Abstract
To mitigate climate warming, many countries have committed to achieve carbon neutrality in the mid-21st century. Here, we assess the global impacts of changing greenhouse gases (GHGs), aerosols, and tropospheric ozone (O3) following a carbon neutrality pathway on climate and extreme weather events individually using the Community Earth System Model version 1 (CESM1). The results suggest that the future aerosol reductions significantly contribute to climate warming and increase the frequency and intensity of extreme weathers toward carbon neutrality and aerosol impacts far outweigh those of GHGs and tropospheric O3. It reverses the knowledge that the changing GHGs dominate the future climate changes as predicted in the middle of the road pathway. Therefore, substantial reductions in GHGs and tropospheric O3 are necessary to reach the 1.5 °C warming target and mitigate the harmful effects of concomitant aerosol reductions on climate and extreme weather events under carbon neutrality in the future.
Generated Summary
This research investigates the impacts of anthropogenic emission changes under a global carbon neutrality scenario using the Community Earth System Model version 1 (CESM1). The study individually assesses the climate effects of changes in greenhouse gases (GHGs), aerosol emissions, and tropospheric ozone (O3). The primary methodology involves performing five sets of climate simulations to analyze the effects of changes in anthropogenic emissions under carbon neutrality, perturbing GHG concentrations, aerosol and precursor emissions, and tropospheric O3 levels following the Shared Socioeconomic Pathway 1-1.9 (SSP1-1.9) scenario. The study focuses on climate changes over the mid (2050) and end (2100) of the 21st century, compared with the present-day climatology (2020). Furthermore, the study uses DAMIP multi-model simulations to quantify the individual contributions of GHGs and aerosols to future surface air temperature changes under the medium emission pathway (SSP2-4.5) and compares them with the anthropogenic effects under carbon neutrality.
Key Findings & Statistics
- The global mean concentration of CO2 is projected to increase from the current level of 414 ppm to 437 ppm by 2050 and then decrease to 400 ppm by 2100 following the SSP1-1.9 pathway.
- The global mean concentration of methane (CH4) will decrease from the current level of 1884 ppb to 1429 ppb by 2050 and further decrease to 1061 ppb by 2100.
- Nitrous oxide (N2O) will increase from the current level of 332 ppb to 344 ppb and 353 ppb by 2050 and 2100, respectively.
- Tropospheric O3 levels are projected to decrease across the globe by mid-century, with negative anomalies in tropospheric column O3 (TCO) in 2050 and 2100 compared to 2020 levels. Decreases in TCO are higher, exceeding -6 DU in 2050 and -10 DU in 2100, over mid-to-high latitudes of the Northern Hemisphere.
- Surface O3 concentrations show marked decreases above -10 ppb in 2050 and -20 ppb in 2100 over mid-to-high latitudes of the Northern Hemisphere.
- Emissions of SO2 are expected to decrease significantly, with the emission decreasing from 3 g m¯² a¯¹ to lower than 1.4 g m¯² a¯¹ in 2050 and lower than 1.0 g m¯² a¯¹ in 2100.
- Black carbon (BC) emissions are projected to decrease from above 1 g m¯² a¯¹ to lower than 0.3 g m¯² a¯¹ in 2050 and below 0.1 g m¯² a¯¹ in 2100.
- Organic carbon (OC) emissions are projected to decrease from more than 0.2 g m¯² a¯¹ in 2020 to lower than 0.14 g m¯² a¯¹ and 0.1 g m¯² a¯¹ in 2050 and 2100, respectively.
- The annual mean aerosol optical depth (AOD) at 550 nm will decrease by 0.08-0.2 over polluted regions, with the maximum reduction above 0.2 in 2050 and above 0.3 in 2100.
- Aerosol reduction-caused warming dominates climate change over all subregions, with surface air temperatures increasing by 0.5-1.4 °C.
- Aerosol reduction-caused warming dominates climate change over all subregions, with surface air temperatures increasing by 0.5-1.4 °C.
- Aerosol reduction-caused warming dominates climate change over all subregions, with surface air temperatures increasing by 0.5-1.4 °C.
- The global mean changes in air temperatures and precipitation in 2050 relative to 2020 simulated by CESM1 are 0.85 °C and 0.07 mm/day, respectively, which are higher than the changes predicted by CMIP6 averages (0.60 ± 0.50 °C and 0.04 ± 0.24 mm/day).
- The global mean future changes in surface air temperatures and precipitations in 2100 relative to 2020 levels are 0.92°C and 0.10 mm/day in CESM1, also exceeding the CMIP6 averages (0.40±0.60 °C and 0.04 ± 0.24 mm/day).
- Mean HWF and HWD will exceed 50 days/year and 28 days/event globally and HWA increases to 1.5 °C/day over mid-to-high latitudes by 2100.
- Pretotal, the annual total precipitations on wet days, are reduced over many land regions in GHG2050 with negative anomalies of more than -20 mm.
- Reductions in aerosols have positive contributions to Pretotal while changes in GHGs and tropospheric O3 have weak negative contributions in 2050.
Other Important Findings
- The future aerosol reductions significantly contribute to climate warming and increase the frequency and intensity of extreme weathers toward carbon neutrality.
- The study challenges the knowledge that changing GHGs dominate future climate changes, as aerosol impacts outweigh those of GHGs and tropospheric O3.
- Aerosol reduction-caused warming dominates climate change over all subregions, with surface air temperatures increasing by 0.5-1.4 °C.
- Changes in extreme weather events, including heat waves and precipitation, are significantly influenced by aerosol reductions.
- The spatial distributions of annual mean precipitation changes caused by anthropogenic emission changes compared to Baseline are shown in Fig. 2. Precipitation anomalies caused by changes in GHGs are primarily identified over the tropical oceans, with significant positive anomalies over the Western Pacific and insignificant negative anomalies over the Eastern Pacific and Indian Ocean (Fig. 2a). The enhanced rainfall over the tropical Western Pacific agrees with previous findings33. By further reducing aerosol precursors, precipitation changes are amplified globally in AerGHG2050 compared to Baseline, with increased precipitation over the Northern Hemisphere, especially along the tropics, and decreased precipitation over many oceans of the Southern Hemisphere (Fig. 2b), suggesting a northward shift of the intertropical convergence zone due to aerosol reductions that largely occur over the Northern Hemisphere34. The spatial pattern of precipitation changes in ALL2050 is similar to that in AerGHG2050 (Fig. 2c), indicating that the reductions in tropospheric O3 levels have little impact on mean precipitations. The regional attributions of mean precipitation changes show that the decreases in aerosols generally increase precipitation over the global land regions, with maximum precipitation increases of up to 0.3 mm/day over Southeast Asia, East Asia, and South Asia. In contrast, changes in GHGs and tropospheric O3 levels generally have much weaker impacts on precipitations, with magnitudes within ±0.05 mm /day (Fig. 2e). In ALL2100, the spatial pattern of precipitation changes is similar to that of ALL2050, but with relatively larger amplitudes, consistent with the much lower aerosol levels by 2100 (Fig. 2d). The crucial impacts of aerosols on precipitations are consistent with previous works that higher aerosol concentrations reduce cloud-droplet size and prolong cloud lifetime, leading to reduced rainfall35,36.
Limitations Noted in the Document
- The equilibrium climate response to GHG changes may be underestimated due to the long spin-up required for the model to reach equilibrium.
- The study notes that the transient and equilibrium responses of surface air temperature and precipitation to CO2 increases are broadly similar in spatial patterns over most of the globe among different forcing scenarios in CESM1 simulations, which may not alter the finding that the climate effects of aerosol reductions far outweigh those of GHGs under carbon neutrality.
- The models used in this study, including CESM1, have large uncertainties in projections of climate response to anthropogenic forcing.
- CESM1 is relatively more sensitive to anthropogenic forcings, which may explain stronger responses in temperature and precipitation to the forcings in ALL2050 than the ensemble means of multi-models in CMIP6 by 2050.
- The interactions between aerosols and the tropospheric gas-phase species were not considered, and climate change can modulate air pollutants.
Conclusion
The research underscores the significant role of aerosol reductions in driving climate change under carbon neutrality, challenging the conventional understanding of GHG dominance. The findings reveal that the future aerosol reductions significantly contribute to climate warming, increasing the frequency and intensity of extreme weather events. This contrasts with the prevailing view that changing GHGs primarily dictate future climate changes. The study highlights that aerosol impacts far outweigh those of GHGs and tropospheric O3. The study emphasizes the need for substantial reductions in both GHGs and tropospheric O3 to meet the 1.5°C warming target, as aerosol reductions alone may lead to warming and exacerbate extreme weather events. These results challenge the conventional wisdom, showing that while carbon neutrality is crucial, the focus should not only be on GHGs but also on the implications of aerosol reductions. The study’s findings have substantial implications for climate policy and mitigation strategies, suggesting that future carbon neutrality efforts must consider the complex interplay of GHGs, aerosols, and their precursors to accurately predict climate outcomes. While the benefits of carbon neutrality on air quality and global health are acknowledged, the study underscores the need for a comprehensive approach to manage the climate effects of anthropogenic emissions. The study emphasizes that the climate effects of aerosol reductions far outweigh those of GHGs under carbon neutrality, thereby recommending that future policies and mitigation strategies must consider the complex interplay of GHGs, aerosols, and their precursors. The results challenge the existing knowledge, which says that the changing GHGs will dominate the future climate changes, and suggest that the aerosol impacts will outweigh the GHGs and tropospheric O3. The study’s main findings suggest the critical necessity of substantial reductions in GHGs and tropospheric O3 to achieve the 1.5 °C warming target and mitigate the harmful effects of concomitant aerosol reductions on the climate and extreme weather events in the future.