Figure 1a maps the difference in CDDs between 1.5ºC and 2.0ºC scenarios, while Table 1a highlights the top ten countries with more than 5 million inhabitants that will experience the largest changes (SI includes the top 100 countries with more than 1M population). To examine variability, we map the standard deviation of results in SI.
The results show that regions surrounding the Equator, particularly the Sub-Saharan region, will experience the largest increase in cooling demand (Fig. 1a). Table 1a shows that nine African countries are among the ten nations with the largest change in CDDs. These countries align in a west-east band in central Africa. They mainly border Mauritania, Niger, and Sudan, identified by Biardeau et al.6 to have the highest extreme heat historically. Mali and Chad experience the combined highest historical CDD6 and increment in CDDs from a 1.5ºC to a 2.0ºC scenario. The central African population not only has had the highest requirements for cooling historically (2009–2018)6 but will also experience the highest surge in these needs.
Notably, the results of relative changes in CDDs (Fig. 1b and Table 1b) show that the Global North will experience dramatic relative increases in the number of days that require cooling. Table 1b is the first to rank the top ten most affected countries by their relative increases in CDDs. Nine of ten are European nations, which are traditionally unprepared for high cooling demand and will require large-scale adaptation to heat resilience.
The United Kingdom will see the largest relative variation in cooling demand (31%) globally. However, current cooling studies for the UK are, at best, limited13. Only one 2009 predictive study is found14, which aligns with the large relative change of our results (but for different temperature increases), and reports that the energy (and emissions) from air conditioners almost double from 2004–2030 in London. For Switzerland (the country with the second largest relative increase in CDDs), two previous studies15,16 have warned of the accelerating demand for cooling (compared to heating demand). Those results were not set in the global context that we provide.
A published study examining predictions of CDD in Europe17 reports absolute and relative changes in CDDs between Representative Concentration Pathways (RCP45 and RCP85) at different years (< 2100) and historical data (1986–2005). They model temperature at different years, rather than our model, which forces specific global warming scenarios. Like our results, they find that the highest absolute increase in Europe is in Mediterranean countries. However, there are no relative changes reported. One study is found to report European CDDs (i.e., Mediterranean) in a 2.0ºC scenario (with spatial resolution > 200km2)18, reporting that the further south (i.e., northern Africa) increases the absolute CDD change.
Other large regions of high CDD relative increase are the mountain ranges of the Andes in South America, crossing the continent from North to South, and the Himalayas in Central Asia, which extend into the Southwest of China. Previous CDD predictions19 for China under different RCP scenarios did not highlight this region for its relative increase in cooling demand.