The influence of radiative forcers such as aerosols can be decomposed into ("fast") radiatively driven and ("slow") SST-mediated response, reflecting the timescales over which they appear. While this decomposition is usually assessed using experiments with fixed emissions. However, industrial and biomass burning (BMB) aerosols have evolved strongly since industrialization, requiring a transient framework. In transient framework, the line between “fast” and “slow” blurs, since gradually changing emissions can themselves introduce slow responses. Thus, both radiative and ocean-mediated effects must be considered on decadal timescales.

Using a novel atmospheric modeling framework, the impacts of evolving aerosol emissions are decomposed into these two pathways, addressing three key questions: What are the dominant patterns of industrial and BMB aerosols? How do they shape precipitation, land temperature, and circulation? What are the relative roles of radiative vs. SST-mediated responses?

Relevant publication:

Zhao, X., Deser, C. and coauthers. Separating radiative and SST-induced impacts of time-evolving industrial and biomass burning aerosols on historical boreal summer climate. J. Climate. (2025)

Industrial aerosols and greenhouse gases (GHGs) are the two dominant anthropogenic climate forcers, co-emitted since the industrial era. Yet, they differ fundamentally: 

• Climate effect: Aerosols exert a net cooling, while GHGs cause warming.

• Radiation interaction: Sulfate scatters and black carbon absorbs solar shortwave radiation, whereas GHGs mainly trap longwave infrared radiation.

• Atmospheric lifetime: Aerosols persist for days to weeks, while GHGs remain for decades to centuries.

These contrasts raise central questions: How do the impacts of short-lived aerosols differ from those of long-lived GHGs? Despite their brevity, why can aerosol effects rival or even exceed those of GHGs on regional climate?

Relevant publication:

Zhao et al. Tropical belt width proportionately more sensitive to aerosols than greenhouse gases. Geophysical Research Letters (2019). 

Most greenhouse gases are nearly transparent to visible light, allowing solar irradiance to warm Earth’s surface. An exception is methane, which absorbs in the near-infrared. Yet, many climate models omit methane’s shortwave absorption, underestimating its stratospheric radiative forcing by ~7-15%. This shortwave absorption reduces the sunlight reaching the surface, leading to surface cooling and precipitation reduction. By contrast, methane’s longwave effect warms the surface and enhances convection. These opposing influences raise key questions: How does methane’s shortwave absorption alter its total radiative forcing? How does it interact with its longwave warming effect to shape surface climate and precipitation?

Relevant publication:

Allen, R.J., and Zhao, X. and coauthers. Surface warming and wetting due to methane’s long-wave radiative effects muted by short-wave absorption. Nature Geoscience (2023).

Allen, R.J., and Zhao, X. and coauthers. Present-day methane shortwave absorption mutes surface warming relative to preindustrial conditions. Atmos. Chem. Phys. (2024).

Climate models predict an El Niño-like SST pattern under anthropogenic warming, with the eastern Pacific warming faster than the west, leading to a weakened Walker Circulation. Long-term sea-level pressure records support this projected trend. Yet, in recent decades, observations show the opposite-a La Niña-like SST pattern and a strengthened Walker Circulation. This strengthening can be reproduced by atmosphere-only simulations forced with observed SSTs, raising doubts about the robustness of coupled models projecting a weakened circulation. These discrepancies pose central questions: What controls the observed SST variations and associated Walker Circulation changes? What are the relative roles of natural variability vs. anthropogenic forcing? Why do coupled and atmosphere-only simulations diverge in their representation of Walker Circulation strength?

Relevant publication:

Zhao., X., Allen, R.J. Strengthening of the Walker Circulation in recent decades and the role of natural sea surface temperature variability. Environ. Res. Commun. (2019).

Understanding how hydroclimate responds to anthropogenic forcing is critical for water resources, agriculture, and adaptation strategies. Evidence shows that aerosols have driven major tropical precipitation changes, including the southward shift of the tropical rain belt and the Sahel drought and recovery before and after the 1970s.  In contrast, the role of aerosols in mid-latitude precipitation remains less clear, despite being co-located with major emission regions. Unlike the thermally constrained tropics, where convection dominates, dynamical processes play a central role at higher latitudes. This motivates a key question: How do anthropogenic aerosols impact mid-latitude precipitation? How does anthropogenic aerosol primarily acting through local thermodynamic effects, alter atmospheric circulation and thereby influence remote precipitation patterns?

Relevant publication:

Allen, R.J., and Zhao, X. Anthropogenic aerosol impacts on Pacific Coast precipitation in CMIP6 models. Environ. Res.: Climate (2022).

Anthropogenic aerosols are widely viewed as the primary driver of air pollution, especially since industrialization shifted aerosol sources in developed regions from natural dominance (pre-industrial) to anthropogenic dominance (industrial era). As a result, current air quality guidelines largely target man-made pollutants, implicitly downplaying natural contributions. Yet, natural aerosols remain crucial: dust alone accounts for ~70% of global aerosol mass. In regions such as Africa, high background dust levels combine with rising anthropogenic pollution, making it difficult to disentangle the role of natural aerosols in air quality degradation. This raises two critical questions: Can air be considered polluted by natural aerosols such as dust? If so, what are the policy implications for air quality management in regions where natural aerosols dominate?

 Relevant publication:

Zhao., X., Allen, R.J. and coauthors. An Implicit Air Quality Bias Due to the State of Pristine Aerosol. Earth's Future (2021).