Drivers

Over the last three decades of the twentieth century, gross domestic product per capita and population growth were the main drivers of increases in greenhouse gas emissions.^[192] CO₂ emissions are continuing to rise due to the burning of fossil fuels and land-use change.^[193] Emissions can be attributed to different regions. The attribution of emissions from land-use change is subject to considerable uncertainty.^[194]

Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, depend upon uncertain economic, sociological, technological, and natural developments.^[195] In some scenarios emissions continue to rise over the century, while others have reduced emissions.^[196] Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century.^[197] Emission scenarios can be combined with modelling of the carbon cycle to predict how atmospheric concentrations of greenhouse gases might change in the future.^{[198][better source needed]} According to these combined models, by 2100 the atmospheric concentration of CO₂ could be as low as 380 or as high as 1400 ppm, depending on the Shared Socioeconomic Pathway (SSP) the world takes and the mitigation scenario.^[199]