Cropping, population boost carbon sinks

Research into the effect of wildfires on global warming has delivered a “good news/bad news/good news” result.

The first bit of good news, write scientists Vivek Arora and Joe Melton from government department Environment and Climate Change Canada, in the journal Nature Communications, is that the area burnt around the world by wildfires has been steadily decreasing since the 1930s. This also means that the amount of carbon dioxide released into the atmosphere by such fires has similarly dwindled.

The bad news, however, is that the result is due largely to the fact that there is, these days, much less wilderness to burn – the result of increasing farmland and human settlement.

But, Arora and Melton report, it’s not all bad news. The overall effect is to increase the amount of carbon being absorbed by the land – decreasing, thus, the amount directly contributing to global warming by accumulating in the atmosphere.

To make their findings, the researchers used sediment-charcoal records combined with satellite observations (in the latter case, from 1997 onwards) and fed the data into a complex Canadian-derived model that simulated additional inputs such as snow cover, soil temperature and soil moisture content.

The results showed that wildfires increased in number and extent from 1850 until the 1930s, at which point the effects of land clearing for both cropping and human settlement pushed the fire numbers downwards, a trend that has continued ever since.{%recommended 6824%}

The reduction in fire outbreaks and area burnt does not directly correlate with the loss of wilderness. Instead, the effect is increased because of additional factors. These include the patchwork nature of cropland geographies, which inhibit the spread of active fires, and the use of deliberate fire-suppression technologies.

Many crop-farming strategies, of course, involve annual stubble burning, but Arora and Melton find that this contributes little to the total amount of fire-generated carbon produced, mainly because stubble comprises a far lower biomass than woodland or forest.

“Even when agricultural fires are considered together with wildfires, the overall effect of increase in cropland area at the global scale is to decrease area burned,” the pair write.

However, conversion of land to cropping alone does not lead to a decrease in carbon emissions, mainly because “the vegetation that is spared burning from wildfires was already deforested in the first place”. In addition, the trees that were cleared to create the fields goes through decomposition for several years afterwards, releasing carbon dioxide into the atmosphere.

However, when the whole model is considered – including cropland clearance, landscape fragmentation and fire suppression – the result is the creation of a significant carbon-sink reservoir.

On a global scale, of the total amount of anthropogenic carbon dioxide, some 45% stays in the atmosphere, while 30% enters the land and the remainder is sequestered in the oceans.

Using data covering the period 1960 to 2009, the researchers found that land clearance and increasing population density were responsible for an extra 130 million tonnes of carbon each year sinking into the land – about 19% of the total.