The year 2023 has been confirmed as the hottest in history, validating warnings from prominent climate scientists that the pace of global warming is accelerating, entering a perilous new phase.
A new study published on April 3rd in Communications Earth & Environment suggests one reason for this acceleration: the skies of the Earth are becoming clearer, allowing more sunlight to penetrate.
Over the past decade, the Earth's skies have become brighter, exacerbating global warming. The research relies on a suite of space instruments propelled by NASA. Since 2001, these instruments have been tracking the delicate balance of energy entering and leaving Earth. Over the last decade, an instrument called the Clouds and the Earth's Radiant Energy System (CERES) detected a significant increase in the amount of solar energy absorbed by Earth, far surpassing the warming expected from greenhouse gas increases. Readings indicate that Earth's reflectivity has diminished, as if it recently donned a darker shirt.
Researchers suggest one reason for this is the reduced reflection of pollution particles due to cleaner fuels and scrubbers used in power plants. They estimate that cleaner air might have accounted for 40% of the increased energy trapping causing global warming from 2001 to 2019.
Reduced pollution can exacerbate climate warming because pollution particles or aerosols not only reflect light back into space but also increase the brightness or persistence of droplets in clouds. ?ivind Hodnebrog, the lead author of the paper and a modeling scientist at the Norwegian International Climate Research Center, stated that the climate models used in the new study attributed remarkable warming to reduced pollution. "It was eye-opening to me how big the effect was."
CERES measures reflected sunlight and infrared heat using six instruments mounted on four satellites, imaging observed scenes including cloud cover. To interpret changes seen in CERES data, Hodnebrog and colleagues ran three different versions of four climate models multiple times, adjusting ocean temperatures to roughly match weather patterns from 2001 to 2019. The first version kept air pollution at 2000 levels, the second kept greenhouse gases at 2000 levels, and the third allowed both to "evolve" to best simulate changes in the real world. By calculating differences between these runs, they roughly teased out reasons for the increase in energy absorption measured by CERES.
However, reduced pollution might not be the sole reason for the brighter skies detected by CERES — this trend began after 2015. These models couldn't account for up to 40% additional absorbed light, and CERES data shows reflectivity declining in both hemispheres, with the greatest reduction in pollution occurring in the Northern Hemisphere. Both observations suggest other factors may be reducing Earth's reflectivity, such as melting ice exposing darker land and warming dispersing low clouds, revealing dark ocean. Changes in wind and ocean currents could also alter cloud behavior, diminishing their reflectivity.
Bjorn Stevens, a climate scientist at the Max Planck Institute for Meteorology in Germany, suggests the models in the new study may overstate the influence of aerosols. "If models are too sensitive to aerosols, they'll steer us in the wrong direction."
Tiffany Shaw, a climate dynamics researcher at the University of Chicago, notes that this new research underscores the crucial role aerosols will play as Earth continues to warm and the necessity of keeping models updated.
Related paper information: https://doi.org/10.1038/s43247-024-01324-8
