Last year was marked by the breaking of temperature records around the world. Despite high greenhouse gas emissions and a strong El Niño, the unexpected level of temperature rise surprised climate researchers. A detailed global study that tracked the Earth's energy balance, helped scientists find another key element of this surprise: Earth reflected less sunlight back into space than usual.

Albedo is defined as the fraction of sunlight that a planet reflects. Light surfaces reflect more, while dark surfaces reflect less. When global albedo decreases, the system receives more solar energy. This additional energy has helped to raise temperatures beyond what would be expected based on greenhouse gas emissions and El Niño alone.

Researchers compared measurements of reflected solar radiation and radiated heat, finding that reflectivity dropped sharply in 2023. The main culprit for this large swing was low cloud cover. Low clouds are usually the most important for albedo because they tend to reflect a lot of sunlight back into space and don't trap much heat. Low cloud cover was found to have decreased in key areas, particularly parts of the northern mid-latitudes and the tropics. With fewer of these bright clouds, more sunlight reached the ocean and land surfaces, leading to rising surface temperatures. This was most pronounced in North Atlantic, where low cloud cover decreased along with unusually high sea surface temperatures.

Feedback and key suspects

Scientists estimate that without the decrease in albedo from late 2020 to 2023, the global temperature in 2023 would have been about a few tenths of a degree Celsius lowerThis calculation accounts for the "unexplained gap" that scientists noticed when comparing observations to common climate factors.

Candidates that could have caused the albedo decrease include:

1. Natural variability: Internal climate fluctuations that can reduce low-level cloud formation in some areas for a year or two.
2. Cleaner air (reduction of aerosols): Tiny particles called aerosols, which are created by things like fuel combustion and traffic, help form droplets in clouds and reflect sunlight. As sulfur pollution along major shipping lanes and over land decreases, the air contains fewer of these particles. Fewer particles can lead to fewer or less reflective low clouds over the oceans.
3. Feedback related to warming: As the oceans and air warm, some areas may favor fewer low-lying clouds. Warm water can contribute to fewer clear, low-lying clouds, which then let in more sunlight, warming the surface even more.

Emerging Hemispheric Asymmetry

Long-term satellite observations as part of the Clouds and the Earth's Radiant Energy System (CERES) project covering a period of 24 years (2001 to 2024) also revealed a broader trend: emerging hemispheric asymmetry in terrestrial radiation. While it was previously assumed that the Southern and Northern Hemispheres (NH and SH) reflect the same amount of solar radiation on average, leading to symmetry in albedo, this assumption is being challenged.

Observations show that while both hemispheres are darkening (absorbing more solar radiation, ASR), The Northern Hemisphere is darkening at a faster rateThe difference in solar radiation absorption trend between NH and SH is 0.34 ± 0.23 Wm⁻² per decade.

This asymmetric trend is associated with changes in aerosol-radiation interactions, surface albedo, and water vapor changes. For example, the reduction in pollution (aerosols) over North America, Europe, and China since 2000 is an expected contributor to the faster NH darkening. Decreases in sea ice and snow cover also contribute to NH darkening. Changes in cloud cover also contribute to the larger hemispheric ASR contrast, although their overall magnitude is small due to the opposite trends in the tropics and extratropics.

Implications for the Future

These findings have key implications for the coming years. If the feedback between warming and low cloud cover strengthens, short-term warming could occur more rapidly than many models have predicted. A sustained decline in albedo would shorten the timeline for crossing important thresholds, such as 1.5°C above pre-industrial levels.

Therefore, it is crucial to continue to accurately measure low cloud properties and continuously monitor Earth's energy flows so that we can see changes in near real time. Whether reflectivity increases, remains at lower levels, or decreases even further will determine the rate at which the climate warms in the coming years. JRi

The entire study was published in journals Science a PNAS.