A new study looking at data from 24 years of NASA satellite surveys has found that the Earth is getting darker and that there are significant differences between the northern and southern hemispheres.
The team examined data collected by the CERES (Clouds and the Earth’s Radiant Energy System) project, the first stage of which was launched in 1997.
“Climate is controlled by the amount of sunlight absorbed by the Earth and the amount of infrared energy emitted into space. These quantities – and their differences – define the Earth’s radiation budget (ERB),” NASA explains about the project. “The Clouds and the Earth’s Radiant Energy System (CERES) project provides satellite observations of the ERB and clouds. It uses measurements from CERES instruments flying on multiple satellites as well as 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. It is therefore extremely important to determine exactly where the Earth is gaining and losing energy amid an ever-worsening climate crisis. Digging into the data, the team discovered 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 related to the distribution of radiant energy within the climate system. On average, the Southern Hemisphere (SH) and the Northern Hemisphere (NH) reflect the same amount of solar radiation, and the NH emits more longwave radiation,” the team explains. “Using satellite observations, we find that while both hemispheres are getting darker, the NH is getting darker at a faster rate.”
According to the team, the northern hemisphere absorbs more incoming solar radiation than the southern hemisphere, while 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 this effect.
Previous studies suggested that the imbalance between the two hemispheres could be compensated for by the circulation of the oceans and atmosphere, but the current study suggests that key differences remain, which have not been compensated for by the usual circulation.
“Since the darkening of the NH (compared to the SH) due to non-cloud property changes (aerosol-radiation interactions, surface albedo, water vapor) is not compensated for by cloud changes, this suggests that there may be a limit to the role of clouds in maintaining hemispherical albedo symmetry,” the team writes.
“The hemispheric difference in surface warming and surface albedo in response to increasing CO2 the forcing observed in climate model simulations as well as any other hemispheric changes in aerosols suggest that we should see an increase in hemispheric albedo asymmetry in the future. However, if clouds compensate for the hemispheric asymmetry (e.g., through circulation changes), but do so on a longer time scale, the trend of the ASR NH-SH difference may reach some upper limit.
Additionally, the team found that the Northern Hemisphere is warming relative to the Southern Hemisphere and that the Northern Hemisphere tropics are becoming wetter, suggesting a change in large-scale atmospheric circulation across the planet.
Further study of this complex system is needed, but research suggests that the Northern Hemisphere may continue to warm more quickly than the Southern Hemisphere and that clouds may play a lesser role in redistributing heat around the planet.
“Our observational results suggest that the NH extratropics are likely to 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 accurately monitor the evolution of TOA radiation, clouds, and atmosphere-ocean circulation.”
The study is published in PNAS.
