Less Than 2 °C Warming by 2100 Unlikely

Area of Science:

• Climate Science
• Environmental Modeling
• Statistical Forecasting

Background:

• Intergovernmental Panel on Climate Change (IPCC) projections offer scenarios for 2100 but lack a fully statistical basis.
• Previous climate projections did not fully incorporate statistical methods for forecasting CO2 emissions and temperature change.

Purpose of the Study:

• To develop a statistically-based probabilistic forecast of carbon dioxide (CO2) emissions and global temperature increase to 2100.
• To provide a data-driven assessment of future climate change, considering population, economic growth, and carbon use.
• To compare probabilistic forecasts with existing IPCC scenarios.

Main Methods:

• Utilized a country-specific version of Kaya's identity for CO2 emissions analysis.
• Developed a joint Bayesian hierarchical model for GDP per capita and carbon intensity.
• Incorporated data from 1960-2010 and United Nations probabilistic population projections.

Main Results:

• The 90% interval for cumulative CO2 emissions encompasses the IPCC's middle scenarios.
• The projected likely range for global temperature increase is 2.0-4.9°C, with a median of 3.2°C.
• Population growth was found to be a minor factor; emission mitigation policies already influence the data.

Conclusions:

• Achieving the 1.5°C warming goal necessitates a significantly faster decline in carbon intensity than observed historically.
• The statistical model provides a probabilistic forecast that differs from non-statistical IPCC scenarios.
• Future climate mitigation strategies must prioritize accelerated reductions in carbon intensity.