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Atmospheric methane (CH4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era1. Fossil fuel extraction and use are among the largest anthropogenic sources of CH4 emissions, but the precise magnitude of these contributions is a subject of debate2,3. Carbon-14 in CH4 (14CH4) can be used to distinguish between fossil (14C-free) CH4 emissions and contemporaneous biogenic sources; however, poorly constrained direct 14CH4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century4,5. Moreover, the partitioning of total fossil CH4 emissions (presently 172 to 195 teragrams CH4 per year)2,3 between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH4 per year6,7. Geological emissions were less than 15.4 teragrams CH4 per year at the end of the Pleistocene, about 11,600 years ago8, but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core 14CH4 measurements to show that natural geological CH4 emissions to the atmosphere were about 1.6 teragrams CH4 per year, with a maximum of 5.4 teragrams CH4 per year (95 per cent confidence limit)—an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH4 emissions are underestimated by about 38 to 58 teragrams CH4 per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH4 budget, and will help to inform strategies for targeted emission reductions9,10. Isotopic evidence from ice cores indicates that preindustrial-era geological methane emissions were lower than previously thought, suggesting that present-day emissions of methane from fossil fuels are underestimated.

Atmospheric concentrations of carbon dioxide increased between the Last Glacial Maximum (LGM, around 21,000 years ago) and the preindustrial era1. It is thought that the evolution of this atmospheric carbon dioxide (and that of atmospheric methane) during the glacial-to-interglacial transition was influenced by organic carbon that was stored in permafrost during the LGM and then underwent decomposition and release following thaw2,3. It has also been suggested that the rather erratic atmospheric δ13C and ∆14C signals seen during deglaciation1,4 could partly be explained by the presence of a large terrestrial inert LGM carbon stock, despite the biosphere being less productive (and therefore storing less carbon)5,6. Here we present an empirically derived estimate of the carbon stored in permafrost during the LGM by reconstructing the extent and carbon content of LGM biomes, peatland regions and deep sedimentary deposits. We find that the total estimated soil carbon stock for the LGM northern permafrost region is smaller than the estimated present-day storage (in both permafrost and non-permafrost soils) for the same region. A substantial decrease in the permafrost area from the LGM to the present day has been accompanied by a roughly 400-petagram increase in the total soil carbon stock. This increase in soil carbon suggests that permafrost carbon has made no net contribution to the atmospheric carbon pool since the LGM. However, our results also indicate potential postglacial reductions in the portion of the carbon stock that is trapped in permafrost, of around 1,000 petagrams, supporting earlier studies7. We further find that carbon has shifted from being primarily stored in permafrost mineral soils and loess deposits during the LGM, to being roughly equally divided between peatlands, mineral soils and permafrost loess deposits today. Comparing the northern permafrost region of the Last Glacial Maximum with the same area today shows that the soil carbon stock has now increased, suggesting that permafrost carbon made no net contribution to preindustrial atmospheric carbon.

Wetlands are the largest global source of atmospheric methane (CH4), a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain, the largest natural geographic source of CH4 in the tropics, consistently underestimate the atmospheric burden of CH4 determined via remote sensing and inversion modelling, pointing to a major gap in our understanding of the contribution of these ecosystems to CH4 emissions. Here we report CH4 fluxes from the stems of 2,357 individual Amazonian floodplain trees from 13 locations across the central Amazon basin. We find that escape of soil gas through wetland trees is the dominant source of regional CH4 emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests6 and tropical peat swamp forests, representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δ13C) of −66.2 ± 6.4 per mil, consistent with a soil biogenic origin. We estimate that floodplain trees emit 15.1 ± 1.8 to 21.2 ± 2.5 teragrams of CH4 a year, in addition to the 20.5 ± 5.3 teragrams a year emitted regionally from other sources. Furthermore, we provide a ‘top-down’ regional estimate of CH4 emissions of 42.7 ± 5.6 teragrams of CH4 a year for the Amazon basin, based on regular vertical lower-troposphere CH4 profiles covering the period 2010–2013. We find close agreement between our ‘top-down’ and combined ‘bottom-up’ estimates, indicating that large CH4 emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH4 budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH4 emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH4 source when trees are combined with other emission sources.

Revisions in isotopic source signatures reveal that global total fossil fuel methane emissions from industry plus natural geological seepage are much larger than thought. Stefan Schwietzke et al. re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions on the basis of long-term global methane and methane carbon isotope records. They find that total fossil fuel methane emissions (fossil fuel industry plus natural geological methane seepage) are not increasing over time, but are 60–110 per cent greater than was previously thought. They also conclude that methane emissions from natural gas, oil and coal production and their usage are 20–60 per cent greater than inventories and that methane emissions from natural gas as a fraction of production have declined from about 8 per cent to 2 per cent over the past three decades. Methane has the second-largest global radiative forcing impact of anthropogenic greenhouse gases after carbon dioxide, but our understanding of the global atmospheric methane budget is incomplete. The global fossil fuel industry (production and usage of natural gas, oil and coal) is thought to contribute 15 to 22 per cent of methane emissions1,2,3,4,5,6,7,8,9,10 to the total atmospheric methane budget11. However, questions remain regarding methane emission trends as a result of fossil fuel industrial activity and the contribution to total methane emissions of sources from the fossil fuel industry and from natural geological seepage12,13, which are often co-located. Here we re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions based on long-term global methane and methane carbon isotope records. We compile the largest isotopic methane source signature database so far, including fossil fuel, microbial and biomass-burning methane emission sources. We find that total fossil fuel methane emissions (fossil fuel industry plus natural geological seepage) are not increasing over time, but are 60 to 110 per cent greater than current estimates1,2,3,4,5,6,7,8,9,10 owing to large revisions in isotope source signatures. We show that this is consistent with the observed global latitudinal methane gradient. After accounting for natural geological methane seepage12,13, we find that methane emissions from natural gas, oil and coal production and their usage are 20 to 60 per cent greater than inventories1,2. Our findings imply a greater potential for the fossil fuel industry to mitigate anthropogenic climate forcing, but we also find that methane emissions from natural gas as a fraction of production have declined from approximately 8 per cent to approximately 2 per cent over the past three decades.

Methane produced in sediments beneath the Greenland Ice Sheet is released to the atmosphere by meltwater in the summer. This suggests that glacial melt could be an important global source of this greenhouse gas. Glacial melt is a source of atmospheric methane.

An analysis of 12,000-year-old Antarctic ice revises our understanding of natural methane emissions to the atmosphere, and constrains estimates of the sensitivity of natural methane sources to abrupt climate-warming events. See Letter p.443 Natural geological sources of methane such as marine seepage and volcanic areas are estimated to contribute around 52 million tonnes per year to global methane emissions, but the true figure remains uncertain. Data retrieved from ancient ice in Antarctica now suggest that natural geological methane emissions were on average no higher than 15.4 million tonnes per year over the Younger Dryas–Preboreal warming event of about 11,600 years ago. Assuming that past geological methane emissions were not lower than today, the finding indicates that current estimates of geological methane emissions might be too high and, in consequence, anthropogenic fossil-fuel-related methane emission estimates might be too low. The study also provides further support for the idea that the rapid increase in atmospheric methane during this past warming event was probably the result of methane emissions from wetlands rather than from old carbon reservoirs such as marine gas hydrates or permafrost regions.

Atmospheric methane (CH(4)) increased through much of the twentieth century, but this trend gradually weakened until a stable state was temporarily reached around the turn of the millennium, after which levels increased once more. The reasons for the slowdown are incompletely understood, with past work identifying changes in fossil fuel, wetland and agricultural sources and hydroxyl (OH) sinks as important causal factors. Here we show that the late-twentieth-century changes in the CH(4) growth rates are best explained by reduced microbial sources in the Northern Hemisphere. Our results, based on synchronous time series of atmospheric CH(4) mixing and (13)C/(12)C ratios and a two-box atmospheric model, indicate that the evolution of the mixing ratio requires no significant change in Southern Hemisphere sources between 1984 and 2005. Observed changes in the interhemispheric difference of (13)C effectively exclude reduced fossil fuel emissions as the primary cause of the slowdown. The (13)C observations are consistent with long-term reductions in agricultural emissions or another microbial source within the Northern Hemisphere. Approximately half (51 ± 18%) of the decrease in Northern Hemisphere CH(4) emissions can be explained by reduced emissions from rice agriculture in Asia over the past three decades associated with increases in fertilizer application and reductions in water use.

Considerable debate surrounds the source of the apparently ‘anomalous’1 increase of atmospheric methane concentrations since the mid-Holocene (5,000 years ago) compared to previous interglacial periods as recorded in polar ice core records2. Proposed mechanisms for the rise in methane concentrations relate either to methane emissions from anthropogenic early rice cultivation1, 3 or an increase in natural wetland emissions from tropical4 or boreal sources5, 6. Here we show that our climate and wetland simulations of the global methane cycle over the last glacial cycle (the past 130,000 years) recreate the ice core record and capture the late Holocene increase in methane concentrations. Our analyses indicate that the late Holocene increase results from natural changes in the Earth's orbital configuration, with enhanced emissions in the Southern Hemisphere tropics linked to precession-induced modification of seasonal precipitation. Critically, our simulations capture the declining trend in methane concentrations at the end of the last interglacial period (115,000–130,000 years ago) that was used to diagnose the Holocene methane rise as unique. The difference between the two time periods results from differences in the size and rate of regional insolation changes and the lack of glacial inception in the Holocene. Our findings also suggest that no early agricultural sources are required to account for the increase in methane concentrations in the 5,000 years before the industrial era.

Past atmospheric methane concentrations show strong fluctuations in parallel to rapid glacial climate changes in the Northern Hemisphere superimposed on a glacial-interglacial doubling of methane concentrations. The processes driving the observed fluctuations remain uncertain but can be constrained using methane isotopic information from ice cores. Here we present an ice core record of carbon isotopic ratios in methane over the entire last glacial-interglacial transition. Our data show that the carbon in atmospheric methane was isotopically much heavier in cold climate periods. With the help of a box model constrained by the present data and previously published results, we are able to estimate the magnitude of past individual methane emission sources and the atmospheric lifetime of methane. We find that methane emissions due to biomass burning were about 45 Tg methane per year, and that these remained roughly constant throughout the glacial termination. The atmospheric lifetime of methane is reduced during cold climate periods. We also show that boreal wetlands are an important source of methane during warm events, but their methane emissions are essentially shut down during cold climate conditions. ©2008 Nature Publishing Group.

Large uncertainties in the budget of atmospheric methane, an important greenhouse gas, limit the accuracy of climate change projections. Thaw lakes in North Siberia are known to emit methane, but the magnitude of these emissions remains uncertain because most methane is released through ebullition (bubbling), which is spatially and temporally variable. Here we report a new method of measuring ebullition and use it to quantify methane emissions from two thaw lakes in North Siberia. We show that ebullition accounts for 95 per cent of methane emissions from these lakes, and that methane flux from thaw lakes in our study region may be five times higher than previously estimated. Extrapolation of these fluxes indicates that thaw lakes in North Siberia emit 3.8 teragrams of methane per year, which increases present estimates of methane emissions from northern wetlands (< 6-40 teragrams per year; refs 1, 2, 4-6) by between 10 and 63 per cent. We find that thawing permafrost along lake margins accounts for most of the methane released from the lakes, and estimate that an expansion of thaw lakes between 1974 and 2000, which was concurrent with regional warming, increased methane emissions in our study region by 58 per cent. Furthermore, the Pleistocene age (35,260-42,900 years) of methane emitted from hotspots along thawing lake margins indicates that this positive feedback to climate warming has led to the release of old carbon stocks previously stored in permafrost.

Methane is an important greenhouse gas and its atmospheric concentration has almost tripled since pre-industrial times. It plays a central role in atmospheric oxidation chemistry and affects stratospheric ozone and water vapour levels. Most of the methane from natural sources in Earth's atmosphere is thought to originate from biological processes in anoxic environments. Here we demonstrate using stable carbon isotopes that methane is readily formed in situ in terrestrial plants under oxic conditions by a hitherto unrecognized process. Significant methane emissions from both intact plants and detached leaves were observed during incubation experiments in the laboratory and in the field. If our measurements are typical for short-lived biomass and scaled on a global basis, we estimate a methane source strength of 62-236 Tg yr(-1) for living plants and 1-7 Tg yr(-1) for plant litter (1 Tg = 10(12) g). We suggest that this newly identified source may have important implications for the global methane budget and may call for a reconsideration of the role of natural methane sources in past climate change.

An abrupt climate warming of 5 to 10 degrees C during the Palaeocene/Eocene boundary thermal maximum (PETM) 55 Myr ago is linked to the catastrophic release of approximately 1,050-2,100 Gt of carbon from sea-floor methane hydrate reservoirs. Although atmospheric methane, and the carbon dioxide derived from its oxidation, probably contributed to PETM warming, neither the magnitude nor the timing of the climate change is consistent with direct greenhouse forcing by the carbon derived from methane hydrate. Here we demonstrate significant differences between marine and terrestrial carbon isotope records spanning the PETM. We use models of key carbon cycle processes to identify the cause of these differences. Our results provide evidence for a previously unrecognized discrete shift in the state of the climate system during the PETM, characterized by large increases in mid-latitude tropospheric humidity and enhanced cycling of carbon through terrestrial ecosystems. A more humid atmosphere helps to explain PETM temperatures, but the ultimate mechanisms underlying the shift remain unknown.

Atmospheric methane is the second most important greenhouse gas after carbon dioxide, and is responsible for about 20% of the global warming effect since pre-industrial times. Rice paddies are the largest anthropogenic methane source and produce 7-17% of atmospheric methane. Warm waterlogged soil and exuded nutrients from rice roots provide ideal conditions for methanogenesis in paddies with annual methane emissions of 25-100-million tonnes. This scenario will be exacerbated by an expansion in rice cultivation needed to meet the escalating demand for food in the coming decades. There is an urgent need to establish sustainable technologies for increasing rice production while reducing methane fluxes from rice paddies. However, ongoing efforts for methane mitigation in rice paddies are mainly based on farming practices and measures that are difficult to implement. Despite proposed strategies to increase rice productivity and reduce methane emissions, no high-starch low-methane-emission rice has been developed. Here we show that the addition of a single transcription factor gene, barley SUSIBA2 (refs 7, 8), conferred a shift of carbon flux to SUSIBA2 rice, favouring the allocation of photosynthates to aboveground biomass over allocation to roots. The altered allocation resulted in an increased biomass and starch content in the seeds and stems, and suppressed methanogenesis, possibly through a reduction in root exudates. Three-year field trials in China demonstrated that the cultivation of SUSIBA2 rice was associated with a significant reduction in methane emissions and a decrease in rhizospheric methanogen levels. SUSIBA2 rice offers a sustainable means of providing increased starch content for food production while reducing greenhouse gas emissions from rice cultivation. Approaches to increase rice productivity and reduce methane emissions as seen in SUSIBA2 rice may be particularly beneficial in a future climate with rising temperatures resulting in increased methane emissions from paddies.

Well-drained non-agricultural soils mediate the oxidation of methane directly from the atmosphere, contributing 5 to 10% towards the global methane sink1,2. Studies of methane oxidation kinetics in soil infer the activity of two methanotrophic populations: one that is only active at high methane concentrations (low affinity) and another that tolerates atmospheric levels of methane (high affinity). The activity of the latter has not been demonstrated by cultured laboratory strains of methanotrophs, leaving the microbiology of methane oxidation at atmospheric concentrations unclear3,4. Here we describe a new pulse-chase experiment using long-term enrichment with 12CH4 followed by short-term exposure to 13CH4 to isotopically label methanotrophs in a soil from a temperate forest. Analysis of labelled phospholipid fatty acids (PLFAs) provided unambiguous evidence of methane assimilation at true atmospheric concentrations (1.8–3.6 p.p.m.v.). High proportions of 13C-labelled C18 fatty acids and the co-occurrence of a labelled, branched C17 fatty acid indicated that a new methanotroph, similar at the PLFA level to known type II methanotrophs, was the predominant soil micro-organism responsible for atmospheric methane oxidation.

The global atmospheric methane burden has more than doubled since pre-industrial times1,2, and this increase is responsible for about 20% of the estimated change in direct radiative forcing due to anthropogenic greenhouse-gas emissions. Research into future climate change and the development of remedial environmental policies therefore require a reliable assessment of the long-term growth rate in the atmospheric methane load. Measurements have revealed that although the global atmospheric methane burden continues to increase2 with significant interannual variability3,4, the overall rate of increase has slowed2,5. Here we present an analysis of methane measurements from a global air sampling network that suggests that, assuming constant OH concentration, global annual methane emissions have remained nearly constant during the period 1984–96, and that the decreasing growth rate in atmospheric methane reflects the approach to a steady state on a timescale comparable to methane's atmospheric lifetime. If the global methane sources and OH concentration continue to remain constant, we expect average methane mixing ratios to increase slowly from today's 1,730 nmol mol−1 to ∼1,800 nmol mol−1, with little change in the contribution of methane to the greenhouse effect.

Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice. The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time. The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 ± 3 °C colder than today.

Microbiologists have succeeded in culturing the most acid-loving methanogen ever discovered. The new species, a member of the Methanomicrobiales group, was found in McLean Bog in New York State. It grows at a preferred pH of around 5, beating the previous record-holder, Methanobacterium espanolae, which has an optimum pH of between 5.5 and 6.0. Although some other methanogens can survive a pH as low as 4.5, the new species is the first to show growth and optimal methanogenesis in such acidic conditions. Microbes living in acidic soils are important sources of atmospheric methane, which is linked to global warming. Acidic peatlands are among the largest natural sources of atmospheric methane and harbour a large diversity of methanogenic Archaea1. Despite the ubiquity of methanogens in these peatlands, indigenous methanogens capable of growth at acidic pH values have resisted culture and isolation2,3,4; these recalcitrant methanogens include members of an uncultured family-level clade in the Methanomicrobiales prevalent in many acidic peat bogs in the Northern Hemisphere1,5,6. However, we recently succeeded in obtaining a mixed enrichment culture of a member of this clade7. Here we describe its isolation and initial characterization. We demonstrate that the optimum pH for methanogenesis by this organism is lower than that of any previously described methanogen.

The longest continuous record of global atmospheric ethane levels is presented, showing that global ethane emission rates decreased by 21 per cent from 1984 to 2010, probably owing to decreased venting and flaring of natural gas in oil fields; decreased venting and flaring also account for at least 30 to 70 per cent of the decrease in methane emissions over the same period. Ethane is the most abundant non-methane hydrocarbon in the remote atmosphere and is a precursor to tropospheric ozone. This paper presents the longest continuous record of global atmospheric ethane levels assembled so far and finds that global ethane-emission rates decreased by 21% between 1984 and 2010. This can probably be attributed to a decrease in fugitive emissions, such as the venting and flaring of natural gas from oil fields, rather than a decline in its other major sources, biofuel use and biomass burning. Because methane shares ethane's main sources of emissions, this new long-term ethane record can be used to investigate changes in global methane levels. This leads the authors to suggest that reduced fugitive fossil-fuel emissions also account for 30–70% of the decrease in global methane emissions. After methane, ethane is the most abundant hydrocarbon in the remote atmosphere. It is a precursor to tropospheric ozone and it influences the atmosphere’s oxidative capacity through its reaction with the hydroxyl radical, ethane’s primary atmospheric sink1,2,3. Here we present the longest continuous record of global atmospheric ethane levels. We show that global ethane emission rates decreased from 14.3 to 11.3 teragrams per year, or by 21 per cent, from 1984 to 2010. We attribute this to decreasing fugitive emissions from ethane’s fossil fuel source—most probably decreased venting and flaring of natural gas in oil fields—rather than a decline in its other major sources, biofuel use and biomass burning. Ethane’s major emission sources are shared with methane, and recent studies have disagreed on whether reduced fossil fuel or microbial emissions have caused methane’s atmospheric growth rate to slow4,5. Our findings suggest that reduced fugitive fossil fuel emissions account for at least 10–21 teragrams per year (30–70 per cent) of the decrease in methane’s global emissions, significantly contributing to methane’s slowing atmospheric growth rate since the mid-1980s.

MEASUREMENTS of methane in modern air1–8 and in air trapped in ice cores9–12 have shown convincingly that the abundance of atmospheric methane has been rising since the Industrial Revolution. This is a matter of concern because of the important role of methane in determining the radiative balance and chemical composition of the atmosphere13. The causes of this increase have not been identified unambiguously because of uncertainties in our understanding of the global budget of atmospheric methane14 and in how it is changing with time. Here we report on measurements of atmospheric methane from an extensive global network of flask sampling sites, which reveal that, although methane continues to accumulate in the atmosphere, there has been a substantial slowing of the global accumulation rate between 1983 and 1990. If this deceleration continues steadily, global methane concentrations will reach a maximum around the year 2006. Our results hint that changes in methane emissions in the latitude band 30–90° N may be of particular significance to this trend.

METHANE concentrations in the atmosphere have increased from about 0.75 to 1.7 p.p.m.v. since pre-industrial times1,2. The current annual rate of increase of about 0.8% yr−1 (ref. 2) is due to increases in industrial and agricultural emissions. This increase in atmospheric methane concentrations not only influences the climate directly, but also indirectly through chemical reactions. Here we show that the climate effects of methane's atmospheric chemistry have previously been overestimated, notably by the Inter-governmental Panel on Climate Change (IPCC)3, largely owing to neglect of the height dependence of certain atmospheric radiative processes. Using available estimates of fossil-fuel-related leaks of methane, our results show that switching from coal and oil to natural gas as an energy source would reduce climate warming. A significant fraction of methane emissions cannot, however, be accounted for by known sources; should leakages from gas production and distribution be underestimated for some countries, then it might be unwise to switch to using natural gas.

Rice agriculture is projected to expand by up to 70% over the next 25 years, and is likely to mean much more intensive use of ammonium-based nitrogen fertilizers. That is a worry because rice paddies are one of the main sources of methane (a greenhouse gas) in the atmosphere, and increased emissions would be undesirable. It seems, however, that extra ammonium may stimulate soil bacteria that oxidize methane, and so reduce emissions.

ATMOSPHERIC concentrations of methane, an important greenhouse gas, have increased significantly over the past few decades1,2. Although attention has been focused on anthropogenic sources, data from ice cores show that large changes in atmospheric methane concentrations have occurred over glacial–interglacial time scales, indicating that there is significant variability in natural methane fluxes3,4. The surface waters of the oceans are often supersaturated with methane, which implies that the oceans are a net source, although their contribution to the global methane budget is small relative to other sources4. Here we report high concentrations of methane in the Arabian Sea, and calculate that the flux of methane to the atmosphere is up to five times greater than the previously reported average ocean flux. Methane production is associated with high phytoplankton biomass, which is closely coupled with the monsoon-driven upwelling of nutrient-rich water. We calculate that the Arabian Sea (representing 0.43% of the total surface area of the world's oceans) could account for between 1.3 and 133% of the current estimates of the open-ocean source of methane. Our results do not alter the view that the oceans are a relatively minor source of atmospheric methane, but the magnitude of the methane fluxes from the Arabian Sea and the link with the monsoon suggest that this region may be particularly sensitive to climate change, with a greater potential for feedback responses than its surface area might suggest.

METHANE is an important greenhouse gas, whose concentration in the troposphere is steadily increasing. To estimate the flux of methane into the atmosphere and its atmospheric lifetime, its rate of removal needs to be accurately determined. The main loss process for atmospheric methane is the reaction with the hydroxyl radical OH. We have measured the rate coefficient for this reaction in carefully controlled experiments and found it to be smaller than currently accepted values. Our results indicate a longer CH4 lifetime (by ∼25%) and a correspondingly smaller flux (by ∼100 Tg CH4 yr−1) than previously calculated.

Methane measurements along the Vostok ice core reveal substantial changes over the past 160,000 years which are associated with climate fluctuations. These results point to changes in sources of methane and also show that methane has probably contributed, like carbon dioxide, to glacial-interglacial temperature changes. © 1990 Nature Publishing Group.

Molecules present in the atmospheres of extrasolar planets are expected to influence strongly the balance of atmospheric radiation, to trace dynamical and chemical processes, and to indicate the presence of disequilibrium effects. As molecules have the potential to reveal atmospheric conditions and chemistry, searching for them is a high priority. The rotational-vibrational transition bands of water, carbon monoxide and methane are anticipated to be the primary sources of non-continuum opacity in hot-Jupiter planets. As these bands can overlap in wavelength, and the corresponding signatures from them are weak, decisive identification requires precision infrared spectroscopy. Here we report a near-infrared transmission spectrum of the planet HD 189733b that shows the presence of methane. Additionally, a resolved water vapour band at 1.9 (micro)m confirms the recent claim4 of water in this object. On thermochemical grounds, carbon monoxide is expected to be abundant in the upper atmosphere of hot-Jupiter planets, but is not identifiable here; therefore the detection of methane rather than carbon monoxide in such a hot planet could signal the presence of a horizontal chemical gradient away from the permanent dayside, or it may imply an ill-understood photochemical mechanism that leads to an enhancement of methane.

The Palaeocene-Eocene thermal maximum (PETM), a period of intense, global warming about 55 million years ago, has been attributed to a rapid rise in greenhouse gas levels, with dissociation of methane hydrates being the most commonly invoked explanation. It has been suggested previously that high-latitude methane emissions from terrestrial environments could have enhanced the warming effect, but direct evidence for an increased methane flux from wetlands is lacking. The Cobham Lignite, a recently characterized expanded lacustrine/mire deposit in England, spans the onset of the PETM and therefore provides an opportunity to examine the biogeochemical response of wetland-type ecosystems at that time. Here we report the occurrence of hopanoids, biomarkers derived from bacteria, in the mire sediments from Cobham. We measure a decrease in the carbon isotope values of the hopanoids at the onset of the PETM interval, which suggests an increase in the methanotroph population. We propose that this reflects an increase in methane production potentially driven by changes to a warmer and wetter climate. Our data suggest that the release of methane from the terrestrial biosphere increased and possibly acted as a positive feedback mechanism to global warming.

The 5% methane present in the nitrogen-rich atmosphere of Titan, Saturn's largest moon, would disappear in tens of millions of years in the absence of a methane source to replenish it. Until the arrival of the Cassini-Huygens mission at Saturn the favoured candidate source had been liquid hydrocarbon seas hundreds of metres thick, but no trace of such seas was found. Now numerical modelling suggests that episodic outgassing of methane stored as clathrate hydrates, associated with an ammonia-rich water ocean is the most likely source, which ties in with a Cassini fly-by image showing a possible cryovolcano. Saturn's largest satellite, Titan, has a massive nitrogen atmosphere containing up to 5 per cent methane near its surface. Photochemistry in the stratosphere would remove the present-day atmospheric methane in a few tens of millions of years1. Before the Cassini-Huygens mission arrived at Saturn, widespread liquid methane or mixed hydrocarbon seas hundreds of metres in thickness were proposed as reservoirs from which methane could be resupplied to the atmosphere over geologic time2. Titan fly-by observations3,4,5 and ground-based observations6 rule out the presence of extensive bodies of liquid hydrocarbons at present, which means that methane must be derived from another source over Titan's history. Here we show that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titan's atmospheric methane. The other possible explanations all fail because they cannot explain the absence of surface liquid reservoirs and/or the low dissipative state of the interior. On the basis of our models, we predict that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.

GLOBAL surface-based measurements of atmospheric methane and carbon monoxide concentrations revealed a marked and unex-pected decrease in their growth rates in 1991 and 1992, particularly in the Northern Hemisphere1,2. Changes in emissions are unlikely to be the sole reason for the sudden reduction in the concentrations of these source gases2,3. The unprecedentedly large depletion of stratospheric ozone observed in 1991 and 1992 (ref. 4) may have contributed to the sharp decrease in the growth rates of both CH4 and CO by exposing the troposphere to more ultraviolet radiation. This would have resulted in increased concentrations of the hydroxyl radical, OH·, which is the major atmospheric sink for both CH4 and CO. Here we present two-dimensional model simulations which allow us to assess the significance of the link between stra-tospheric ozone depletion and the observed trends of CH4 and CO. We find that the low values in stratospheric ozone concentration can account for almost half of the 1992 decrease in the CH4 and CO growth rates.

SOILS currently consume about 30–40 Tg methane per year1,2, which is comparable to the net annual increase in atmospheric methane concentration from 1980 to 19903. Most soils consume methane2,4–9, but the extent varies with soil water content, land use and ammonium inputs5,10–13. Ammonium concentrations in many soils have increased in recent years as a result of land-use changes and increases in ammonium concentration in precipitation14,15. Ammonium strongly inhibits soil methane consumption, but the mechanism is uncertain. Even if enhanced ammonium concentrations are subsequently reduced, inhibition can still persist for months to years12,13. Here we show, from field and laboratory experiments, that the extent of ammonium inhibition increases with increasing methane concentration. We propose that nitrite formation from methanotrophic ammonium oxidation accounts for much of the observed inhibition, and that the persistence of inhibition with reduced ammonium concentrations is due to the limited capacity of methanotrophs to grow or recover in present concentrations of atmospheric methane. We suggest that past increases in atmospheric methane concentration may have increased the inhibitory effect of ammonium, thereby decreasing soil methane uptake capacity, and that this mechanism could also provide a positive feedback on future atmospheric methane concentrations.

The previously increasing atmospheric methane concentration has inexplicably stalled over the past three decades. This may be due to a fall in fossil-fuel emissions or to farming practices that are curtailing microbial sources. See Letters p.194 & p.198 Methane is a greenhouse gas with a significant warming effect on climate — only water vapour and carbon dioxide are more important — yet the factors influencing its atmospheric concentration are poorly understood. In particular, a rapid rise in methane levels in the mid-twentieth century gradually (but temporarily) levelled off around the turn of the millennium; the reasons for this decline in growth rate are still being debated. Two new studies shed light on this conundrum — but reach conflicting conclusions. Fuu Ming Kai et al. measure differences in the concentration and isotopic signature of methane between the Northern and Southern Hemispheres and conclude that reduced microbial activity in wetlands was primarily responsible. Changing rice agricultural practices seem to explain about half of the Northern Hemispheric trend. By contrast, Murat Aydin et al. combine measurements of ethane trapped in Antarctic ice with a simple atmospheric model and conclude that the slow-down was caused by reduced methane emissions from fossil-fuel production. In News and Views, Martin Heimann discusses the differing findings of these two studies.