The Business-As-Usual (BAU) scenario describes the development of the concentration of greenhouse gas emissions in the atmosphere under the assumption that no further efforts to reduce emissions will be made. It is used in climate models to examine inaction in the areas of climate politics, social behavior, and technological progress and the consequences this inaction will have for climate impacts that will affect societies and natural systems in the future.
In the reports of the Intergovernmental Panel on Climate Change (IPCC), the BAU scenario is used synonymously with a high or very high emission baseline scenario (RCP 8.5) that would lead to an estimated global mean temperature increase of 4.3°C relative to preindustrial levels. This scenario would result in climate conditions that threaten civilization, for example much more frequent and severe weather extremes and drastically rising sea levels. It could possibly lead to tipping points in the Earth system being exceeded. This, in turn, would increase the climate risks for global food security or spur political instability. In such a scenario, for example, large parts of the equatorial region could become uninhabitable due to excessively high temperatures and humidity.