Methane emissions from forests ecosystems enhance global warming?
:
Keppler et.al.(2006). Methane emissions from terrestrial plants under aerobic conditions. Nature 439:
187-191
Quantifications of possible effects and comments
Summary
The results of this study could affect attractiveness and returns of CDM carbon offset projects and future
negotiations about carbon sinks in the second commitment period. If confirmed independently, they represent
a paradigm shift regarding methane formation in plant physiology. When extrapolated, net greenhouse gas
removals by growing forests, e.g. existing forests or forests created by afforestation projects under the Clean
Development Mechanism, could be reduced by maximally 4-8% though aerobic emissions of methane by
trees. Net actual effects are likely to be lower. However, transposing and up-scaling results obtained in
containers on small samples, seedlings, and on only one tree species to global forests and other biomes is not
a valid “first estimate”, but at best a hypothesis which should be tested. Results do not invalidate the role of
forests as carbon sinks; they also do not support the view that deforestation mitigates global warming via
reduced emissions of methane.
Background
The paper postulates a sizeable, previously unknown methane source of 60-240 Mt from
forests and other biomes, and an additional source of 1-7 Mt CH4 from litter. Plant physiology
has up to now not recognized the possibility of aerobic methane formation by intact plants or
litter. Methane, a powerful greenhouse gas with a global warming potential of 23 times that of
CO2, has been thought to originate in rice patties, natural wetlands, landfills, natural gas
reservoirs, biomass burning and in the digestive tracts of cattle.If these methane emissions can
be corroborated, the discovery amounts to a paradigm shift in plant physiology.
Possible consequences
If these methane emissions occur in all biomes in the field, then the role of the terrestrial
vegetation and specifically the role of forests in climate change must be revised. Possible
consequences are:
• The role of forests as sources and sinks of greenhouse gases in global warming would
need to be adjusted and other sources of methane recalculated.
• The 2006 IPCC Guidelines would have to be revised and default values included.
• Reporting by countries under UNFCCC of greenhouse gas emissions and removals by
forests according to 2003 IPCC Good Practice Guidance would have to be revised;
• Carbon sequestration achieved by afforestation projects under the Clean Development
Mechanism needs to be adjusted. Economic returns from such projects could change1.
• Methane emissions from forests would affect net greenhouse gas removals and
emissions from domestic afforestation, reforestation, deforestation and from forest
management activities under the Kyoto Protocol in industrialized countries.
The postulated emissions would decrease sink strength by existing forests by 4-8% ( Table1).
Net anthropogenic greenhouse gas removals in CDM afforestation and reforestation projects
and economic returns would not necessarily decrease to that extent, as only the net increase in
methane emissions compared to the pre-existing non-forest vegetation enters calculations. Net
anthropogenic greenhouse gas removals could increase, if methane emissions by planted trees
are lower than those of the predecessor vegetation.
Additional comments:
1
The “baseline net greenhouse gas removals by sinks” would not be affected, since only changes in carbon
stocks are considered. “Actual net greenhouse gas removals by sinks” and “net anthropogenic greenhouse gas
removals by sinks” would change, as these additional emissions of methane would need to be included.
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Results were achieved on small trees and litter samples in the lab or in small chambers
outdoors. The assumption that emission rates of methane for “short-lived biomass” are
representative and can be up-scaled to the level of global biomes is no more than an
untested and speculative hypothesis2.
Results were achieved with ambient air that had been purified of methane. This might
have influenced CH4 emissions observed. Forest soils for example, are sources or
sinks for methane dependent on the methane content of the ambient air.
It is unclear how the paper (table 1) calculates emissions by plant detrius. It provides
no stocks as a basis for calculating CH4 emissions from leaf litter. It disregards belowground litter.
The report does not mention that soils of forests, savannah, shrublands and deserts
sequester 23-56 Mt of CH4 annually, partially compensating for any methane
emissions.
Tropical deforestation, e.g. by fire, is followed by crops or secondary vegetation that
would also release methane; any release of CH4 could be larger in absolute terms than
that for natural tropical forests (Table1). Moreover, biomass burning including forest
clearing and wildfires releases 10 kg CH4/ ton d.m. of combusted biomass stock,
amounting to globally 55 – 100 Mt CH4 per year3 with high variability. Deforestation
appears as a simplistic explanation for reduced atmospheric growth rates of methane.
Net primary productivity includes the current year’s leaf-, root- and other litter. The
paper considers leaf litter separately without providing litter stocks.
Vegetation
CH4 as % of net
primary
productivity**
typical
NPP*
CH4
emissions
global
warming
equivalent
reduction of
carbon
offset
%
t C/ha
t per ha
t C/ ha
%
tropical natural mature forests
0.357
1
0.00357
0.08211
8.211
tropical plantations
0.357
12
0.04284
0.98532
8.211
temperate forests
0.218
6
0.01308
0.30084
5.014
boreal forests
0.115
2.5
0.002875
0.066125
2.645
mediterranean shrubs
0.193
3
0.00579
0.13317
4.439
tropical savannah and grasslands
0.195
3
0.00585
0.13455
4.485
temperate grasslands
0.132
2.5
0.0033
0.0759
3.036
deserts, semi-deserts
0.108
0.4
0.000432
0.009936
2.484
crops
0.175
3.2
0.0056
0.1288
4.025
* net primary production (NPP), includes biomass increment above-and belowground, detrius above-and below-ground and
biomass consumed by herbivores
** ratio between annual CH4 production and NPP from table 1 in the paper
2
In a possible analogy, the so-called “fertilization effect” of raised CO2 levels in the atmosphere on trees was
discovered and grossly overestimated based on glass-house experiments involving young trees.
3
in addition to roughly 1 kg/t of nitrous oxides with a global warming potential roughly 300 times larger than
CO2