Global Aerosol Synthesis and Science Project (GASSP) - to reduce the uncertainty in aerosol radiative forcing

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

The motivation for this project is that aerosols have persistently been assessed by the IPCC as the largest uncertainty in the radiative forcing of climate over the industrial period. This means that our ability to understand temperature changes over the industrial period is hampered by very poorly constrained aerosol processes in models. The main uncertainty is due to the effect that aerosols have on clouds - the so-called aerosol indirect effect by which anthropogenic aerosols make clouds more reflective. In the IPCC assessment, the range of predictions of the aerosol indirect forcing lies between -0.4 to -1.8 Wm-2, a far larger range than associated with CO2 forcing (1.6-1.9 Wm-2). Thus, to improve our understanding of climate change, we need to reduce the uncertainty in the aerosol indirect effect.

The controlling factor in the indirect effect is the concentration in the atmosphere of "cloud condensation nuclei" (CCN). CCN are a subset of the aerosol particles in the atmosphere, typically larger than 50 nm diameter and sufficiently water soluble to form cloud drops. Only recently, global models have been developed that are able to explicitly simulate CCN concentrations. This opens up the possibility of reducing model uncertainty by exploiting extensive measurements of CCN that have been made over many years.

We propose to undertake the first ever comprehensive synthesis of global CCN and related aerosol observations within the UK aerosol-chemistry-climate model. The overall aim is to reduce uncertainty in the indirect effect by constraining modern aerosol as much as possible based on present observing systems and models. We will reduce the uncertainty by producing a global model of CCN with well defined uncertainties that are constrained by worldwide observations. We will then use the "calibrated" aerosol model to quantify the indirect radiative forcing and its uncertainty. We will also use the new and better model to understand the sources of CCN in different environments, and thereby the factors that will drive future changes in the concentration. As a spin-off of the project we will also be able to use the model and data to identify the regions or environments in which new measurements would have the greatest impact on reducing the uncertainty further.

An important new aspect of the project will be the use of new uncertainty information about the global model. In most similar studies it has been possible to run the model only a few times. However, in reality the model has a wide uncertainty range due to the very large number of uncertain processes in the model. In this project we will use new information that tells us how the model behaves under all possible assumptions of uncertainty. From this collection of model runs we will be able to identify the best possible model in all parts of the world. This procedure is known as "calibration", and it has not been attempted before for a complex global model. With this approach we can be sure the model is as close to observations of CCN as can presently be achieved.

Planned Impact

GASSP is very well placed to have a direct impact on national and international climate and weather prediction centres, and therefore climate policy and the IPCC. The key to our engagement is that we are proposing to synthesise a vast number of existing measurements to constrain and improve aerosol forcing in the UK's climate model HadGEM. The unique compilation of harmonised observational data, synthesised to suitable formats / statistics, will then be available for wider community use and directly contribute to key international activities, such as the International Aerosol Intercomparison Project (AeroCom, http://aerocom.met.no/). Our engagement activities are a direct extension to our well-established collaborations.

Specifically in this project, our user engagement will include:

European Centre for Medium-Range Weather Forecasts (ECMWF): Our impact on the ECMWF will be through the EU Global Monitoring for Environment and Security MACC-II project (http://www.gmes-atmosphere.eu/). MACC combines state-of-the-art atmospheric modelling with Earth observation data to provide information services covering European Air Quality, Global Atmospheric Composition, Climate, and UV and Solar Energy. As part of MACC-I Leeds incorporated the GLOMAP aerosol model (the same as in HadGEM) in the ECMWF forecast model to enable future aerosol microphysics data assimilation and operational forecasts of aerosols and air quality. MACC makes a direct connection to the public and environmental agencies. The GASSP project will have an impact on the ECMWF by providing a comprehensive microphysics dataset to evaluate their model. Three months of the project has been set aside for this work at the end of the project (see Pathways to Impact).

Met Office (formal project partner, see letter of support): Engagement with the Met Office will continue through our ongoing NCAS-funded collaboration (UKCA). The data synthesis will provide a level of model evaluation superior to that of any other model, and will lead to improvement of the UK's climate modelling capability. Met Office staff will be invited to the GASSP data workshop in month 26. The workshop will produce a community publication on the observational needs for reducing the uncertainties in aerosol radiative forcing. This paper will have a long-term impact on Met Office model development.

The international Aerosol Model Intercomparison Project (AeroCom) (formal project partner, see Kinne letter of support): AeroCom brings together the majority of global models in the world and its results have provided significant input to the IPCC concerning aerosol forcing of climate. In GASSP we will produce extensive harmonised datasets and new evaluation methodologies that can be taken up immediately by AeroCom. In our Pathways to Impact activity we are proposing to hold a session at an AeroCom workshop to discuss how our new knowledge on model uncertainty can be exploited by AeroCom. Through this activity, GASSP can have an impact on model evaluation and future development internationally.

Publications

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Hodnebrog Ø (2019) Increased water vapour lifetime due to global warming in Atmospheric Chemistry and Physics Discussions

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Hodnebrog Ø (2019) Water vapour adjustments and responses differ between climate drivers in Atmospheric Chemistry and Physics

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Lund M (2018) Short Black Carbon lifetime inferred from a global set of aircraft observations in npj Climate and Atmospheric Science

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Reddington C (2017) The Global Aerosol Synthesis and Science Project (GASSP): Measurements and Modeling to Reduce Uncertainty in Bulletin of the American Meteorological Society

 
Description Aerosols impact our climate in a variety of ways but current estimates of their over effect are highly uncertain. As models are used to assess this effect, a better understanding of model uncertainty is desired. GASSP sets out to do this by charting the impact of individual physical and chemical processes and their uncertainty. Furthermore, GASSP will attempt to reduce those uncertainties through the use of detailed observations. GASSP follows up where AEROS stopped and greatly expands on it by: 1) compiling a unified database of disparate aerosol observations; 2) developing tools to assess and (hopefully) reduce model uncertainty using those observations.

Our work in GASSP provides the first comprehensive assessment of representation errors in in-situ and remote sensing measurements, suiteable for the assessment of global climate models.
Exploitation Route There will be a unified datatset of very disparate observations from a large variety of platforms (satellite, planes, surface sites) for other scientists to use. Two new methodologies (emulators, pathway analysis) have been advanced to improve our understanding of aerosol models.
Sectors Agriculture, Food and Drink,Energy,Environment,Healthcare

URL https://www2.physics.ox.ac.uk/research/climate-processes/projects/gassp
 
Description GASSP findings have been shared with peer-scientists through papers and conference talks. Many groups are interested in using both the unified observational database and the newly developed tools for assessing model processes (emulators, pathway analysis). Our developed methods will underpin new protocols for the assessment of aerosol-climate models using observations. The GASSP database has already been used to test and improve the MetOffice models and we are currently extending this analysis to the key global models worldwide.
First Year Of Impact 2013
Sector Environment
Impact Types Societal

 
Description Contribution to Chapter on Clouds and Aerosols in the Intergovernmental Panel on Climate Change AR5 report
Geographic Reach Multiple continents/international 
Policy Influence Type Participation in a advisory committee
 
Description NERC Standard Grant
Amount £796,596 (GBP)
Funding ID NE/P013406/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 07/2017 
End 06/2020
 
Title Updated aerosol activation scheme for UK HadGEM Earth System Model 
Description ACID-PRUF work led to an improved aerosol activation scheme used in the UK HadGEM Earth System Model family, also used in the MetOffice Unified Model 
Type Of Material Computer model/algorithm 
Year Produced 2012 
Provided To Others? Yes  
Impact Improved representation of aerosol-cloud interactions in current climate models 
URL https://ukesm.ac.uk
 
Description Aerosol Clouds Precipitation Climate initiative 
Organisation Global Energy and Water Exchanges Project
Country United States 
Sector Charity/Non Profit 
PI Contribution I am on the steering committee of the international Aerosol Clouds Precipitation Climate initiative sponsored by iLeaps, GEWEX and IGAC.
Collaborator Contribution I am on the steering committee of the international Aerosol Clouds Precipitation Climate initiative sponsored by iLeaps, GEWEX and IGAC.
Impact Workshops and two intercomparison studies.
Start Year 2016
 
Description GEWEX Aerosol Precipitation (GAP) initiative 
Organisation Global Energy and Water Exchanges Project
Country United States 
Sector Charity/Non Profit 
PI Contribution I am co-chairing a new initiative on aerosol precipitation interactions as part of the international Global Energy and Water cycle Exchanges (GEWEX) project.
Collaborator Contribution Co-chairing new initiative. Currently writing white paper.
Impact GAP workshop in Oxford 2017
Start Year 2016
 
Description School outreach talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Outreach talk to six-form students at Cherwell School on "Clouds, the known unknown in the climate system"
Year(s) Of Engagement Activity 2015
 
Description School outreach talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact School outreach talk at St Barnabas Primary School on Earth Observations
Year(s) Of Engagement Activity 2017
 
Description School science fair 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Participation in Science Fair at Wolvercote Primary School.
Year(s) Of Engagement Activity 2017