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Constrained aerosol forcing for improved climate projections

Periodic Reporting for period 1 - FORCeS (Constrained aerosol forcing for improved climate projections)

Reporting period: 2019-10-01 to 2021-03-31

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). The FORCeS project 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 of precipitation formation, ice formation and updrafts). The FORCeS team 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 are ongoing, and detailed models are being used to develop new process-based metrics and constraints for ESM evaluation. FORCeS has established an active, engaged and productive community bringing together experts focusing on detailed aerosol- and cloud-related processes with those working on Earth system prediction. It has furthermore established communication pathways with key stakeholders, including the publication of a first policy brief.
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 already made substantial progress towards these outcomes. Two field campaigns in Arctic and sub-Arctic environments were successfully executed, despite the covid-19 pandemic. These observations provide unique and novel insights into aerosol-cloud interaction processes. Laboratory experiments have generated additional knowledge on detailed aerosol processes. The ongoing evaluation of experimental results will form a basis for improving the ESMs. A future campaign will also provide information on aerosol and fog properties in a highly polluted region of Europe.
FORCeS participants have contributed with model simulations relevant for the sixth Assessment Report (AR6) of the IPCC, expected to be published in 2021. A collaborative effort between modelling and experimental groups has generated concrete steps towards implementation of the process-knowledge into FORCeS ESMs. We have benchmarked the current versions of FORCeS ESMs to prepare for the evaluation of new process descriptions. New and existing datasets suitable for model evaluation have been compiled, which will be crucial for constraining FORCeS ESMs to produce reduced uncertainty related to aerosol radiative forcing. FORCeS has also evaluated the ESMs against new types of metrics, referred to as “emergent constraints” in the climate community. This evaluation will guide future model improvements and help identify promising avenues for new constraints. In the next period, FORCeS will develop constraints over the multitude of scales that govern aerosol-cloud-climate interactions.
The objectives of FORCeS are specifically 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). A first landmark paper is under production, which outlines the major components and reasons for the large persistent uncertainty in aerosol radiative forcing. Simultaneously, FORCeS has taken steps towards mitigating some of these issues, for example by 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. After a dialogue with stakeholders, FORCeS released its first policy brief aimed at providing decision- and policymakers with information on the potential conflicts between the clean air goals of the European Green Deal and climate neutrality. By engaging in a “Science2Policy” event, FORCeS has contributed to a collection of key policy messages and research needs in climate science. 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 and help ensuring Europe’s leading position in climate science.
Sampling in San Pierto Capofuime (Photo: Leone Tarozzi, CNR) and Ny Ålesund (Photo: Paul Zieger, SU)