Scientists from the Institute of Atmospheric Physics under the Chinese Academy of Sciences have come to a surprising conclusion while conducting a study: rising atmospheric carbon dioxide concentrations can not only amplify global warming but also cause localized summer cooling, namely, in India. This discovery changes our understanding of how greenhouse gases affect the climate and why their effect can vary significantly from region to region.
Modern phenomena known to science include so-called warming holes, zones in which temperatures rise more slowly and even decline amid overall warming. The most extensively studied examples are based in the North Atlantic and the Southeastern United States, where warming holes have been linked to ocean circulation and the use of aerosols. The former refers to a slowdown in heat transfer by ocean currents, which results in less surface warming. The latter has to do with atmospheric particles that reflect solar radiation and increase cloud cover, reducing the heat flow to the surface.
The Indian phenomenon has proven fundamentally different. Seeking to understand its nature, the scientists used climate models from the international CMIP6 project, performing a series of calculations to isolate the influence of CO₂, excluding the impacts of the ocean and other factors. They assumed a fixed ocean temperature in some scenarios and used fully coupled atmosphere-ocean models in others. The results turned out to be consistent in both cases: while land temperatures in summer generally increased, air temperatures over India went down.
This mechanism is related to differences in CO₂ impacts on land and ocean. Continents, including Eurasia, warm faster than the surrounding seas. This aspect intensifies the temperature contrast between land and ocean, which serves as a key factor in monsoon circulation. This results in stronger southwesterly winds that bring moist air from the Arabian Sea. After reaching land, these air masses rise and collide with the Himalayan and Hindu Kush mountain ranges, leading to intense cloud formation. The higher the CO₂ concentration, the stronger the effect.
Cloud cover is crucial in this case. It reduces the influx of solar radiation to the surface, reflecting some of it back into space. Energy balance analysis shows that it is the reduction in solar heating that becomes the main factor causing cooling. However, this effect is limited in time and space: it manifests in the summer, during the monsoon season, and is mostly observed in the surface layer of the atmosphere. At higher altitudes, the warming trend persists. The researchers estimate that this mechanism may have produced cooling of about 0.1–0.3°C since the mid-20th century, partially mitigating the effects of global warming in the region.
The scientists also tested their hypothesis by comparing India with East Asia, another monsoon region. Despite similar humidity conditions, no cooling of this kind has been observed there. The key difference lies in topography. In India, the Himalayas and the Hindu Kush increase the lifting of air masses and enhance cloud cover, while there are no such barriers in East Asia. This proves that crucial factors include not only humidity but also the combination of atmospheric circulation and geography.
The results of the study have important practical implications. They explain why summer temperatures in India increase more slowly than the global average and also show that the impact of CO₂ on the climate can be ambivalent. Furthermore, they suggest a counterintuitive hypothesis: a reduction in carbon dioxide emissions could weaken this cooling mechanism, and summer temperatures in India could increase faster than expected in simplified climate scenarios.
