Black carbon aerosols and Arctic climate

Human-made climate changes are primarily due to increasing CO2 emissions, but other components also contribute. Soot particles, or black carbon, is emitted from combustion and heat the climate by absorbing sunlight. When black carbon is deposited on snow and ice, the surface albedo changes. The new darker surface can absorb more sunlight, thus enhancing the warming, as part of a positive feedback. The Arctic is therefore sensitive to black carbon particles. Reduction of black carbon emissions has the potential to both dampen the warming quickly and at the same time improve air quality. This has led to wide national and international efforts to promote measures to reduce black carbon emissions, such as the International Climate and Clean Air Coalition, announced by the US Department of Foreign Affairs in 2012. Quantifying the impact of such measures is highly uncertain. We are uncertain on how much black carbon is emitted from different sectors, how black carbon particles are transported to the Arctic, and how black carbon absorbs sunlight, affects clouds, and deposits on snow and ice. Black carbon is not emitted alone. Sulphate compounds and organic carbon, which have a cooling effect on the climate, are often co-emitted. The total effect of emission cuts will therefore depend on the amount of black carbon emitted compared with other particles. The goal of the BlackArc project is to reduce the uncertainties in the modelling of black carbon particles and to improve our understanding of the climate effect of black carbon in the Arctic. This can then serve as a foundation for black carbon measures. Quantifying the impact of climate measures requires accurate models. In two of the studies in the BlackArc project we have documented how state-of-the-art models simulate the transport of black carbon and other particles to the Arctic (and Antarctica) compared to ground-based observations and satellites. The simulations are available in the AeroCom project, with 16 models contributing. The models simulate a positive radiative effect in the Arctic spring, when there is plenty of sunlight and still a lot of snow and ice left from winter. This positive radiative effect on the energy balance in the Arctic is dominated by black carbon particles. Compared with available observations in the Arctic, the 16 models show a large spread in the seasonal variation of particles in the Arctic, but the model mean is close to the observations. A process that was found to be especially important for correctly transporting black carbon to the Arctic was wet removal by rain and snow. In this project we have also quantified how black carbon emissions and co-emitted species from different sectors, such as industry, transport, households, forest fires and flaring, affect the surface temperature in the Arctic, broken down by different countries and regions. The calculations have been made with four different models with the newest emission inventories. We did the calculations with several models to account for model uncertainty. We find that the largest contribution to increased surface temperature in the Arctic is from household pollution in Southeast Asia owing to the large absolute amount of emissions. On the other hand, the Arctic is most sensitive (per kg of emissions) to sectors further north, such as flaring in Russia. These emissions are transported into the Arctic at lower altitudes, thus affecting the surface temperatures more directly. We have made the sensitivity factors available online for others to use with new emissions data sets.

Prosjektet har bidratt til økt kunnskap om hvordan utslipp av sotpartikler fra kilder langt unna Arktis kan påvirke klimaet i Arktis. I prosjektet har vi beregnet hvilke utslipp som gir størst oppvarming i Arktis, og vi har laget en tabell for temperaturendrng per utslipp for en rekke utslippskategorier. Disse koeffisientene ligger fritt tilgjengelig på nett. Dataene har blant annet blitt brukt av Environmental Protection Agency i USA for å beregne klimaeffekten i Arktis av ulike utslippstiltak. Koeffisientene vil også danne grunnlaget for utslippsberegninger i neste AMAP-rapport for kortlevde klimadrivere. Dette mobilitetsstipendet har vært sentralt for prosjektleders nettverksbygging rundt fagmiljøer i USA. Arbeidet i prosjektet er relevant for organisasjonen Air Pollution in the Arctic: Climate, Environment and Societies (PACES) og prosjektleder skal arrangere neste PACES konferanse i Oslo. Et mål for PACES er å knytte tettere bånd mellom ulike fagmiljøer som jobber med Arktis.

How do Black Carbon (BC) aerosols influence Arctic climate change? The outcome of the BlackArc project is to reduce the uncertainty in modelling BC and to better constrain the climate impact of BC with an Arctic focus. The ultimate goal is to provide foundation for meaningful mitigation options for BC. BlackArc consists of 4 main objectives: 1.Identify which countries and emissions sectors that are the main contributors to Arctic climate change by BC and co-emitted species. 2.Improve parameterizations in the models to reduce bias in modelled BC concentrations. 3.Quantify the impact of the aerosol mixing state on the absorption per mass of BC. 4.Classify how the forcing by BC in different part of the atmosphere (latitudinally and vertically) contribute to Arctic warming. These objectives will be investigated in detail by comparing NorESM with the NASA GISS model coupled to an advanced aerosol microphysics module (MATRIX) and new field measurement campain data from the TCAP campaign provided by NASA GISS.