Bacchus brightens up atmospheric aerosols

publiziert: Donnerstag, 11. Feb 2016 / 13:53 Uhr

Although Bacchus, the Roman god of wine, has lent his name to an ongoing research project on the interactions between aerosols, clouds and climate, he seems not to have clouded the scientists? capacity for analytical thought: After two years of research, the BACCHUS project team has now issued its mid-term summary for policy makers.

Are you one of those people who associate the name Bacchus with the Roman god of wine and revelry? ETH Zurich, 19 European and one Israeli research team - a gathering of leading experts in aerosol and climate research - have given this term an entirely different meaning: our BACCHUS stands for "Impact of Biogenic versus Anthropogenic emissions on Clouds and Climate: towards a Holistic UnderStanding". It is the acronym of a four-year EU FP7 research project which began in 2013 and is coordinated by Professor Ulrike Lohmann at the Institute of Atmospheric and Climate Science at ETH (see this blog post on BACCHUS). Two years into the project, we have now published a first report for policy makers and interested non-professionals, which presents the main findings in a clear and concise way.

Aerosols and clouds as uncertainty factors

Cloud droplets and ice crystals form around an individual tiny seed - called an aerosol. These aerosols can be of natural origin, such as dust, pollen, or salt from sea spray, or they can be man-made, emitted by diesel cars or industry, for instance. As the latest Intergovernmental Panel on Climate Change (IPCC) report states, the interactions between aerosols and clouds are a major uncertainty in climate projections. This uncertainty is partly due to a lack of fundamental understanding of ice-containing clouds and of the coupling between the biosphere and the atmosphere.

BACCHUS explores these two topics in detail, addressing important questions such as: how do clouds respond to a changing climate? Does melting Arctic sea ice lead to changes in Arctic cloud formation and hence to an increased or decreased warming of the Arctic? How do plants react to changes in rising concentrations of carbon dioxide? Will they emit more biogenic aerosol particles that may change cloud properties? Will the altered clouds produce more rain, which in turn would lead to an enhanced biomass production?

Measuring ice-forming cloud seeds

One of our main achievements is that we have collected and stored worldwide observational data on seeds for ice crystals, known as ice nucleating particles (INP), in the newly launched BACCHUS INP database. To complement already existing data, BACCHUS partners took part in several international measurement campaigns in 2015.

Understanding processes in clouds

Our scientists evaluate these data in a joint effort to better understand differences in INP and their interactions with clouds in three different key cloud regimes: tropical thunderstorm clouds in a region of high aerosol variability from wildfires and urban influence, sub-tropical shallow fair weather clouds, and Arctic summer clouds, that cause overcast and hazy conditions in a region almost entirely free of anthropogenic aerosols. For these case studies, the researchers apply process models which include the details of cloud droplet, ice crystal, rain and snow formation in an individual cloud. Due to their high spatial and temporal resolution, the models are ideal for directly comparing with the measurements conducted and compiled in BACCHUS.

Simulating feedback processes in climate models

In the first two years of the project we have developed improved descriptions of aerosol and cloud processes - called parameterizations - for use in global Earth System models (ESMs). ESMs enable us to simulate the interactions between the sub-systems that influence climate, i.e. the atmosphere, ocean, biosphere, land surface, and soil. We have developed parameterizations for releases of organic aerosols from the ocean to the atmosphere, as well as for emissions of aerosols from continental plants (biogenic volatile organic compounds, BVOCs). Two of the BACCHUS ESMs are now able to simulate releases of BVOCs to the atmosphere in a changing climate. This is one fundamental step towards improved climate change scenarios that will be carried out in the second half of BACCHUS.

Where do we go from here?

Climate change is one of the most significant challenges that humankind currently faces. For the second half of BACCHUS, we expect to further unravel the complexities of aerosol and cloud processes and hence reduce the uncertainties of future climate predictions. Our projections will contribute to international assessments such as IPCC, and thus may guide politicians and other stakeholders to develop strategies for air pollution control, mitigation and adaptation of climate change. Let?s drink to that!

(Christina Schnadt/ETH-Zukunftsblog)

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