Much of this Century’s Warming May Result from Decreased Air Pollution

The presence of sulfate air pollution causes clouds to darken and reduces sunlight. This factor could contribute to recent temperature increases beyond just greenhouse gas effects.

“Two-thirds of the global warming observed since 2001 is attributed not to rising CO2 levels, but to decreasing SO2 levels,” says Peter Cox from the University of Exeter, UK.

While some sunlight is reflected and some is absorbed before being released as heat, increased carbon dioxide levels enhance the retention of this heat. This greenhouse effect is a primary driver of global warming, but the albedo, or reflectivity of the planet, significantly influences temperature.

Since 2001, satellite instruments like Ceres have measured sunlight reflection and absorption. These observations reveal a decline in sunlight reflectivity, indicating a darker planet with diminishing albedo, leading to more intense warming.

Factors contributing to this reduced albedo include diminished snow and sea ice as well as fewer clouds. However, Cox and Margaux Marchant’s analysis of Ceres data spanning 2001 to 2019 suggests that the most significant contributor is the darkening of clouds.

Industrial and maritime sulfate emissions are known to enhance the density of cloud droplets, improving their reflectivity. This principle underpins a proposed geoengineering technique called Marine Cloud Brightening. However, recent shifts away from high-sulfur fuels like coal have led to reductions in these emissions.

Thus, Merchant and Cox explored whether the observed loss of cloud brightness is linked to reduced SO2 levels and found correlations. They presented initial findings at the Exeter Climate Forum recently.

These findings are promising, as the accelerated warming trends indicate that some researchers fear the global climate sensitivity (the temperature rise associated with increased atmospheric CO2) could be at the upper range of estimates. While the short-term effects of reduced pollution contribute to warming, this suggests greater warming potential as CO2 emissions rise if cloud darkening results from increased CO2.

“If this darkening signifies a genuine shift in cloud feedback indicating greater sensitivity than previously thought, rather than a mere result of decreased SO2 emissions, it is promising news,” stated Laura Wilcox from the University of Reading, UK, who was not involved in the research.

Wilcox notes limitations in the datasets utilized by Marchant and Cox; for instance, the SO2 contamination data may have changed since their analysis.

Furthermore, two recent studies suggest dimming is largely due to reduced cloud cover, not darker clouds. “The factors behind these recent darkening trends are currently being intensely debated,” she says.

Overall, Wilcox adds that her research supports the view that the recent acceleration of global warming is chiefly driven by reduced air pollution, and this effect is likely to be temporary.