Global dimming: how tackling air quality exacerbates global warming 

In a painful irony, improving global air quality through regulation and decarbonising could actually raise average surface temperatures by counteracting a phenomenon called 'global dimming'.

Global dimming refers to the observed reduction in sunlight reaching Earth's surface, primarily due to atmospheric pollutants like sulfate aerosols.

These particles, resulting from industrial activities, reflect and absorb solar radiation, leading to a cooling effect that has, to some extent, masked the full impact of greenhouse gas-induced warming.

As efforts to improve air quality reduce these pollutants, we face a paradox: the diminished cooling effect reveals the true extent of global warming, potentially accelerating climate change.

What is global dimming?

From the 1950s to the 1980s, scientists observed a decline in solar irradiance at Earth's surface, a phenomenon termed "global dimming." This trend was largely attributed to increased industrial activity that released significant amounts of aerosols into the atmosphere.

These aerosols scatter and absorb sunlight, leading to a net cooling effect. Additionally, they influence cloud formation, increasing cloud reflectivity and further reducing the amount of solar energy reaching the surface.

However, since the 1980s, efforts to reduce air pollution have led to a decrease in aerosol emissions, resulting in a phenomenon known as "global brightening."

While beneficial for public health and ecosystems, this reduction unearths a paradox: the cooling effect of aerosols has been masking the full extent of global warming. As aerosol levels decrease, the suppressed warming is unleashed, potentially leading to a more rapid increase in global temperatures.

A paradox of climate action

Efforts to mitigate air pollution have led to a reduction in aerosol emissions, resulting in a phenomenon known as "global brightening." While beneficial for public health and ecosystems, this reduction unearths a paradox: the cooling effect of aerosols has been masking the full extent of global warming. As aerosol levels decrease, the suppressed warming is unleashed, potentially leading to a more rapid increase in global temperatures.

Reducing aerosol emissions is essential for improving air quality and public health. However, without simultaneous and aggressive reductions in greenhouse gas emissions, the decrease in aerosols could lead to an abrupt and severe acceleration of global warming. This scenario underscores the urgency of comprehensive climate strategies that address both air quality and greenhouse gas emissions. 

This, from the latest Assessment Report from the Intergovernmental Panel on Climate Change, lays out the numbers of this paradox:

'It is likely that well-mixed GHGs contributed a warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C'¹

Does global brightening necessitate geoengineering?

In response to this challenge, some scientists have proposed solar geoengineering techniques, such as stratospheric aerosol injection (SAI), to artificially enhance Earth's reflectivity and counteract warming.

SAI involves releasing reflective particles into the stratosphere to mimic the cooling effects of volcanic eruptions. While theoretically effective, this approach carries significant risks, including disruptions to global weather patterns, impacts on agriculture, and the ethical dilemma of intentionally modifying the climate.

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Moreover, reliance on geoengineering does not address the root cause of climate change—greenhouse gas emissions—and could lead to unforeseen consequences if not carefully managed.

The phenomenon of global dimming highlights the complex interplay between air pollution and climate change. As we strive to improve air quality through the reduction of aerosol emissions, it is imperative to simultaneously implement robust strategies to reduce greenhouse gas emissions.

Failing to do so could result in the unintended consequence of accelerated global warming, exacerbating the very challenges we aim to mitigate. This paradox serves as a stark reminder of the need for comprehensive and integrated approaches to climate policy.

¹ Summary for Policymakers. Sixth Assessment of the Intergovernmental Panel on Climate Change. 2022.