Climate change

A nasty surprise in the greenhouse

The Kyoto Protocol aims to reduce emissions of greenhouse gases such as methane. But it seems that the fall in human-induced methane emissions in the 1990s was only transitory, and atmospheric methane might rise again.

Methane is a potent greenhouse gas — per molecule, more than 20 times as powerful as carbon dioxide1. Moreover, when methane emissions rise, so too does the concentration of the pollutant ozone in the troposphere, the lowest layer of Earth's atmosphere2. Methane also consumes hydroxyl radicals, whose oxidative effects are essential to atmospheric cleansing. On page 439 of this issue3, Bousquet et al. recount the results of an international effort to measure atmospheric methane concentrations and combine these data with a computer model of atmospheric chemistry and transport. The bad news is that the slowdown in global methane emissions in the past few decades was only temporary: reports of the emissions' control have been exaggerated.

At present, about two-thirds of global methane comes from anthropogenic sources, and most emissions occur in the Northern Hemisphere (Fig. 1, below). Of naturally produced methane, the largest proportion stems from bacteria in wetlands that produce the gas when decomposing organic material. The growth rate of atmospheric methane was more than 10% per decade before 1980, but by the 1990s it had dropped to nearly zero (Fig. 2, below)4. Bousquet and colleagues3 compute the global methane source distribution, especially its variability over recent decades. This is a rather controversial issue, as it is difficult to determine whether this variability should be attributed to fluctuations in the sources or in the sinks; the sink mechanisms are dominated by the good offices of the atmospheric hydroxyl radicals5.

Figure 1: Methane hotspots.
figure1

Average methane mixing ratios in the boundary layer (the layer of the atmosphere in immediate contact with Earth's surface) in 2003, calculated with a chemistry–transport model. The atmospheric lifetime of methane is almost a decade, so it disperses globally. Regions of strong emissions are nevertheless manifest, leading to the largest variability in the Northern Hemisphere and an inter-hemispheric difference of 5–10%. The recently proposed release of methane by terrestrial vegetation is not included, as its magnitude is still uncertain. (Figure courtesy S. Houweling5.)

Figure 2: Meridional slide.
figure2

The latitudinal methane distribution in the marine boundary layer over the past 10 years, based on measurements in the air-sampling network of the Earth System Research Laboratory of the US National Oceanic and Atmospheric Administration. Annual fluctuations and the higher methane concentrations at higher northern latitudes are clear, but over the period surveyed, the year-on-year trend is small. Bousquet et al.3 suggest that recent dry years have led to a drop in natural methane emissions, masking a rise in emissions induced by human activities. (Figure courtesy E. Dlugokencky4.)

The authors used a so-called inversion modelling technique, which starts from observed concentrations at Earth's surface and back-calculates using models of transport and loss processes to optimize source estimates. The measurements stem from a global network of monitoring stations, and include isotope data (in particular, the relative proportion of carbon-13) that provide an additional clue as to what methane came from where. Methane from biomass burning, fossil-fuel-related sources and bacterial processes have distinct isotopic signatures; methane emissions from wetlands, for example, are substantially depleted in carbon-13.

The approach is novel because the model computations optimized both methane emissions and methane loss through hydroxyl oxidation. The crux of the findings is that fluctuations of natural emissions, in particular by wetlands in the tropics, are a dominant factor in the variability of methane from year to year. These emissions are in turn sensitive to meteorological parameters: during dry periods, methane flux from wetlands is depressed.

Thus, during the most recent part of the analysis period — from 1999 onward — extended droughts have reduced natural methane emissions. This has concealed the fact that anthropogenic emissions have resumed their increase, an increase perhaps associated with the accelerating use of fossil fuels by booming Asian economies. Continued monitoring of atmospheric methane, and especially its relation to wetland inundation and drying, will be needed to substantiate this prediction.

Bousquet and colleagues' study3 is not incompatible with the recent suggestion that terrestrial vegetation is a strong methane source6. It sounds paradoxical that adding a large new source such as this, which contributes 30% of the global methane flux to the model, and decreasing the others proportionately can be done without fundamentally changing the inversion calculations. But the combination of the relatively strong meridional methane gradient (Fig. 2) and too few measurement stations in the tropics makes constraining the size of single-source categories such as forests and wetlands tricky.

Further confirmation of a large vegetation source comes from satellite measurements showing that methane concentrations are enhanced over the tropical rainforests7. It is remarkable not only that this large new methane source has just been discovered, but also that mechanisms for producing methane deviate from the known anaerobic formation by microbes. The role of oxygen in methanogenesis, including its application in renewable energy production, should receive much more attention in the coming years.

Atmospheric methane is a factor in the amplification of climate change, because the amount of methane released by wetlands and vegetation responds sensitively to temperature and moisture conditions. This establishes a positive-feedback mechanism that has contributed to rapid climate shifts during the last glacial cycle8. But Bousquet and colleagues' analysis3 also allows for a negative-feedback mechanism, put in place by atmospheric chemistry. In dry periods of reduced methane emissions, methane removal by hydroxyl radicals also decreases. The dryness aggravates vegetation fires, which release large amounts of carbon monoxide, and this pollutant gas also consumes hydroxyl. With less hydroxyl around, less methane is broken down, and the decrease in methane concentration is not as large as might be expected.

This phenomenon is not natural, however: most fires are ignited by humans. Furthermore, vast amounts of methane are deposited as hydrates in permafrost regions and in marine sediments. It is as yet unclear to what extent the melting of permafrost and increasing ocean temperature will affect these methane reservoirs, destabilize the hydrates and exacerbate greenhouse warming. These processes influence both atmospheric methane and hydroxyl, and are potentially important feedback mechanisms that require further research.

It will be both essential and difficult to control greenhouse-gas emissions and verify nationally reported inventories. Human-induced emissions scale with fossil-fuel consumption, and the global yearning for energy, especially in nations that do not recognize the Kyoto Protocol on climate change, gives rise to concern about climate change. The uncertainty range associated with climate projections1 implies that large changes will be as likely as modest ones. Such large and possibly disruptive climate changes ask for short-term solutions.

Unfortunately, the response times of most greenhouse gases in the atmosphere and their climatic effects are slow — decades to centuries. Measures to control methane emissions should be placed high on the agenda because they, in contrast, become effective within a few years. In a situation where climate change is accelerating, there is no time to lose: we need effective solutions, and we must act fast.

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Lelieveld, J. A nasty surprise in the greenhouse. Nature 443, 405–406 (2006). https://doi.org/10.1038/443405a

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