Sunlight follows a rhythm set by Earth’s orbit and tilt: long summer days, dark winters, and a familiar seasonal cycle from low to high latitudes. But the amount of sunlight that actually reaches the surface is also filtered by the atmosphere. As the climate warms, rising water vapor and changing clouds can reshape this surface sunlight map, making some regions dimmer and others brighter.
The research team, led by Dr. Fengfei Song, Shichu Liu, Dr. Yuwei Wang, Dr. Lu Dong, Dr. Yu-Fan Geng, and Professor Ying Zhang from Ocean University of China, used climate model simulations to examine how downward surface solar radiation may change by the late 21st century. Their findings show that future sunlight reaching Earth’s surface would not change uniformly: polar regions are projected to become dimmer, while the Northern Hemisphere mid-latitudes are projected to become brighter, especially during local summer.
Downward surface solar radiation, or DSSR, is the main energy source for Earth’s surface. It helps control surface temperature, snow and ice melt, evaporation, hydrological cycles, and solar power potential. Scientists have found that the amount of sunlight reaching Earth’s surface has not been constant in the past: it declined across many regions from the 1950s to the 1980s and then partly recovered. This history is often referred to as “global dimming and brightening.” What remains less clear is how surface sunlight will change in the future, especially from region to region and season to season as the planet warms.
To uncover the mechanism, the team separated changes in downward surface solar radiation into clear-sky and cloud-induced components. One force acts almost everywhere: as the atmosphere warms, it holds more water vapor, which absorbs more incoming solar radiation and reduces the sunlight reaching the surface under clear skies. But clouds decide how this broad dimming signal plays out from region to region. In the polar regions, warming increases cloud liquid water, making clouds more reflective; this cloud-induced dimming joins the water vapor effect, so both pathways lead to less surface sunlight. In the Northern Hemisphere mid-latitudes, the story is different. Water vapor still works to dim the surface, but fewer clouds allow more sunlight through. There, the brightening from reduced cloud cover outweighs the dimming from water vapor, producing a net increase in surface solar radiation.
“Think of sunlight as making a journey through the atmosphere before it reaches the ground. In a warmer climate, that journey changes. More water vapor acts as a broad absorber of sunlight. Over the polar regions, clouds become richer in liquid water and more reflective, adding another barrier. But over parts of the Northern Hemisphere mid-latitudes, fewer clouds open a clearer path for sunlight.” Dr. Song explained. “What makes this pattern especially important is that it is not tied to just one emissions pathway. We see it under low-, medium-, and high-emission scenarios, and satellites have captured a similar signal over the past two decades. This suggests that the reshaping of Earth’s surface sunlight map may not be only a future possibility — it may already be beginning.”
The study suggests that this redistribution of sunlight may influence polar surface energy balance, snow and ice processes, hydrological cycles, and solar energy resources. By showing where and why surface sunlight changes, the findings add another layer to understanding how the climate system adjusts to a warmer world.
National Science Review
Experimental study