Change in cooling degree days with global mean temperature rise increasing from 1.5 °C to 2.0 °C

Rising extreme heat is already driving an unprecedented surge in cooling demand, with energy needs for cooling by 2050 projected to equal the combined 2016 electricity capacity of the United States, European Union and Japan. Given growing consensus that there is currently no credible pathway to avoid warming to 1.5 °C, the paper asks: how much more cooling would be required if the Paris Agreement’s preferred 1.5 °C limit is overshot and global mean temperature increases to 2.0 °C? The study uses cooling degree days (CDDs)—the sum over days where mean outdoor temperature exceeds a baseline (here 18 °C)—to quantify and compare cooling demand across scenarios. The purpose is to map global changes in annual CDDs and identify countries facing the largest absolute and relative increases when moving from a 1.5 °C to a 2.0 °C warmer world, highlighting planning, resilience, and adaptation needs.

Previous work has mostly reported CDDs from historical observations, with some model-based regional studies. Global model analyses have typically focused on specific years rather than explicit global warming levels, leaving a gap for 1.5 °C and 2.0 °C scenarios. For Europe, prior studies reported absolute CDD increases under RCP scenarios without assessing relative changes; Mediterranean countries showed the largest absolute increases. Studies focused on Switzerland and the United Kingdom are scarce, despite emerging evidence of rising cooling demand. Prior work on China under different RCPs did not highlight the Himalayas/Southwest China for relative increases; the present study identifies those regions as emerging hotspots for relative change. This paper therefore complements the literature by providing a global, scenario-based, ensemble approach assessing both absolute and relative changes in CDDs.

- Scenarios and design: Three global warming scenarios following the HAPPI protocol were analysed: historical (2006–2016), 1.5 °C, and 2.0 °C above pre-industrial levels. - Climate simulations: 2,100 simulations in total (700 ensemble members per scenario) were run using the HadAM4 atmosphere-only general circulation model within the climateprediction.net (CPDN) environment. Outputs provide 6-hourly mean temperatures at a horizontal resolution of approximately 0.833° longitude by 0.556° latitude, globally. - Bias correction and data processing: Model outputs were bias-corrected using ERA5 hourly data. Processing and analysis were performed in Python (v3.9) and QGIS (v3.28). Standardized procedures were followed, and additional code for bias correction is publicly available. - Cooling demand metric: Annual cooling degree days (CDDs) were computed using a temperature baseline of 18 °C (CDD18). Differences (ΔCDD18) were calculated between the 1.5 °C and 2.0 °C scenarios. - Country-level aggregation and statistics: Area-weighted mean CDDs were computed per country. Absolute changes (abs-ACDD18) identify where exposure to hotter weather will be severe. Relative changes (rel-ACDD18) highlight adaptation challenges where cooling has historically been limited. To avoid distortions from grid cells transitioning from near-zero to notable CDDs, rankings use country-level area-weighted means rather than grid-specific relative values. Variability was assessed via ensemble statistics; standard deviations are mapped in supplementary materials.

- Largest absolute increases: Regions near the Equator, especially Sub-Saharan Africa, experience the greatest absolute ΔCDD18 from 1.5 °C to 2.0 °C. Top ten countries by absolute increase (area-weighted mean CDD18; countries with >5 million population): Central African Republic (266), Burkina Faso (254), Mali (253), South Sudan (251), Nigeria (245), Congo (241), Democratic Republic of the Congo (240), Chad (236), Uganda (232), Cameroon (228). - Largest relative increases: The Global North shows dramatic relative increases in CDDs. Top ten countries by relative increase (countries with >5 million population): Switzerland (30%), United Kingdom (30%), Norway (28%), Finland (28%), Sweden (28%), Austria (24%), Canada (24%), Denmark (24%), New Zealand (24%), Belgium (21%). - Additional hotspots: Mountain ranges in the Andes (South America) and the Himalayas (Central Asia into Southwest China) show large relative increases, regions not previously emphasized in some China-focused RCP studies. - Implications: Central African countries, already exhibiting high historical CDDs, will face the highest surge in heat exposure and associated adaptation requirements. Countries traditionally prepared for heating (e.g., many in Europe) will require rapid adaptation for cooling. - Context: With global surface temperature already ~1.09 °C above pre-industrial (2011–2020), total future cooling demand to reach a 2.0 °C world would exceed the ΔCDD18 mapped between 1.5 °C and 2.0 °C.

The analysis directly addresses how much additional cooling demand arises when moving from 1.5 °C to 2.0 °C global warming by quantifying ΔCDD18 across countries. The largest absolute rises concentrate in Sub-Saharan Africa, indicating severe increases in heat exposure with major implications for energy systems, equitable access to cooling, and development planning. Meanwhile, the largest relative increases occur across the Global North—particularly several European countries—where historical preparedness for cooling is limited; these regions will need substantial adaptation of buildings, infrastructure, and urban planning to enhance heat resilience. The identification of high relative increases in the Andes and Himalayas informs regional sustainability planning not highlighted in some prior studies. Overall, even a half-degree additional warming markedly amplifies cooling needs globally, underscoring urgent adaptation and mitigation efforts to limit warming and prepare energy systems and built environments for increased cooling demand.

This study provides a global, scenario-based assessment of how cooling degree days change when warming increases from 1.5 °C to 2.0 °C, using the largest ensemble to date with high temporal resolution. It identifies Sub-Saharan African countries as facing the greatest absolute increases and several European and other high-latitude countries as facing the largest relative surges, alongside additional hotspots in major mountain ranges. The results emphasize that each increment of warming substantially increases cooling demand, requiring immediate, unprecedented, and localized adaptation strategies, while reinforcing the importance of limiting warming toward 1.5 °C. Future research should integrate additional climatic and socio-technical variables (e.g., humidity, solar irradiance, wind), urbanization patterns, and evolving comfort expectations to refine demand projections and guide sustainable cooling policies.

- The analysis quantifies ΔCDD18 between 1.5 °C and 2.0 °C scenarios; the total difference from today’s climate to a 2.0 °C world would be larger than mapped. - CDDs capture temperature-based exposure but do not incorporate other drivers of cooling demand such as humidity, solar irradiance, wind, technology adoption, building characteristics, or behavioral and cultural comfort differences. - Country rankings use area-weighted means to avoid grid-level distortions; while appropriate for comparability, this may mask subnational heterogeneity. - Model output resolution (~0.833° × 0.556°) may not capture local microclimates. - Although more simulations were available, 700 runs per scenario were retained to maintain ensemble parity across scenarios; excluded runs might marginally affect uncertainty characterization.