Less Than 2 °C Warming by 2100 Unlikely

Abstract

The recently published Intergovernmental Panel on Climate Change (IPCC) projections to 2100 give likely ranges of global temperature increase in four scenarios for population, economic growth and carbon use¹. However these projections are not based on a fully statistical approach. Here we use a country-specific version of Kaya's identity to develop a statistically-based probabilistic forecast of CO2 emissions and temperature change to 2100. Using data for 1960-2010, including the UN's probabilistic population projections for all countries^2-4, we develop a joint Bayesian hierarchical model for GDP per capita and carbon intensity. We find that the 90% interval for cumulative CO2 emissions includes the IPCC's two middle scenarios but not the extreme ones. The likely range of global temperature increase is 2.0-4.9°C, with median 3.2°C and a 5% (1%) chance that it will be less than 2°C (1.5°C). Population growth is not a major contributing factor. Our model is not a "business as usual" scenario, but rather is based on data which already show the effect of emission mitigation policies. Achieving the goal of less than 1.5°C warming will require carbon intensity to decline much faster than in the recent past.

Figure 1

Carbon intensity, expressed in tonnes of CO₂ per US$10,000 in 2010 Purchasing Power Parity for USA, China, India, and Nigeria. This illustrates the tendency of carbon intensity to decline after a peak has been reached.

Figure 2

Out of sample predictive validation of model for world CO₂ emissions: (a) Model estimated from data from 1950-1980 and used to generate predictive distributions for 1980-2010, excluding countries in the former USSR due to lack of data. The solid red line is the predictive median, the heavily shaded region is the likely range (90% interval), the lightly shaded region is the 95% interval, and the black line shows the observations. (b) Model estimated from 1950-1990 data, predictions for 1990-2010. (c) Model estimated from 1950-2000 data, predictions for 2000-2010.

Figure 3

Probabilistic forecast to 2100, with IPCC RCP scenarios: (a) CO₂ emissions by year; (b) cumulative CO₂ emissions by year; (c) logarithm of the components of the Kaya identity by year, normalized to zero in 1960: population, GDP per capita, carbon intensity; (d) histogram of the predictive distribution of the global mean temperature increase relative to 1861–1880 (°C). In (a) and (b), the solid red line is the predictive median, the heavily shaded region is the likely range (90% interval), the lightly shaded region is the 95% interval, and the IPCC RCP scenarios are the dashed lines.

Figure 4

Probabilistic CO2 emissions forecasts for leading countries and regions, with Paris Climate Agreement targets. In each panel, the large black dot shows the preliminary estimate of CO2 emissions for 2015, while the large blue dot shows the Paris Climate Agreement target for 2030 (2025 for the U.S.). The targets for China and India are in terms of carbon intensity rather than total CO2 emissions, and no comparable 2015 numbers for these two countries are available.