the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 May 2026
East Asian Anthropogenic Aerosols Strongly Influence Past and Present Southern African Hydroclimate and Ecosystem Changes
Abstract. Southern Africa (SA) is highly sensitive to rainfall changes, as water availability significantly influences agriculture, ecosystems, and the region's socio-economic stability. Observations reveal substantial multi-decadal changes in December–February precipitation during the second half of the 20th century, characterized by enhanced rainfall over the southern part of SA (hereafter SSA), including Madagascar (MDG), and drying to the north. More recently, however, this long-term wetting tendency has reversed, with widespread drying observed across much of the region since the mid-2000s. Despite their global significance, their impact of anthropogenic aerosols on southern African precipitation has received limited attention, and the underlying mechanisms remain unclear. We show that East Asian (EAS) anthropogenic aerosols played a key role in driving enhanced precipitation over SSA and MDG between 1945 and 2005, alongside the influence of internal variability. Increased EAS sulfate emissions strengthened interhemispheric temperature and pressure gradients, inducing a southward shift of the Intertropical Convergence Zone and associated Hadley circulation, thereby enhancing moisture convergence over SSA and MDG. After the mid-2000s, rapid reductions in EAS aerosols reversed this circulation response and contributed to declining precipitation. Applying this physical framework to near-future scenarios from the Regional Aerosol Model Intercomparison Project further suggests that continued reductions in aerosols will lead to further hydroclimatic adjustments. The recent decline in precipitation (2006–2020) coincides with reduced gross primary productivity and leaf area index, as well as intensified fire-weather conditions. These findings highlight the sensitivity of southern African hydroclimate and ecosystems to remote anthropogenic aerosol emissions.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2198', Anonymous Referee #1, 22 Jun 2026
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RC2: 'Comment on egusphere-2026-2198', Anonymous Referee #2, 02 Jul 2026
Major Comments
The manuscript concludes that East Asian anthropogenic aerosols played a dominating role in the observed Southern African precipitation changes. While physical processes of the teleconnection are provided to explain the major points, I find that the current analysis and experiment design are not sufficient to fully support this attribution. Several major concerns are needed to be addressed before being considered to be published at ACP.
The first concern arises regarding the potential roles of aerosol changes from other regions and sources, including North America, Europe, South Asia, local anthropogenic sources and natural sources, which significantly influences the attribution to EAS aerosol changes.
Model reliability (observation-model discrepancy) and spread among model members and generations are another critical source of concern regarding the rigor of the attribution.
Additionally, given the relatively small region this work focuses on, the potential impacts of internal variability should also be discussed and, to the extent possible, excluded to attribute local precipitation changes to remote effects from East Asian aerosol changes.
Specific comments are listed below:
1. Potential Influences from aerosols emitted from other sources
This work analyzes precipitation trend from 1940-2005 which covers several distinct periods of aerosol emission changes. During 1940-1970, anthropogenic aerosol emissions increased over Europe (EU), North America (NAM) and East Asia (EAS) aerosols; after ~1970s, aer emissions decreased over EU, NAM; during this whole period, Indian aerosol emission changes concurrently as in EAS. What is even more complicated, natural aerosols emission show pronounced decreasing trend since 1970-80s, sea salt emission is also indicated changed over Southern Ocean -- adjoining neighboring SA and MDG. Previous studies suggested potential impacts by these aerosol sources:
(1) EU and NAM aerosol reductions are indicated to play critical roles in driving Africa precipitation changes. Regional aerosol perturbation research show the EU and NAM aerosol emission reduction can cause increasing trends of precip (annual mean) over SA and MDG (Westervelt et al., 2018; https://doi.org/10.5194/acp-18-12461-2018), with a similar spatial pattern of observed precip trend shown in this paper.
(2) Although South Asian aerosol emissions are at lower levels compared to East Asia, their geographic location is expected to directly influence Indian Ocean SSTs, which are critical for moisture transport to Southern Africa.
Additionally, the CMIP5 PDRMIP results partially support this concern. In Fig. 4, the results show opposite SA precipitation responses to Global aerosol increase (Global SUL×5) (Fig. 4a; significant drying trend) and Asia aerosol increase (Asia SUL×10) (Fig. 4b; significant wetting trend). The effects of aerosol changes from regions other than Asia (REST) can be qualitatively estimated as the difference between precip responses to Global and Asia SUL increases. The results (Fig4 a vs. b) suggest the effects of REST SUL x5 (leading to decreasing trend) potentially play more critical roles compared to Asia SUL x5 (increading trend) in driving SA precip changes. Therefore, the effects of aerosol changes over other regions are non-negligible, including historical EU and NAM aerosol declines and local aerosol increases.
These potential influences need to be addressed before attributing SA precipitation changes to East Asia aerosol increasing.
2. Model uncertainties and Inter-Model spread
GCMs’ ability to capture observed historical precipitation changes are widely indicated to be limited by previous studies (e.g., Knutson and Zeng, 2018; ttps://doi.org/10.1175/JCLI-D-17-0672.s1), which need to be addressed as they are the basis of this work. This study also shows obs-model discrepancies. Given by Fig. 1a, observations generally show no statistic significant increasing trend in SA (except very small part of SSA), and significant increasing trend in MDG precip are observed. However, Fig. 1b shows a a cross-model increasing trend over this SA, and poor model agreement in MDG precip trend, which are different from the observations. Another concern is the model uncertainty and spread. Fig. 1b shows limited model agreement over MDG, even a relatively low threshold of 70% models agreement is used. This raises questions about the model uncertainty, influencing the robustness of model-based statements.
Additionally, it is problematic to blend CMIP6 and CMIP5 results for attributions without sufficient discussion and justification. The major statements of this work are based on combination of CMIP6 DAMIP results and CMIP5 PDRMIP results. The differences in precipitation response across model generations should be fully discussed and, to the extent possible, accounted for, given the nonnegligible differences in CMIP5 and 6 model physics.
Furthermore, beyond the model difference across generations, DAMIP and PDRMIP simulations in this study are from different model members -- 3 out of 7 PDRMIP models are not included in DAMIP -- which could cause systematic problems when directly combining and comapring DAMIP and PDRMIP results for analysis and attribution.
3. Impacts from internal variability
The proposed mechanistic picture of "East Asian Aerosol Increase (1945-2005) → Northern Hemisphere cooling → Interhemispheric temperature gradient → Southward ITCZ shift + Hadley circulation displacement" is overall physically reasonable. However, the connection between large-scale circulation responses and regional precipitation changes over SA and MDG remains uncertain, as precipitation changes in these relatively small regions are widely recognized to be influenced by internal variability. The potential influence of internal variability is suggested by several evidence given by the preliminary results, such as the pronounced decadal variations and insignificant long-term trends (Fig 1 a, c) in SA precipitation
Furthermore, the distinct characteristics in precipitation variations over two adjacent regions of SA and MDG (Fig 1 c vs. d) also challenging the point of "East Asian aerosol-driven large-scale circulation changes dominate local precipitation". If SA and MDG regional precipitation trends were commanly dominated by large-scale circulation changes driven by East Asian aerosols, the precipitation variations in these adjacent regions would be expected to show similar long-term trends and multi-annual to decadal variability characteristics. The discrepancies observed between SA and MDG therefore weaken the statement and suggest the potential influences from local factors or internal variability that are not fully captured by the proposed mechanism.
Citation: https://doi.org/10.5194/egusphere-2026-2198-RC2
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Review’s comments for the manuscript egusphere-2026-2198, entitled “East Asian Anthropogenic Aerosols Strongly Influence Past and Present Southern African Hydroclimate and Ecosystem Changes"
General comments
By using observations and climate model simulations, this study investigates drivers and physical mechanisms of the austral summer multidecadal precipitation variations over Southern Africa during the second half of the 20th century. Observations show enhanced precipitation over the southern part of SA (hereafter SSA) and drying to the north. Climate model simulations suggest that East Asian (EAS) anthropogenic aerosols played a key role in driving enhanced precipitation over SSA between 1945 and 2005. Further analysis indicates that the recent decline in precipitation (2006–2020) coincides with reduced gross primary productivity and leaf area index, as well as intensified fire-weather conditions. The study is a good contribution in understanding austral summer precipitation changes over SA. Therefore, the paper is acceptable for publication after minor revision by addressing the following major and specific comments.
Major comment
Specific comments