24 Years Of NASA Satellite Data Suggest The World Is Getting Darker, And It's Happening Faster In The North

A new study looking at data from 24 years of NASA satellite surveys has found that the Earth is getting darker, and there are significant differences between the Northern and Southern Hemispheres.

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The team looked at data collected by the Clouds and the Earth's Radiant Energy System (CERES) project, which launched its first stage in 1997.

"Climate is controlled by the amount of sunlight absorbed by Earth and the amount of infrared energy emitted to space. These quantities – together with their difference – define Earth’s radiation budget (ERB)," NASA explains of the project. "The Clouds and the Earth’s Radiant Energy System (CERES) project provides satellite-based observations of ERB and clouds. It uses measurements from CERES instruments flying on several satellites along with data from many other instruments to produce a comprehensive set of ERB data products for climate, weather and applied science research."

The energy budget is an important driver of ocean and atmospheric circulation, and so figuring out exactly where Earth is gaining and losing energy is of huge importance during the ever-worsening climate crisis. Delving into the data, the team found that Earth's albedo, or the amount of solar radiation it reflects back into space, is changing.

"The general circulation of the atmosphere–ocean system is closely linked with the distribution of radiant energy within the climate system. On average, the southern hemisphere [SH] and northern hemisphere (NH) reflect the same amount of solar radiation, and the NH emits more outgoing longwave radiation," the team explains. "Using satellite observations, we find that while both hemispheres are darkening, the NH is darkening at a faster rate."

According to the team, the Northern Hemisphere is absorbing more incoming solar radiation than the Southern Hemisphere, whilst its outgoing longwave radiation is higher. A combination of factors, such as cloud cover, snow cover, and water vapor in the atmosphere all contribute to the effect. 

Earlier studies suggested that the imbalance between the two hemispheres can be offset by circulation of the oceans and atmosphere, but the current study suggests that key differences remain, which weren't compensated for by usual circulation. 

"Since the NH darkening (relative to the SH) due to noncloud property changes (aerosol–radiation interactions, surface albedo, water vapor) is not compensated by cloud changes, this suggests that there may be a limit to clouds’ role in maintaining hemispheric symmetry in albedo," the team writes. 

"The hemispheric difference in surface warming and surface albedo in response to increasing CO₂ forcing seen in climate model simulations together with any further hemispheric changes in aerosol suggests we should see an increase in hemispheric albedo asymmetry in the future. However, if clouds compensate for hemispheric asymmetry (e.g., through circulation changes), but do so over a longer timescale, the trend in the NH–SH ASR difference may reach some upper limit."

As well as this, the team found that the Northern Hemisphere is warming relative to the Southern Hemisphere, and the Northern Hemisphere tropics are getting wetter, suggesting a change in large-scale atmospheric circulation on the planet. 

Further study of this complex system is needed, but the research suggests that the Northern Hemisphere could continue to warm more quickly than the Southern Hemisphere, and clouds may play a diminished role in redistributing heat around the planet.

"Our observational results suggest the NH extratropics will likely darken relative to the SH extratropics, but the short observational record precludes a definitive conclusion," they add. "Clearly, a longer observational record is needed to precisely monitor the evolution of TOA radiation, clouds, and atmosphere–ocean circulation."

The study is published in PNAS.