP IAMs are more disaggregated and seek to provide projections of mitigation strategies in terms of consequences for energy and land use. They can often also provide projections of physical impacts such as reductions in crop growth, land inundated by sea level rise, and additional deaths from heat stress. They are typically used to provide emission scenarios for future climate change projections and to evaluate efficient mitigation strategies. In contrast, BC IAMs provide a more aggregated representation of climate change mitigation costs and aggregate impacts by sector and region into a single economic metric. They are typically used to assess the social costs of carbon.
• Optimization versus simulation
  Many models optimize a welfare or cost measure to identify efficient courses of action to reach a long-term goal, and associated prices and system changes. A second category of simulation models uses pre-defined rules to describe the evolution of the human-earth system.
• Global vs national
  Global IAMs are used to assess the collective climate change impact of human activity around the world. While they are becoming increasingly detailed in their description of regions, sectors and processes they are naturally limited in terms of detail due to their global nature. With the emergence of a hybrid international climate policy architecture combining global targets with nationally determined actions and commitments as laid out in the Paris Agreement, national IAMs are being developed to capture national transition pathways and associated policy strategies with the necessary level of detail.