Abstract. Savanna fires contribute significantly to greenhouse gas emissions. While it
is recognized that these fires play a critical role in the global methane
cycle, there are too few accurate estimates of emissions from West Africa,
the continent's most active fire region. Most estimates of methane emissions
contain high levels of uncertainty as they are based on generalizations of
diverse landscapes that are burned by complex fire regimes. To improve
estimates we used an approach grounded in the burning practices of people
who set fires to working landscapes. We collected and analyzed smoke samples
for 36 experimental fires using a canister method for the early dry season
(EDS) and mid-dry season (MDS). We also collected data for savanna type,
grass type, biomass composition and amount consumed, scorch height, speed of
fire front, fire type, and ambient air conditions for two sites in Mali. We
report values for fire intensity, combustion completeness, patchiness,
modified combustion efficiency (MCE), emission factor (EF) and methane
emission density. Our study found that mean methane EFs ranged from 3.83 g kg−1 in the EDS to
3.18 g kg−1 in the MDS, but the small sample did not provide enough power for this
effect to be significant. We found head fires had nearly double the
CH4 EF of backfires (5.12 g kg−1 to 2.74), a significant
difference. Byram's fire intensity was a significant driver of CH4 EF
but with weak effect. Methane emission density increased marginally from
0.839 g m−2 in the EDS to 0.875 g m−2 in the MDS, a difference that
was not significant. Head fires, however, had much higher emission densities
than backfires – 1.203 vs. 0.708 g m−2 – respectively, a significant
difference. We suggest the reason for the higher methane emissions from
head fires, which have higher intensity, is the longer flame lengths that
burn green leaves on trees, releasing methane. We conclude that policies
aimed at shifting the burning regime earlier to reduce methane emissions
will not have the desired effects, especially if fire type is not
considered. Future research should consider the state and amount of leafy
biomass combusted in savanna fires.
Methane gas emissions from savanna fires: what analysis of local burning regimes in a working West African landscape tell us
