Environmental Engineering Reference
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Other studies of African burning show how uncertain that total is. The median
burning interval is about four years, but some grasslands in the Sahel are not
burned for up to 20 years, while annual i res are common in the Guinean zone. This
makes for substantial interannual l uctuations, and different assumptions regarding
the density of the burned phytomass result in annual aggregates whose extreme
difference has been more than eightfold, 0.22 versus 1.85 Gt/year (Barbosa,
Stroppiana, and Grégoire 1999). The latest published annual rate is for the years
2001-2005, when about 195 Mha of African grasslands were burned annually,
releasing about 725 Mt C (Lehsten et al. 2009).
But adding this uncertain total to the HANPP is questionable because nearly all
of the released carbon will be incorporated into new grass growth following the
burning, and because many tropical and subtropical grasslands have been always
subject to widespread natural seasonal i res: i re has been a recurrent event in tropi-
cal forests of Amazonia (Cochrane and Schulze 1999), as well as a dominant driver
of carbon balance in Canadian boreal forests (Bond-Lamberty et al. 2007), and
hence it would not be easy to quantify only the net increase in i re activity due to
deliberate burning. Moreover, the productivity of many i re-adapted ecosystems
actually benei ts from regular burning, and so an accounting challenge would be
even greater: quantifying only that part of deliberate burning that reduces overall
productivity. Similarly, the productivity of i re-adapted forests may actually increase
after a i re as fast-growing new trees have lower autotrophic respiration than does
old-growth forest.
Moreover, an analysis of a global sedimentary charcoal data set shows that the
recent rates of anthropogenic burning are actually much lower than in the past
(Marlon et al. 2009a). The record shows a prolonged decline in biomass burning
that lasted since the beginning of the Common Era to about 1750; then came a
marked rise peaking around 1870, followed by a sharp downturn. Yet the post-1870
period has seen the most rapid land-use changes as well as rising temperatures, and
hence the downturn cannot be explained by reduced human activity or a cooler
climate: the fragmentation of vegetated areas, the emergence of generally less l am-
mable landscapes, and active i re suppression are the most likely causes. These
long-term trends in anthropogenic burning were coni rmed for the last 650 years
by an analysis of concentrations and isotopic ratios of atmospheric CO preserved
in a South Pole ice core (Wang et al. 2010): it shows a pronounced decline in South-
ern Hemisphere burning between 1350 and 1650, followed by an undulating rise
peaking during the late nineteenth century, followed by a decline to levels lower
than at any time since 1350.
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