Significance

The study finds strong evidence that anthropogenic warming has increased economic inequality between countries. Warming has reduced GDP per capita by roughly 17–31% across the poorest four deciles of the global income distribution, raising the 90:10 ratio by about 25% and the 80:20 ratio by about 45% relative to a world without warming. These effects emerge from a nonlinear temperature–growth relationship in which warming depresses growth in already hot countries and boosts growth in cooler ones. Expanding low-carbon energy sources may provide development gains by limiting future warming-related losses.

Introduction

This report examines how historical warming has influenced global economic inequality. Poorer countries are more vulnerable to climate impacts because they tend to be warmer and have limited capacity to adapt. Higher temperatures reduce productivity, agricultural output, and health outcomes. The strongest warming has occurred in low-latitude regions that are disproportionately poor. Prior research shows that economic output responds nonlinearly to temperature, with hot shocks lowering growth. While most earlier studies emphasize future projections or sector-specific damages, this study estimates historical distributional effects by combining counterfactual temperature reconstructions with established statistical estimates of temperature–growth relationships.

Materials and methods

The analysis uses Historical and Natural CMIP5 climate simulations to build 21 counterfactual temperature trajectories for each country, removing anthropogenic forcing using the delta method. These trajectories are paired with observed population-weighted temperatures from 1961–2010. The authors replicate a nonlinear temperature–growth regression and generate 1,000 bootstrapped response functions. Combined with the climate model ensemble, this produces more than 20,000 simulated economic outcomes per country. Impacts are calculated as percentage differences between observed 2010 GDP per capita and counterfactual values. Two samples—1961–2010 (86 countries) and 1991–2010 (151 countries)—enable comparison across time periods. Between-country inequality is assessed using population-weighted 90:10 and 80:20 GDP per capita ratios.

Results

The parabolic temperature–growth relationship shows that warming benefits cool countries and harms warm ones. From 1961–2010, tropical and subtropical nations experience median losses above 25%, with over 99% likelihood of negative effects. High-latitude countries exhibit gains of similar scale with more than 90% likelihood of positive outcomes.

Impact magnitudes correlate closely with temperature and income: poorer, hotter countries suffer the largest losses, while many richer, cooler countries gain. The longer sample reflects larger cumulative effects. The poorest decile shows a median −17% impact, and the next three poorest deciles −30% to −31%. Countries in the upper half of the income distribution experience small losses or gains.

Global inequality has almost certainly increased: the 90:10 ratio is roughly 25% larger, and the 80:20 ratio 45% larger, than in the counterfactual without warming. There is about a 90% likelihood that warming slowed income convergence.

Discussion

Uncertainties arise from variation in optimal temperatures, internal climate variability, and data limitations, but the direction of inequality impacts is robust. Including lagged growth still yields a 66% likelihood that warming increased inequality. Trade patterns may alter country-level outcomes, and within-country inequality cannot be evaluated with available data. Overall, warming has reduced growth in poorer, warmer countries and increased growth in some richer, cooler ones.

Conclusions

The results indicate that wealthy countries have benefited from fossil-fuel-driven warming, while many poor countries have become relatively poorer. Expanding low-carbon energy could reduce future warming-related growth losses and support development.