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While the study is not yet published and will probably be behind a paywall when it is, the abstract is pretty clear:
However, global atmospheric absorption attributable to black carbon is too low in many models, and should be increased by a factor of almost three.[...]
Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m-2, is the second most important human emission in terms of its climate-forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing.Some 7.5 million tonnes of soots are released into the atmosphere each year, coming from cooking fires, open burning (like clearing land or wildfires) and diesel engines. This means that there's actually room to reduce soot production, with diesel engines producing about 70 percent of the soot emissions in Europe, North America, and Latin America. In Asia and Africa, wood burning domestic fires make up 60% to 80% of soot emissions. Coal fires are also a significant source of soot in China, parts of Eastern Europe, and former Soviet bloc countries.
Coal, one of the great evils in fuels, should already be seeing its use cut. Trimming diesel use would also provide other health benefits, as micro-particulates emitted from diesel engines have been linked to a number of respiratory ailments. And the whole cook-fire issue is another reason added to the heap of reasons for bringing the undeveloped world forward.
So bad news: soot is worse than we thought--by a whole lot. Good news? Soot production is something that could potentially be managed. Not that I expect we will....