Air Pollution, Not Clouds, Drove China’s Decades of Solar Dimming and Brightening

The sunlight that reaches the ground over China has swung dramatically over the past seven decades, and a new study argues that the culprit was never the clouds. Instead, a team of atmospheric scientists has traced the country’s long-term swings in surface solar radiation, or SSR, to a handful of airborne pollutants, with organic carbon, black carbon and sulfur dioxide emerging as the dominant forces. The findings, published in Climate Dynamics, offer the most detailed regional accounting yet of what scientists call global dimming and brightening, and they carry direct consequences for air quality policy, solar power planning and climate projections in one of the world’s most rapidly changing environments.

Surface solar radiation is the amount of the Sun’s energy that actually arrives at the ground after passing through the atmosphere. It is shaped by everything that stands in the way: clouds, water vapor, aerosols and the gases that generate them. Since the mid-twentieth century, researchers worldwide have documented a widespread dimming of SSR from roughly the 1950s through the 1980s, followed by a partial recovery, or brightening, in many regions. China, with its extraordinary industrialization and equally extraordinary clean-up efforts, has become one of the most important natural laboratories for understanding these swings. Yet long-term regional studies have been hampered by sparse observations, inhomogeneous records and the difficulty of separating the influence of clouds from that of pollution.

The new research, led by Anna Li of Sun Yat-sen University together with colleagues including Martin Wild of ETH Zurich, a leading authority on dimming and brightening, tackles these obstacles head-on. The team worked with a homogenized and reconstructed SSR dataset known as SSRIH20CR, which extends consistently back to the mid-twentieth century across China and its subregions. Homogenization is critical here: raw station records of radiation are riddled with artificial jumps caused by instrument changes, station relocations and calibration shifts. By correcting these artifacts and reconstructing missing periods, the dataset allows genuine climatic signals to emerge from the noise of measurement history.

With a trustworthy record in hand, the researchers turned to attribution, the hardest part of the problem. They combined two complementary statistical approaches: partial least squares regression, a linear technique that handles the strong correlations among pollutant emissions, and a random forest model, a machine-learning method capable of capturing nonlinear relationships between radiation and its potential drivers. Using both perspectives matters, because the relationship between an emission and the sunlight at the surface is rarely a simple straight line. Aerosols scatter and absorb light directly, but they also alter cloud properties, seeding more numerous smaller droplets that make clouds brighter and longer-lived, and even suppressing cloud formation through soot-induced heating of the atmosphere.

The headline result is a decisive verdict against clouds as the driver of long-term change. Total cloud cover, the team found, correlates well with the year-to-year variability of SSR, which is exactly what one would expect: a cloudier year is a dimmer year. But when it comes to the multi-decadal trend, cloud cover accounts for only about 4.1 percent of the variation. In other words, clouds explain the weather of the radiation record but not its climate. The slow, decades-long evolution of sunlight over China was written by something else entirely, and that something was pollution.

Among the aerosols and their gaseous precursors, three species stand out. Organic carbon, black carbon and sulfur dioxide contributed 24.7 percent, 22.3 percent and 13.7 percent respectively to the national variability of SSR. Organic carbon, largely a product of biomass and fossil fuel combustion, scatters sunlight efficiently. Black carbon, the sooty component of smoke, absorbs it, warming the atmosphere and modifying clouds. Sulfur dioxide, emitted mainly by coal burning, transforms in the atmosphere into sulfate particles, which are powerful scatterers. Together these three pollutants dominate the decadal fluctuations of radiation over China as a whole, and their fingerprints are strongest in the country’s densely populated and heavily industrialized eastern regions, where they collectively account for 49.8 percent of the variability.

The picture changes subtly as one moves west. In China’s western regions, the dominant drivers are ammonia, black carbon and organic carbon, which together contribute 53 percent of the variability. The prominence of ammonia, a gas released largely by agriculture, points to the importance of ammonium-containing particles formed when ammonia reacts with acidic species in the atmosphere. This east-west contrast is a reminder that China’s atmospheric chemistry is not monolithic: industrial coal combustion writes the radiation history of the eastern seaboard, while agriculture and combustion together shape the skies of the interior. Any national policy aimed at managing sunlight, whether for solar energy or for climate purposes, must therefore be tailored regionally.

Two other findings sharpen the story. First, total column water vapor, the total amount of water in the atmospheric column, contributes less than 5 percent to the long-term variation of SSR over China, a nearly negligible role compared with the aerosols. Second, the authors are careful to flag the limits of their method: because statistical attribution cannot fully disentangle physically entangled processes, the estimated contributions represent relative importance rather than strict physical attribution. That honesty matters, because aerosol-cloud interactions remain among the largest uncertainties in climate science, and no statistical model can fully replace a mechanistic simulation. Still, the convergence of linear and nonlinear methods on the same answer lends considerable weight to the conclusion.

The implications ripple outward in several directions. For the solar energy industry, the study confirms estimates that air pollution has cost China substantial photovoltaic production over the decades, and that the clean air actions launched in 2013 have effectively been a solar energy subsidy, brightening the skies over the country’s eastern power markets. For climate science, the work reinforces the view that reductions in scattering and absorbing aerosols, while unambiguously beneficial for human health, unmask additional greenhouse warming by removing a veil that has been dimming the surface. China’s experience suggests that as other developing regions industrialize and then clean up, they will trace a similar arc from dimming to brightening, with radiation trends tracking their emissions of organic carbon, black carbon and sulfur dioxide far more closely than any change in the clouds above.

Subject of Research: Long-term variations and drivers of surface solar radiation over China

Article Title: Evolution and drivers of surface solar radiation over China since the mid-twentieth century

Article References: Li, A., Li, Z., Zhang, H., Jiao, B., Wei, S., Wild, M., & Li, Q. (2026). Evolution and drivers of surface solar radiation over China since the mid-twentieth century. Climate Dynamics, 64(10), Article 414. https://doi.org/10.1007/s00382-026-08318-6

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08318-6

Keywords: surface solar radiation, global dimming and brightening, aerosols, black carbon, organic carbon, sulfur dioxide, cloud cover, air pollution, China, climate dynamics, random forest, attribution