"We attribute this shift in part to enhanced forest law enforcement and lower deforestation rates that reduced forest fires in the latter 2000s," John Ten Hoeve of Stanford University told environmentalresearchweb. "The increase in savanna/agricultural fires in 2007 and 2010 is due in part to drought conditions during these years, but also to increased agricultural and pastoral development on already degraded land rather than newly deforested land, a change which has been shown in other recent studies."

According to Ten Hoeve, thick smoke palls made of tiny aerosol particles cover much of the Amazon region during the biomass burning season. The smoke comes from burning of both forest and savanna/agricultural areas. Recently cut forest is burnt to develop agricultural or pastoral land while fires are lit on existing agricultural or pastoral land to mobilize nutrients, control pests or remove brush and litter.

"Fire ignitions are largely anthropogenic but fire spreading is mainly affected by natural factors such as drought," he explained. "Another implication of this study is if droughts worsen over the Amazon with climate change – two of the worst droughts in the last century occurred in 2005 and 2010 – the increase in forest-fire spreading may offset current and future reductions in anthropogenic fire ignitions."

Brazil recently introduced a target of an 80% reduction in deforestation by 2020, the researchers said. Exporters and buyers, meanwhile, have brought in soy and cattle moratoriums that restrict export or the purchase of these commodities from newly deforested land. "However, because this shift is due to political and societal factors, a relaxation of these policies could send the recent decreasing trend [in forest burning] in the opposite direction," said Ten Hoeve.

When it comes to carbon emissions, Brazil is aiming to reduce its greenhouse-gas output by around 35% below business as usual by 2020, mainly through forest conservation programmes such as REDD (Reducing Emissions from Deforestation and Forest Degradation). Although forest burning has decreased in recent years, savanna/agricultural burning has increased.

"Fuel characteristics are different between forest and savanna lands so the emissions inventory becomes more complex, but the REDD programme does not account for changing emissions on savanna or agricultural lands," said Ten Hoeve. "In order to ensure that Brazil's emissions target is met, the Brazilian government must address the usage of fire as a tool for land maintenance on savanna/agricultural lands as well as for land clearing on forested lands."

The team found that aerosols from biomass burning showed lower absorptive properties over the period of the study as burning shifted from forest to savanna/agricultural lands. This could have implications for regional aerosol–cloud–climate interactions.

Now the team plans to focus on these interactions and investigate the effect of changing aerosol type. "We recently published a combined remote sensing/modelling study finding evidence of complex aerosol effects on clouds during the biomass-burning season over Amazonia, and we plan to extend a similar analysis to other biomass-burning regions of the world, such as Southeast Asia," added Ten Hoeve.

The team, who reported the study in Environmental Research Letters (ERL), also plans to determine the co-benefits of reducing emissions from global biomass burning for climate and for human health.