The important role of African emissions reductions in projected local rainfall changes

The study investigates how African emissions reductions, particularly aerosols associated with decarbonization and sustainable development, affect regional climate with a focus on precipitation. Although Africa contributes less than 4% of global fossil-fuel CO₂ emissions, its aerosol-to-CO₂ ratio is high, raising concerns about a near-term 'climate penalty' from reducing cooling aerosols that currently mask greenhouse warming. The research question is whether African-led mitigation can materially influence local climate—especially rainfall—despite limited influence on global temperatures. Building on discussions of short-lived climate pollutant (SLCP) mitigation and the African Union’s Agenda 2063 development vision, the study explores whether sustainable pathways can deliver air-quality and precipitation benefits that counteract projected drying trends under business-as-usual, thereby providing substantial regional climate leverage for Africa.

Prior work highlights that reductions in cooling aerosols can unmask greenhouse warming, producing a near-term temperature 'climate penalty' (e.g., IPCC AR6 SPM). Historical aerosol controls contributed to observed warming in the US, Europe, and China. Although instantaneous global removal of cooling aerosols would sharply increase warming, realistic phaseouts yield modest additional warming (~0.02–0.10°C). Many studies have linked aerosol changes in Europe, North America, and Asia to regional precipitation shifts, including the Sahel drought, often via altered meridional temperature gradients and ITCZ shifts. Most analyses emphasize remote aerosol effects (e.g., European/North American sulfate) rather than African aerosol changes. Limited multimodel work suggests that decreases in biomass burning aerosols increase Sahel rainfall, though with large inter-model spread. CMIP6 models generally project boreal-summer drying in northwest and southern Africa and increases in central and east Africa; the GISS model broadly aligns with these spatial tendencies but with differences in extent and magnitude. The literature thus supports a strong role for aerosols in modulating African rainfall, with uncertainties in magnitude and regional patterns.

Emissions scenarios were developed using the Low Emissions Analysis Platform (LEAP) for Africa with national-scale resolution. Three scenarios were analyzed: (1) Baseline (business-as-usual) with no additional climate or air pollution policies; (2) Agenda 2063, representing decarbonization and sustainable development measures (37 measures across transport, residential, industry, power, agriculture, and waste, including SLCP controls, energy efficiency, renewables, CCUS, and behavioral shifts reducing food waste and meat consumption); and (3) NDAR (No Decarbonization Aerosol Reductions), which includes all Agenda 2063 reductions except for aerosols and ozone precursors associated with decarbonization, instead setting those species to SLCP-scenario levels to isolate the effect of co-emitted cooling aerosol reductions from decarbonization. Outside Africa, anthropogenic emissions follow SSP3-7.0. Natural sources (e.g., lightning NOx, biogenic VOCs) are climate-sensitive; wildfire is prescribed. Emissions differences at mid-century (2050–2060; vs. Baseline) include Agenda 2063: SO₂ −57%, NOx/CO/OC/BC −70–75% (OC −15% explicitly noted), NH₃ −25%, methane −37%, CO₂ −5%; NDAR: SO₂ −9%, NOx/CO/OC/BC −60–65%, NH₃ −5%, methane −5%, CO₂ −1% (Table 2). In the Baseline relative to 2015–2019, African SO₂ grows to 16.2 Tg/yr by 2060, driven by increased coal use (without added controls), diesel growth, and coal liquefaction. Climate-composition simulations used GISS-E2.1-G (CMIP6 version) at 2°×2.5° atmosphere and 1°×1.25° ocean with 40 vertical layers. The aerosol module represents sulfate, black/organic carbon, nitrate, ammonium, dust, and sea-salt; aerosols affect cloud albedo (no cloud lifetime effect). Chemistry includes ~200 reactions among ~40 species. Ensembles of 10 simulations per scenario span 2015–2064, initialized from distinct states of historical runs. Monthly outputs of 2-m air temperature and precipitation were analyzed. Statistical significance is based on 10-member ensembles (95% confidence). Radiative forcings were computed via offline (CH₄/CO₂), online double-call (ozone), and shortwave flux analyses for aerosol-radiation interactions, excluding ozone and sea-ice feedback components. Analyses emphasized regional/seasonal precipitation (June–August) over Africa, especially 0–20°N.

• Temperature: Under Baseline, Africa warms at a fairly constant rate; mean annual warming of 1.5°C (±0.2°C) in 2050–2064 relative to 2015–2029. Agenda 2063 shows only a modest, statistically insignificant additional warming relative to NDAR (~0.07°C warmer than NDAR; ~0.05°C Africa-mean climate penalty vs Baseline), consistent with small net forcings: Agenda 2063 vs Baseline total forcing 0.08 ± 0.08 W m⁻²; Agenda 2063 vs NDAR 0.16 ± 0.07 W m⁻². Temperature responses vary by region/season but remain weak and within internal variability.
• Precipitation: In stark contrast to temperature, precipitation is strongly affected. Baseline projections show substantial June–August drying over tropical Northern Hemisphere Africa, especially West Africa, with a regional mean decrease of 0.15 (±0.08; 95% CI) mm/day over 0–20°N and localized declines up to ~5× larger in parts of West Africa. Agenda 2063 essentially eliminates this projected drying, yielding a slight increase in JJA precipitation over 0–20°N with high statistical significance.
• Role of aerosols: Comparing Agenda 2063 and NDAR indicates that reductions in cooling aerosols contribute about half of the total precipitation increase and account for essentially all avoided drying in West Africa. In the model, reductions in cooling aerosols are responsible for ~33–90% of avoided drying, with the remainder mainly from reduced absorbing aerosols. Aerosol direct (clear-sky) forcing under NDAR is ~70% of that under Agenda 2063. AOD over 10°S–10°N, 20°W–40°E increases from ~0.34–0.38 (2015–2019) to ~0.74–0.77 (2050–2060) in Baseline; Agenda 2063 lowers AOD by ~0.35–0.39 relative to Baseline, NDAR by ~0.27–0.31.
• Mechanisms: Reductions in absorbing carbonaceous aerosols decrease atmospheric shortwave absorption aloft over tropical NH Africa, reducing stability and enhancing convection (JJA), closely matching precipitation response patterns. Decreases in scattering aerosols/cloud cover increase surface shortwave flux, locally enhancing land heating and precipitation but with smaller influence than absorbing aerosol changes. Meridional Atlantic temperature gradient changes occur under Agenda 2063, suggesting additional remote effects, especially in far West Africa.
• Seasonal/Regional nuance: Aerosol reductions have minimal significant impact on winter precipitation in southern and northern Africa; strongest benefits occur in boreal summer over 0–20°N (notably West Africa).

The findings directly address whether African mitigation can influence local climate: despite weak temperature effects, African aerosol reductions substantially alter precipitation, reversing projected JJA drying over tropical NH Africa under a sustainable development pathway. Local aerosol-radiative interactions, especially reduced shortwave absorption by carbonaceous aerosols, appear to dominate the precipitation response across much of Africa, with remote ocean-mediated mechanisms contributing in West Africa. The results align qualitatively with prior studies linking aerosols to Sahel rainfall via ITCZ shifts and radiative effects, while highlighting the underexplored importance of African-origin aerosols. In the broader CMIP6 context, spatial patterns of baseline rainfall changes in GISS are broadly consistent with multi-model tendencies, though magnitudes and extents differ. Policy-wise, the conventional temperature-focused 'climate penalty' narrative is incomplete: aerosol reductions can yield climate benefits through improved rainfall, complementing substantial air-quality and health gains. Thus, African policy choices around sustainable development and decarbonization could provide significant leverage over regional hydroclimate outcomes, including potential crop benefits.

This study shows that African emissions reductions, particularly of aerosols co-emitted with fossil fuel use and solid biofuels, can substantially mitigate projected boreal-summer drying over tropical Northern Hemisphere Africa, even though their effect on temperatures is modest and statistically weak. Under Agenda 2063, JJA drying seen under a business-as-usual baseline is essentially eliminated, with reduced cooling and absorbing aerosols accounting for most of the avoided drying. These results suggest that sustainable development strategies—shifting away from fossil fuels, improving efficiency, adopting clean household energy, and changing food systems—may yield significant regional rainfall benefits alongside air-quality and health improvements. Future work should employ multi-model ensembles to better quantify the magnitude and robustness of precipitation responses to African aerosol changes, explore regional mechanisms (local vs remote) in more detail, and assess implications for agriculture and water resources, including potential hysteresis in rainfall responses.

• Model dependence and spread: Precipitation responses to regional aerosol changes vary substantially across models; results here are from a single model (GISS-E2.1-G) and may represent a middle-of-the-road response but with uncertainties in magnitude.
• Emissions baseline: The baseline African SO₂ trajectory is high relative to SSPs; resulting aerosol perturbations and precipitation responses may be toward the upper end of literature estimates.
• Aerosol representation: The model includes aerosol effects on cloud albedo but not cloud lifetime; global aerosol optical depth is 12–33% lower than multiple observational analyses, potentially biasing aerosol–radiation interactions.
• Statistical and internal variability: Temperature differences among scenarios are small and within internal variability; some regional precipitation differences have large uncertainty ranges.
• Forcing and mechanisms: While analyses point to aerosol-driven mechanisms (especially absorbing aerosols), disentangling local and remote effects (e.g., Atlantic gradients) remains uncertain and model-specific.
• Time horizon and energy balance: Simulations end in 2064 with net positive radiation, implying continued adjustment beyond the analyzed period.