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Constrained aerosol forcing for improved climate projections

Periodic Reporting for period 2 - FORCeS (Constrained aerosol forcing for improved climate projections)

Berichtszeitraum: 2021-04-01 bis 2022-09-30

Atmospheric aerosol particles, i.e. liquid and solid particles in air, influence Earth’s climate and impact air quality. The overall objective of the FORCeS project is to understand and reduce the long-standing uncertainty of the effects of anthropogenic aerosol particles on climate, which is crucial to increase confidence in climate projections. These projections provide a basis for mitigation planning as well as key information on emission pathways that will facilitate achieving the targets of the Paris Agreement (PA). FORCeS improves the scientific understanding of key processes governing the climate impacts of atmospheric aerosols, including climate feedbacks related to aerosols and clouds. It brings together leading European scientists with trans-disciplinary expertise to i) exploit in-situ and remote sensing atmospheric data that have emerged during the recent decades; ii) perform dedicated laboratory and field experiments; iii) utilize a range of state-of-the-art computational models; and iv) apply novel theoretical methods including machine learning techniques. The process analysis within FORCeS targets to improve a set of leading European climate models, all providing essential information to climate assessments such as reports by the Intergovernmental Panel on Climate Change (IPCC). The knowledge gap between process scale and model application on the climate scale is currently a main reason preventing the climate science community to push the frontier in understanding aerosol-cloud-climate interactions. FORCeS bridges this knowledge gap by systematically constraining processes on scales ranging from hours to decades, ultimately leading to the desired refinement of model-estimated aerosol radiative forcing and climate sensitivity. To communicate FORCeS science and ensure maximum impact, FORCeS reaches out to decision makers and stakeholders providing added-value information through e.g. workshops where climate science and climate policy experts meet.
FORCeS has generally progressed according to plan and taken important steps towards understanding and reducing the uncertainty in estimates of aerosol-cloud-climate interactions. We have focused on improving the understanding of the behavior of key aerosol components (organic compounds, nitrate, light-absorbing components) and processes (ultrafine aerosol dynamics, interactions with clouds), as well as cloud processes (interaction with aerosol particles, microphysics cloud droplets and precipitation formation as well as ice formation in the atmosphere). FORCeS is using this improved understanding to evaluate and develop leading Earth System Models (ESMs), which form the methodological basis for e.g. the future projections discussed by the IPCC. To evaluate and constrain the model predictions, FORCeS has collected observational data over various scales and developed robust theories for interpreting the observations. To fill key gaps in observational constraints, novel laboratory experiments and field campaigns have been completed successfully, and detailed models are being used to develop new process-based metrics and constraints for ESM evaluation. Historical aerosol loadings and sophisticated statistical methods have been used together with present-day observations to highlight the most important revisions needed in the ESMs, and corresponding refinements in the models are being made.
The expected key outcomes from the FORCeS project include:
i. New fundamental understanding of physical and chemical processes involving aerosols and clouds;
ii. Climate models with new and improved description of aerosols and aerosol-cloud interactions;
iii. Improved predictions of climate evolution in the context of climate policy, particularly the PA;
iv. Improved quantification and reduction of uncertainty related to aerosol radiative forcing, climate sensitivity and transient climate response.
FORCeS has made substantial progress towards these outcomes. All the planned laboratory and field experiments have been completed, despite the covid-19 pandemic. These observations provide unique and novel insights into aerosol-cloud interaction processes. Based on the fundamental knowledge base, we have made concrete recommendations on the representation of key components and processes governing aerosol-cloud interactions in ESMs. These recommendations are being implemented and tested in the FORCeS ESMs. Several new and complementary approaches to constraining equilibrium climate sensitivity (ECS) and transient climate response (TCR) – parameters that are central for climate projections and hence steps towards meeting the PA targets – have emerged from FORCeS. In the last phase of FORCeS, the updated ESMs will be applied and compared against the benchmarking done during the first reporting period of the project to yield new improved estimates of the near-term climate projections.
The objectives of FORCeS are designed to meet the following impacts:
i. Supporting major international scientific assessments;
ii. Increase confidence in climate projections;
iii. Providing added value to decision- and policymakers;
iv. Sustaining Europe’s leadership in climate science.
FORCeS has made substantial progress towards these impacts. FORCeS results will be included in the upcoming IPCC AR6, and project participants have been highly involved in writing this report. FORCeS members have also contributed to the upcoming 2021 assessment report by the Arctic Monitoring and Assessment Program (AMAP). FORCeS has furthermore taken strategic steps towards enhancing the dialogue between different aerosol-cloud-climate research communities. These efforts will, together with the above work, lead to the desired increased confidence in climate projections. In a dialogue with stakeholders, FORCeS released its first two policy briefs (“Is there a conflict between the clean air goals of the European Green Deal and climate neutrality” and “Air quality and climate policies: Moving forward after COVID-19”), which have framed their questions in the context of the Paris agreement and disseminated the knowledge from the FORCeS community to the relevant policymakers. The third and final policy brief of the project will also have the Paris agreement as its important frame of reference. From a wider perspective, FORCeS provides essential information for developing cost effective multi-beneficial abatement strategies, providing better health and resilient food production, supporting several of the United Nations’ Sustainable Development Goals. The novel and innovative methods applied in FORCeS will improve three leading European ESMs. Together with educating and mentoring a cohort of active Early Career Scientists (ECSs), they help FORCeS promote and sustain Europe’s leading position in climate science.

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