“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).”
and … what’s the cause of such cloud cover decline?
“and … what’s the cause of such cloud cover decline?”
“Internal unforced variability.” Not humans.
“There is evidence that the change in cloud fraction might be the result of natural variability on decadal time scales, consistent with suggestions by Wielicki et al. [2002a, 2002b] and Chen et al. [2002] within the context of internal unforced variability.”
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2002GL016128
There may also be external factors such as the solar magnetic activity and cosmic rays :
– the Svensmark theory (confirmed by the CLOUD experiment (Jasper Kirkby) conducted at the CERN – Genève) states that cosmic rays increase aerosols formation rate of a sufficient size so that water vapor can condensate on them to create droplets (clouds formation enhancement).
– the solar magnetic activity, but also solar mass ejections and solar flares, affect the rate of cosmic rays which collide with the Earth atmosphere.
Very informative and useful post. Thank you so much for sharing this.
more absorbed solar radiation
does not explain the majority
of global warming since 1975
which was at night (Tmin)
in fact a decrease of cloudiness
would make nights cooler
only greenhouse warming can explain
the cooling of the stratosphere
“the majority
of global warming since 1975
which was at night (Tmin)”
If CO2 is the main warming factor, how it is possible that it affect only night temperatures ?
“in fact a decrease of cloudiness
would make nights cooler”
Atmospheric water vapor condensation during the cooling phase (at night) decreases the cooling rate (Tmin is increased) and this may be achieved without clouds but with more water vapor (which is the case in a global warming trend and particularly in a SST warming trend).
“only greenhouse warming can explain
the cooling of the stratosphere”
The stratospheric cooling may be due to many other factors such as less UV from the Sun (thus less warming by O3 formation), stratospheric aerosols from volcanic activity, solar and terrestrial magnetic field modification, cosmic rays evolution, etc.
You are choosing to disagree with 99.9% of climate scientists
& dismissing 100% of lab spectroscopy measurements of CO2 since the mid-1800s
CO2 warming is more effective at night because the atmospheric boundary layer is 10% as thick as night. In daytime, solar heating drives convection, making this layer several kilometers deep. At night, without solar input, the boundary layer collapses to just a few hundred meters The same amount of trapped heat warms a smaller volume of air, leading to a greater temperature rise per unit of added CO₂ compared to daytime.
stratospheric cooling has another major cause: the depletion of the ozone layer. While carbon dioxide is the dominant driver of cooling, ozone loss also plays an important role
Atmospheric water vapor condensation during the nighttime cooling phase slows down the rate of temperature drop. Atmosphere warming does increase atmospheric absolute humidity. But water vapor is a climate feedback, not a direct cause of atmospheric warming … something else must be the direct climate forcing (cause) of night warming, and the best explanation is CO2 emissions
More absorbed solar radiation cannot explain at least half of the warming since 1975 (Tmin). The greenhouse gas portion of the warming is supported by lab spectroscopy, more warming at night, a negative greenhouse effect over most of antarctica and a cooling stratosphere.
The decline in the percentage of cloudiness would cause daytime warming and nighttime cooling. That is not happening == nights are warming faster than days
Warming at night is a sign of URBAN expansion, not CO2.
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Interesting article.
The relationship between solar radiation, cloud cover, and temperature trends is clearly an important area of research, and studies like these highlight the complexity of the climate system and the need to consider multiple interacting factors when interpreting long-term warming patterns.
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The 1980 ish timeframe corresponds with the initiatives globally to remove sulfur from fuels … which would lead to less cloud cover, fewer reflective sulfate aerosols.
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