Nonlinear Climate Responses to Increasing CO₂ and Anthropogenic Aerosols Simulated by CESM1

• Published in: Journal of climate Vol. 33; no. 1; pp. 281 - 301
• Main Authors: Deng, Jiechun, Dai, Aiguo, Xu, Haiming
• Format: Journal Article
• Language: English
• Published: Boston American Meteorological Society 01.01.2020
• Subjects: Aerosols; Air temperature; Anthropogenic factors; Arctic sea ice; Atmosphere; Atmospheric precipitations; Carbon dioxide; Carbon dioxide atmospheric concentrations; Climate; Climate change; Climatic analysis; Computer simulation; Cooling; Emissions; Experiments; Fluxes; Global warming; Greenhouse effect; Ice environments; Precipitation; Regional analysis; Regional climates; Sea ice; Seasons; Sensitivity; Simulation; Surface temperature; Surface-air temperature relationships; Temperature effects; Wind; East Asia; Arctic region
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/jcli-d-19-0195.1

Atmospheric CO₂ and anthropogenic aerosols (AA) have increased simultaneously. Because of their opposite radiative effects, these increases may offset each other, which may lead to some nonlinear effects. Here the seasonal and regional characteristics of this nonlinear effect from the CO₂ and AA forcings are investigated using the fully coupled Community Earth System Model. Results show that nonlinear effects are small in the global mean of the top-of-the-atmosphere radiative fluxes, surface air temperature, and precipitation. However, significant nonlinear effects exist over the Arctic and other extratropical regions during certain seasons. When both forcings are included, Arctic sea ice in September–November decreases less than the linear combination of the responses to the individual forcings due to a higher sea ice sensitivity to the CO₂- induced warming than the sensitivity to the AA-induced cooling. This leads to less Arctic warming in the combined-forcing experiment due to reduced energy release from the Arctic Ocean to the atmosphere. Some nonlinear effects on precipitation in June–August are found over East Asia, with the northward-shifted East Asian summer rain belt to oppose the CO₂ effect. In December–February, the aerosol loading over Europe in the combined-forcing experiment is higher than that due to the AA forcing, resulting from CO₂-induced circulation changes. The changed aerosol loading results in regional thermal responses due to aerosol direct and indirect effects, weakening the combined changes of temperature and circulation. This study highlights the need to consider nonlinear effects from historical CO₂ and AA forcings in seasonal and regional climate attribution analyses.