“These findings indicate that the primary driver of air temperature increase in Kraków is the rise in SD (solar radiation influx) rather than [CO2] radiative forcing (∆F).” 

New research shows internal or natural changes in cloud structure have allowed more solar radiation to reach the surface in recent decades as documented by the widespread increase in sunshine duration (SD).

The increase in SD is the primary driver of warming since the 1980s, explaining both the temperature variability and trend.

“…the main factor driving the rise in temperature after 1988 was also a radical increase in sunshine. These results indicate the fundamental role of changes in cloud structure, and consequently sunshine, in shaping the observed rise in air temperature.”

“The regression analysis revealed that the variability of the three considered factors – SD, the intensity of the western circulation in winter (NAO), and radiative forcing (∆F) – completely explained (within the estimation errors) the observed increase in annual air temperature in Kraków.”

Cloud cover changes are far more influential than modern CO2 concentration increases in shaping climate.

“…even small changes in cloud cover can have a greater effect on earth’s radiation balance than corresponding changes in greenhouse gas concentrations.”

“van Wijngaarden and Happer (2025) estimated that the role of low cloud cover in shaping the transfer of solar energy to Earth’s surface is even greater, and a reduction in low cloud cover by just a few percent produces the same effect as doubling the CO2 concentration.”

Indeed, CO2 forcing has only have a “marginal” (~6%) contribution to warming trends, and only a 3.6% contribution to the variability.

“The analysis indicates that the strong rise in SD has resulted from changes in cloud structure since the late 1980s. … SD variability explains 58% of the variance, NAO index variability accounts for 7.7%, and ∆F variability contributes 3.6%.”

“The role of the anthropogenic factor, namely the increase in CO2 concentration in the atmosphere, described by radiative forcing, in driving the increase in air temperature in Kraków is statistically significant but marginal in terms of its importance. It does not introduce interannual variability and only slightly increases the trend of temperature rise. Similarly, in Wrocław, located ~250 km from Kraków, a small impact of radiative forcing on temperature increase (~6%) was detected (Marsz et al. 2021).”

Image Source: Marsz et al., 2025

Another new study indicates an overall increase in solar radiation reaching the surface since 1983, which is consistent with other studies that have found “a generalized increase in downwelling shortwave radiation (DSWR) across Brazil between 1980 and 2016.

“Trend analysis indicate a generalized increase in SDU [Sunshine Duration] over Brazil since the early 1980s.”

Image Source: Gava et al., 2026

And yet another new study (Budnukaeku, 2026) finds a significant increase in sunshine duration across Nigeria between 1970 and 2022 due primarily to declining cloud cover.

“This study investigates the temporal variability of sunshine duration and cloud cover across Nigeria from 1970 to 2022, leveraging satellite-based and ground-observed datasets to elucidate climatic trends and their implications for renewable energy, agriculture, and climate adaptation strategies. Using data from the Meteosat-based SARAH-2 climate data record, ERA5 reanalysis, and Nigerian Meteorological Agency (NIMET) ground stations, we analyze long-term trends, seasonal patterns, and spatial disparities in sunshine duration and cloud cover. Results indicate a significant increase in sunshine duration in northern Nigeria, averaging 0.5–0.7 hours per decade, driven by decreasing cloud cover, particularly during the dry season (November–March).”