ADVANCE (Aerosol-cloud-climate interactions derived from Degassing VolcANiC Eruptions)

Lead Research Organisation: University of Exeter
Department Name: Mathematics

Abstract

Anthropogenic emissions that affect climate are not just confined to greenhouse gases. Sulfur dioxide (SO2) and other pollutants form atmospheric aerosols that scatter and absorb sunlight, and influence the properties of clouds, modulating the Earth-atmosphere energy balance. Anthropogenic emissions of aerosols exert a significant, but poorly quantified, cooling of climate that acts to counterbalance the global warming from anthropogenic emissions of greenhouse gases. Uncertainties in aerosol-climate impacts are dominated by uncertainties in aerosol-cloud interactions (ACI) which operates through aerosols acting as cloud-condensation nuclei (CCN) which increases the cloud droplet number concentration (CDNC) while reducing the size of cloud droplets and subsequently impact rain formation which may change the overall physical properties of clouds. This consequently impacts the uncertainty in climate sensitivity (the climate response per unit climate forcing) because climate models with a strong/weak aerosol cooling effect and a high/low climate sensitivity respectively are both able to represent the historic record of global mean temperatures.

On a global mean basis, the most significant anthropogenic aerosol by mass and number is sulphate aerosol resulting from the ~100Tg per year emissions of sulphur dioxide from burning of fossil fuels, but these plumes are emitted quasi-continuously owing to the nature of industrial processes, meaning that there is no simple 'control' state of the climate where sulphur dioxide is not present. On/off perturbation/control observations have, to date, been limited to observations of ship tracks but the spatial scales of such features are far less than the resolution of the weather forecast models or of the climate models that are used in future climate projections. This situation changed dramatically in 2014 with the occurrence of the huge fissure eruption at Holuhraun in 2014-2015 in Iceland, which was the largest effusive degassing event from Iceland since the eruption of Laki in 1783-17849. The eruption at Holuhraun emitted sulphur dioxide at a peak rate of up to 1/3 of global emissions, creating a massive plume of sulphur dioxide and sulphate aerosols across the entire North Atlantic. In effect, Iceland became a significant global/regional pollution source in an otherwise unpolluted environment where clouds should be most susceptible to aerosol emissions. Thus, the eruption at Holuhraun created an excellent analogy for studying the impacts of anthropogenic emissions of sulphur dioxide and the resulting sulphate aerosol on ACI.

Our research will comprehensively evaluate impacts of the Holuhraun aerosol plume on clouds, precipitation, the energy balance, and key weather and climate variables. Observational analysis will be extended beyond that of our pilot study to include high quality surface sites. Two different climate models will be used; HadGEM3, which is the most up to date version of the Met Office Unified model and ECHAM6-HAM, developed by MPI Hamburg. These models are chosen because they produce radically different responses in terms of ACI; ECHAM6-HAM produces far stronger ACI impacts overall than HadGEM3. Additionally, the UK Met Office Unified Model framework means that the underlying physics is essentially identical in low-resolution climate models and high-resolution numerical weather predication models, a feature that is unique in weather/climate research. In the high resolution numerical weather prediction version, parameterisations of convection can be turned off and sub-gridscale processes can be explicitly represented. Thus the impacts of choices of parameterisation schemes and discrete values of variables within the schemes may be evaluated.

The research promises new insights into ACI and climate sensitivity promising us great strides improving weather and climate models and simulations of the future.

Planned Impact

ADVANCE: Impact Summary

The Beneficiaries of the Research and how they will benefit from it:-

A) Scientific community:
ADVANCE will be of significant scientific interest nationally and internationally as evident by the level of interest and support from the international project partners. ADVANCE data will be placed on the JASMIN (BADC) for use initially by project partners, but will be available to the entire community. Specifically:-
1) Project partners in the satellite community and surface observation community; the extensive use of satellite and surface observations to validate models will show the considerable benefit in making both routine and specialised measurements over a long time interval.
2) The AeroCom Phase II project comprises a group of 22 international global modelling centres/models; 15 of which have agreed to participate in the proposed Holuhraun experiment under CONSTRAIN. They will benefit from a comprehensive validation of ACI in their individual models.
3) The atmospheric aerosol and aerosol-cloud-interaction community. This community has long been searching for the systematic investigation of aerosol indirect effects performed over a numerical weather prediction and global climate scales that ADVANCE will provide.
4) The community working on climate sensitivity. Without suitably constrained aerosol-cloud-interaction (ACI) forcings, climate sensitivity will remain uncertain because models with a high/low climate sensitivity and a strong/weak aerosol radiative forcing can adequately reproduce the evolution of the past climate in terms of the global mean temperature. By reducing the ACI uncertainty one can better constrain climate sensitivity.
5) The climate change community. Once ACI and climate sensitivity are better constrained, the spread in future climate change scenarios will be reduced. Underlying uncertainties will then become dominated by radiative forcing pathways; uncertainties in the future climate will be due to human emissions, rather than uncertainties in our knowledge of how the climate responds to a unit of climate forcing.

B) Operational Weather Forecasting Centres:
These include, i) The Met Office (see LoS) and associated Unified Model partners including: Australia: ii) Bureau of Meteorology, iii) CSIRO; India,iv) MoES, v) NCMRWF; New Zealand, vi) NIWA; South Korea, vii) KMA; Poland, viii) ICM; South Africa, ix) SAWS; USA, x) USAF. The Met Office will benefit from the extensive use of not only the HadGEM3 climate model, but of the limited area numerical weather prediction model leading to better weather, seasonal forecasts, and climate projections at local, regional, and continental scales. In particular, the role of aerosols on precipitation initiation and intensity will be clarified enabling direction to be set at the strategic level across Unified Model partners.

C) General public / media: There has been significant publicity in recent years on aerosols, clouds and climate. However, there remains a significant lack of knowledge surrounding these issues and clear information needs to be conveyed to the public to enable greater appreciation of the uncertainties in regional and global climate prediction.

D) Policymakers: There is considerable interest from policy-makers on the magnitude of ACI and climate sensitivity. Climate change is arguably one of the most pressing issues over the next two decades as humanity struggles (and very likely fails) to meet the 1.5C above pre-industrial than has been set by the Paris COP21. ADVANCE promises a significant reduction in uncertainty in climate model projections allowing adoption of more robust adaptation and mitigation strategies.

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