Human caused emissions of small, airborne particles - called aerosols - strongly affect our climate, and have historically counteracted some of the warming from greenhouse gas emissions. Today, aerosols are thought to cool the Earth's surface by up to half a degree Celsius, and to influence rainfall patterns, the Asian and African monsoons, and the variability of the weather.
Asian countries, notably India and China, have been major sources of aerosol emissions over the last decades. Now, however, concerns over air quality are leading to rapid changes in the amounts, geographical patterns and composition of such emissions. Effects on the climate can be expected, both locally and further away from the sources, but as yet the details of this potentially quite strong influence are not known.
During 2021-2022, the CATHY partners have laid the foundations for the main work in the project, both technically and through attention-building measures in the academic community. Three activities can be particularly noted:
1) Development of a Green's Function that links changes in aerosol emissions in Asia to climate effects locally and further afield. Here, most of the simulations have been carried out, and work on analyses is well under way
2) Planning and start-up of a new, international model collaboration called RAMIP (Regional Aerosol Model Intercomparison Project). The first results are already available, and will be analyzed in 2023/2024
3) Publication of a commentary article in Nature on the importance of including aerosol changes in climate risk calculations, together with the preparation of a longer roadmap article from a larger professional community (now under review)
In addition, work is under way on publications within several of CATHY's work packages. We have held both physical and virtual meetings, and the international collaboration within the project is both active and dynamic.
Emissions of Asian Anthropogenic Aerosols (A3) are rapidly changing - most notably black carbon and sulphate aerosol precursors from India and China. The resulting range of climate impacts and societal hazards may dominate regionally over greenhouse gas induced trends for the next several decades, but the implications are as yet insufficiently explored. CATHY (Climate implications of rapid changes in Asian Anthropogenic Aerosol emissions: Temperature, Hydrological cycle and variabilitY) tackles the urgent need for quantifying climate related hazards resulting from ongoing and projected changes in A3 emissions.
Why: Today, the Earth’s climate is significantly influenced by Asian aerosol emissions, with impacts affecting up to two billion people in the near-source regions alone. This regional influence is set to change, along with teleconnections affecting remote regions (Arctic, Europe, North Africa). The implications for climate and society are however highly uncertain, due to limited knowledge of physical processes, modelling capability, and future policy choices. Improved quantification of the aerosol-climate influence on physical hazards – in Asia and around the globe – is therefore critical.
How: CATHY will merge observed changes, projected emission patterns, and advances in modelling and analysis techniques. Key opportunities for scientific breakthroughs include recent trends in Chinese and Indian emissions, Large Ensemble modelling and the diversity of aerosol assumptions made in the Shared Socioeconomic Pathways.
Novelty: CATHY will disentangle dynamical responses to localized forcing through development of a novel reduced-complexity Green’s function, investigate remote impacts via a machine learning based circulation pattern analysis, separate the active mechanisms differentiating absorbing and scattering aerosols, and frame the resulting improved understanding as quantification of physical hazards driving climate risk.
