Scenarios are among the most visible and widely used products of the IPCC. Many kinds of scenarios are used in climate research, but emissions scenarios and the socio-economic assumptions that underpin them have a distinct status because the IPCC orchestrated their development. They have evolved from assessment cycle to assessment cycle and serve as ‘boundary objects’ across Working Groups and as instruments of policy relevance. The field of Integrated Assessment Modelling has emerged to produce these scenarios, thereby taking centre stage within the IPCC assessment process. Because these scenarios harmonise assumptions about the future across disciplines, they are essential tools for the IPCC’s production of a shared assessment of climate research and for ensuring the policy relevance of this assessment. Yet, the reliance on a relatively small set of complex models to generate scenarios spurs concerns about transparency, black-boxed assumptions, and the power of IAMs to define the ‘possibility space’.

To explore our collective future, scientists develop plausible emission, pollution, and land use scenarios. Some of these scenarios describe rapid economic and population growth, and concomitant increases in fossil fuel emissions. Other scenarios describe a rapid transition to clean energy and increased efficiency. Data from the 2019 Global Carbon Project carbon budget indicate that 2019 emissions were very close to scientists’ most dangerous emissions pathway. Like a marble careening across the top of an inverted paper Dixie cup, these rapid emissions could lead to runaway global warming, risking a planetary plummet if temperature increases shrink ice caps, reduce the ocean and land’s ability to sequester carbon dioxide, and/or increase fire emissions. Examining national emission statistics, we see that the United States is by far the greatest per capita emitter. There is some good news, however. Global energy use intensity, the amount of carbon emissions per unit of gross domestic product, is increasing rapidly. We can make more goods and services using relatively less emissions. Solar energy is rapidly becoming more affordable. But emissions have already contributed to an exponential increase in disasters costing some $2.2 trillion over the past two decades.