The Earth's northern hemisphere is getting darker.

A new study based on more than two decades of satellite data shows it's happening faster than in the south.

That's because the Earth is reflecting less sunlight back into space.

Oivind Hodnebrog, a senior researcher at Norway’s Center for International Climate Research, says the findings show that the amount of solar radiation coming in versus heat radiated back into space is shifting.

"What we find in this paper is it's increasing more in the northern hemisphere than the southern hemisphere," he says.

"It’s a bit like having a black T-shirt on a summer day. You get warmer because you absorb more sunlight because it’s a dark T-shirt. If you have a white T-shirt, more of the sunlight is reflected away, so then it’s lighter, less dark," he adds.

Scientists think several factors are behind the shift.

Hodnebrog says one of the biggest drivers is a reduction in aerosol pollution.

These tiny reflective particles were once common over Europe, North America and China.

Cleaner air means fewer particles to bounce sunlight back into space. It also changes how clouds form, making them slightly darker and less reflective.

Hodnebrog says melting Arctic sea ice and snow are adding to the effect by exposing darker ocean and land surfaces that absorb more heat. Water vapor in a warming atmosphere also plays a role.

While global darkening appears to be driven largely by cloud changes, the north–south difference comes mainly from cleaner air, melting ice and changes in surface reflectivity, he says.

"When you remove these reflecting particles, that means that more of the sunlight is absorbed. They do not only reflect sunlight, but also affect the clouds. So, when you have aerosol pollution, it makes the clouds brighter, which leads to a cooling. And again, when you clean up the aerosols, the clouds also get darker, and that’s a major effect," he says.

Hodnebrog says that the imbalance may not be permanent.

He says how it develops will depend on several factors, including future emissions of greenhouse gases and aerosols. Also crucial, he says, is how the climate system responds to the resulting warming.

However, he says the trend signals a shift in the climate system that could influence rainfall, ocean currents and long-term weather patterns.