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. • 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 below- ground 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, 3 amounting to globally 55 – 100 Mt CH4 per year 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.

CH4 as % of net global reduction of typical CH4 Vegetation primary warming carbon NPP* emissions productivity** equivalent 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