NetBC: Closing knowledge gaps in order to quantify the net climate impact of atmospheric Black Carbon

Soot: A challenging piece of the climate puzzle Few man-made emissions have as many different connections to the climate as soot - or black carbon aerosols - but many of them are still scientifically uncertain. Through the NetBC project, we have focused on some of these: How long soot particles stay suspended after emission, and where they end up; how effective they are as seeds for ice particles high in the atmosphere; and the total climate impact of current anthropogenic soot emissions. Soot is released to the atmosphere from incomplete combustion. Sources are everything from cars, industry and forest fires, to cooking in wood-burning stoves - and disposable grills. After emission, the small particles remain suspended for a few days, and are transported by weather and wind. In the atmosphere, soot absorbs solar radiation, and thus has a warming effect in the same way as a greenhouse gas. In addition, soot can affect the formation of clouds, how white they become and how long they live. Soot can land on snow and cause it to melt faster, and it changes how easily ice crystals form in cold air. NetBC has contributed to the scientific understanding of the climate impact of soot through four related activities: We have contributed to aircraft measurements of the concentration of soot in the atmosphere, and analyzed the results from a number of measurement campaigns. Here, we have, among other things, been involved in uncovering large differences in the amount of soot above the Atlantic and Pacific Oceans, through collaboration with NASA and the airplane measurement campaign ATom. Furthermore, we have analyzed what a wide range of flight measurements tell us about the airborne lifetime of soot after emission. The more efficiently soot is washed out by rain, or chemically converted into the atmosphere, the shorter its lifetime will be - and the less soot there will be over remote places such as the world's oceans - and the Arctic. Through NetBC, we have helped to reveal that the lifetime of soot is shorter than what most of today's climate models estimate. This, in turn, means that calculations of the effects of today's total man-made emissions - from particles and greenhouse gases - have an uncertainty that we must work harder to reduce, if we want good estimates for future climate risk. Next, through comparisons of calculations from two major climate models, we have found similarities in how soot affects temperature and precipitation formation depending on how high it flies in the atmosphere. This enables us to better reduce the uncertainties inherent in today's model calculations, and will form the basis for further work in this direction. Through NetBC, we have also focused on the ability of soot to initiate the formation of ice particles in the atmosphere. These particles become ice clouds, or clouds that consist of a mixture of ice and water droplets, and these in turn affect the climate. Through calculations with one of the world's most advanced climate models in this area - in collaboration with the University of Oslo and Yale - we have revealed that soot particles from man-made emissions today probably constitute a cooling - and one that can be almost as strong as warming they stand for through the absorption of solar energy. This will help to weaken the total estimates of the climate impact from soot in today's climate. Finally, we have delivered, and contributed to, a number of summary works that look at the total climate impact of soot, from all its various compounds with solar radiation, heat from the surface, the atmosphere and clouds. One of these is the IPCC Sixth Assessment Report, where our results have been visible in the relevant chapters. We have revealed that the total temperature effect of current soot emissions is unlikely to exceed 0.1 ° C, which is weaker than most previous estimates, a result that will be important for future assessments of limiting soot emissions as a climate measure. In all, NetBC has allowed us to take several steps further in uncovering and delimiting the various ways in which soot emissions affect today's climate. The results from the project enable us to provide better and more detailed climate risk assessments, and to shift the focus from global assessments to the local effect that changes in soot emissions will have - for example in India and China - in the coming decades.

Vitenskapelige resultater: NetBC har bidratt til å redusere usikkerheten rundt klimavirkningen til menneskeskapte utslipp av sot, og å bedre kunnskapen om effekten av absorbsjon av energi fra aerosoler i atmosfæren. Resultatene er brukt i den ferske Sjette Hovedrapporten til FNs Klimapanel (IPCC). Innsamling av målinger: NetBC har delfinansiert målekampanjer med fly, ledet fra Storbritannia, og arbeidet med å analysere resultatene. Nettverk: NetBC har arrangert en rekke internasjonale workshops og konferanser, både alene og som sesjoner på etablerte konferanser om årsmøtene til AGU og EGU. Karriereutvikling: NetBC har understøttet flere yngre forskere, og har fremmet deres vitenskapelige synlighet og nettverksmuligheter. Konkret inkluderer dette deltakelse i IPCC og i samarbeid som AeroCom og RAMIP. Videre arbeid: NetBC har åpnet nye vitenskapelige problemstillinger, som hvordan raske endringene i sotutslipp over Asia kan påvirke nedbør.

Is Black Carbon (BC) a good candidate for climate mitigation policy? BC is a major topic among policy makers considering the role of aerosols and other short lived atmospheric components in climate mitigation scenarios. Through measurements and modelling, NetBC will deliver urgently needed science input to policy makers on the envelope of possibilities that BC provides. The net climate impact of BC cannot presently be rigorously constrained. Global BC emissions, its distribution through the atmosphere, and its climate interactions are poorly known. Recent research, e.g. by the NetBC team, has also highlighted the importance of additional effects, such as the semi-direct effect and the impact of BC on cirrus clouds. Hence, there is an urgent need for additional BC measurements, theoretical understanding, and - crucially - combinations of the two. Vertical profiles of BC concentrations, which have very recently become available through improved flight measurement techniques, have been shown to be a powerful tool for simultaneously constraining: - BC atmospheric lifetime, as determined by ageing, wet removal, transport and other detailed processes - BC emissions - atmospheric BC radiative forcing, which is determined both by the physical properties of BC itself, its altitude, location relative to clouds etc. None of the above factors are presently known with sufficient precision. Good measurements of BC concentration profiles are still scarce. Modelling of BC profiles is an active research topic, but further effort on using measurements to rigorously constrain models and the climate impact of BC is highly needed. NetBC will - provide the community with precise measurements of BC vertical profiles, over both source and remote regions - provide modelling studies of BC direct, indirect and semi-direct forcing, using in-house tools, in support of new and existing measurements - provide a revised estimate of the net atmospheric radiative forcing from BC today