Image caption The burning of wood is a major source of black carbon the world over.

Black carbon, or soot, is making a much larger contribution to global warming than previously recognised, according to research.

Scientists say that particles from diesel engines and wood burning could be having twice as much warming effect as assessed in past estimates.

They say it ranks second only to carbon dioxide as the most important climate-warming agent.

The research is in the Journal of Geophysical Research-Atmospheres.

Black carbon aerosols have been known to warm the atmosphere for many years by absorbing sunlight. They also speed the melting of ice and snow.

Half a degree

This new study concludes the dark particles are having a warming effect approximately two thirds that of carbon dioxide, and greater than methane.

If we did everything we could to reduce these emissions we could buy ourselves up to half a degree less warming Prof Piers Forster, University of Leeds

"The large conclusion is that forcing due to black carbon in the atmosphere is larger," lead author Sarah Doherty told BBC News.

"The value the IPCC gave in their 4th assessment report in 2007 is half of what we are presenting in this report - it's a little bit shocking,"

The researchers say black carbon emissions in Europe and North America have been declining due to restrictions on emissions from diesel engines. But they have been growing steadily in the developing world. However as these type of particles don't last very long in the atmosphere, cutting their number would have an immediate impact on temperatures.

Image caption Cutting emissions from diesel engines could have a big effect

"Reducing emissions from diesel engines and domestic wood and coal fires is a no-brainer as there are tandem health and climate benefits," said Professor Piers Forster from the University of Leeds.

"If we did everything we could to reduce these emissions we could buy ourselves up to half a degree less warming, or a couple of decades of respite," he added.

The report warns that the role of black carbon is complex and can have cooling and warming effects.

"Mitigation is a complex issue because soot is typically emitted with other particles and gases that probably cool the climate," said Prof Forster,

"For instance, organic matter in the atmosphere produced by open vegetation burning likely has a cooling effect. Therefore the net effect of eliminating that source might not give us the desired cooling," he added.

Black carbon is said to be a significant source of rapid warming in the northern United States, Canada, northern Europe and northern Asia. The particles are also said to have an impact on rainfall patterns in the Asian monsoon.

Last year a six nation coalition of countries began a combined effort to curb the impact of short lived climate agents such as black carbon.

The authors say that while cutting back on soot is important, cutting carbon dioxide emissions is the best way to address climate change in the long term.