Burning of biomass contributes a significant amount of greenhouses gases to the atmosphere, which in turn influences regional air quality and global climate. Since the advent of humans, there has been a significant increase in the amount of biomass burning, particularly after the industrial revolution. What might not be immediately obvious is that, (naturally occurring) fires also play a part in emitting particulates and greenhouse gases which can absorb solar radiation and contribute to changing Earth’s climate. Producing a reliable record of pre-industrial fire history, as a benchmark to better understand the role of fires in the carbon cycle and climate system, is the focus of research recently published in the open access journal, Climate of the Past.
Did you know the combustion of biomass can emit up to 50% as much CO2 as the burning of fossil fuels? The incomplete burning of biomass during fires also produces significant amounts of a fine particle known as black carbon (BC). Compare BC to more familiar greenhouse gases such as methane, ozone and nitrous oxide and you’ll find it absorbs more incoming radiation than the usual suspects. In fact, it is the second largest contributor to climate change.
The boreal zone contains 30% of the world’s forests, including needle-leaved and scale-leaved evergreen trees, such as conifers. They are common in North America, Europe and Siberia, but fires styles in these regions are diverse owing to differences in weather and local tree types. For instance, fires in Russia are known to be more intense than those in North America, despite which they burn less fuel and so produce fewer emissions. All boreal forest fires are important sources of pollutants in the Arctic. Models suggest that in the summertime, the fires in Siberian forests are the main source of BC in the Artic and shockingly, exceed all contributions from man-made sources!
To build a history of forest fires over a 2000 year period the researchers used ice cores from the Greenland ice sheet. Compounds, such as ammonium, nitrate, BC and charcoal (amongst others), are the product of biomass burning, and can be measured in ice cores acting as indicators of a distant forest fires. Measure a single compound and your results can’t guarantee the signature is that of a forest fire, as these compounds can often be released during the burning of other natural sources and fossil fuels. To overcome this, a combined approach is best. In this new study, researchers measured the concentrations of levoglucosan, charcoal and ammonium to detect the signature of forest fires in the ice. Levoglucosan is a particularly good indicator because it is released during the burning of cellulose – a building block of trees – and is efficiently injected into the atmosphere via smoke plumes and deposited on the surface of glaciers.
The findings indicate that spikes in levoglucosan concentrations measured in the ice from the Greenland ice sheet correlate with known fire activity in the Northern Hemisphere, as well as peaks in charcoal concentrations. Indeed, a proportion of the peaks indicate very large fire events in the last 2000 years. The links don’t end there! Spikes in concentrations of all three measured compounds record a strong fire in 1973 AD. Taking into account errors in the age model, this event can be correlated with a heat wave and severe drought during 1972 CE in Russia which was reported in The New York Times and The Palm Beach Post, at the time.
The results show that a strong link exists between temperature, precipitation and the onset of fires. Increased atmospheric CO2 leads to higher temperatures which results in greater plant productivity, creating more fuel for future fires. In periods of draught the risk of fire is increased. This is confirmed in the ice core studied, as a period of heightened fire activity from 1500-1700 CE coincides with an extensive period of draught in Asia at a time when the monsoons failed. More importantly, the concentrations of levoglucosan measured during this time exceed those of the past 150 years, when land-clearing by burning, for agricultural and other purposes, became common place. And so it seems that the occurrence of boreal forest fires has, until now, been influenced by variability in climate more than by anthropogenic activity. What remains unclear is what the effects of continued climate change might have on the number and intensity of boreal forest fires in the future.
By Laura Roberts Artal, EGU Communications Officer
Zennaro, P., et al.: Fire in ice: two millennia of boreal forest fire history from the Greenland NEEM ice core, Clim. Past, 10, 1905-1924, doi:10.5194/cp-10-1905-2014, 2014.