Projecting future climate change, and what drives it, is difficult, with many uncertainties. Computer models, however, can be useful tools for exploring the long-term implications of climate change and evaluating policy options. For example, models can help construct plausible scenarios of future greenhouse gas (GHG) emissions based on socioeconomic, environmental, and technological trends and drivers.

Integrated assessment models (IAMs), coupled models of the economy, energy, land use, and climate systems, are used by the Intergovernmental Panel on Climate Change (IPCC), the main international scientific body for assessing global climate change. This report explores the results of a selected set of IAM scenarios consistent with keeping the increase in global mean surface temperature to 1.5°C or 2°C above preindustrial levels in 2100, the temperature goal of the Paris Agreement. The modeling indicates that the more stringent the temperature target, the earlier the dates would have to be for global peak and net-zero carbon dioxide (CO2) emissions. In order to hold likely (with at least a 66% probability) warming to below 2°C in 2100, the model results suggest that global annual CO2 emissions would need to decline to net-zero between 2080 and 2100. To keep likely warming below 1.5°C in 2100, the models project that global CO2 emissions would generally have peaked around 2020 and would reach net-zero by 2060. In these scenarios, carbon removal would need to balance positive GHG emissions. The IPCC scenarios indicate that the later the peak in CO2 emissions, the sharper the reductions would be later in the century to hold the temperature increase below any given target.