Atmospheric Sciences Research Spotlight

Using Isotope Fingerprints to Solve a Methane Mystery

Atmospheric methane levels are rising, and isotopic ratios within the greenhouse gas suggest that the tropics may be to blame.

Source: Global Biogeochemical Cycles


Although methane decays more quickly in the atmosphere than carbon dioxide does, it rapidly absorbs heat and is a potent greenhouse gas.  In 2007, after nearly 2 decades of slowed growth in atmospheric methane levels, the amount of this gas in the air began to rapidly rise.

Historically, rises in methane levels have been driven by fossil fuel leaks. However, earlier this year, researchers at the National Institute of Water and Atmospheric Research in New Zealand found that biogenic sources, which they thought were from agriculture, were the primary cause of the uptick.

Here Nisbet et al. discuss the rise, with new data on the concentrations and isotopic makeup of atmospheric methane. They also conducted a detailed mathematical analysis that allowed them to evaluate methane emissions on shorter timescales and differentiate between the latitudes of emissions.

Specifically, their inquiry delved into the rate of methane flux. Data collected by National Oceanic and Atmospheric Administration’s Cooperative Air Sampling Network show that atmospheric methane increased by 5.7 ± 1.2 parts per billion (ppb) every year between 2007 and 2013; then, in 2014, methane levels jumped up by 12.5 ± 0.4 ppb. Was the jump caused by an increase in emissions or a decrease in methane removal rates?

Researchers work to pinpoint possible sources of atmospheric methane, including papyrus swamps
A papyrus swamp at Kajjansi, Uganda, just a few kilometers north of the equator. The swamp is very soggy indeed—step into it, and you sink! The papyrus is about 3 meters high. Swamps like these may be fueling a recent uptick in methane levels, new research reports. Credit: Euan Nisbet

Over the study period, the amount of carbon-13 (13C) in atmospheric methane decreased compared to the amount of carbon-12 (12C). Different sources of atmospheric methane have distinctive ratios of 13C to 12C, depending on where the methane came from. Since both are stable isotopes of carbon, the team took advantage of the fact that the isotopic ratio in a sample of methane is essentially a fingerprint of the gas’s source. By studying the amount of 13C in methane samples from the Arctic, the tropics, and the Southern Hemisphere, the authors were able to eliminate certain potential sources as the primary cause for the increasing amounts of methane.

Emissions from fracking in the United States or coalfields in China, for example, don’t match the isotopic fingerprint of the atmospheric methane samples: Methane emissions from gas and coalfields are typically richer in carbon-13 than the samples collected throughout the study, indicating that fossil fuel emissions are not the main driver of the recent increase.

The team also found little evidence that changes in methane removal rates could explain the increase, but the isotopic evidence reported in the study did provide strong evidence to support a biogenic, rather than anthropogenic, cause for the rising methane levels. However, although previous research, which did not include latitudinal analysis, suggested that agriculture was responsible for much of the rise, here the scientists found that tropical latitudes played a major role in the growth and pointed to tropical wetlands, which are the dominant low-latitude source of methane. To point definitely to the sources, more work is needed to determine the carbon isotopic signatures of methane from both tropical cows and wetlands.  Although wetlands may be emitting a larger share of the new methane than agriculture, it is likely that both are contributing to the rise, in response to recent intense weather changes.

Tropical wetlands are extremely sensitive to climate variations, meaning that changes in temperature or precipitation rapidly translate to changes in methane emissions. Warmer or wetter years mean more emissions. Over the study period, rising temperatures, increased precipitation, and frequent flooding increased methane emissions in many tropical regions.

Because methane levels continue to rise at nearly unprecedented rates, the findings have important implications for policy makers around the globe. Although the growth is biogenic, fossil fuel leaks remain a major source. Control of methane emissions from the oil and gas industries is becoming stricter, but much more can be done worldwide to cut methane from gas and coal use. Stifling emissions from biogenic sources like cows will require much more creative solutions from governments. Most worryingly, if wetlands are primarily responsible for the new growth, is this the start of a strong tropical climate change warming feedback? (Global Biogeochemical Cycles, doi:10.1002/2016GB005406, 2016)

—Kate Wheeling, Freelance Writer

Citation: Wheeling, K. (2016), Using isotope fingerprints to solve a methane mystery, Eos, 97, doi:10.1029/2016EO065017. Published on 16 December 2016.
© 2016. The authors. CC BY-NC-ND 3.0
  • neubarth

    davidlaing You probably know this already but it is hard to get straight answers to your question. Supposedly according to the IPCC, CH4 hasa Global Warming Potential 21 to 25 times that of CO2 after the methane has been in the atmosphere. Another number that is quoted is that CH4 has 84 to 85 the GWP of CO2 after it has been in the atmosphere for 25 years.

    Very strange, isn’t it when you consider that methane supposedly lasts in the atmosphere for 9 to 12 years after it first shows up in the atmosphere. So how can Methane have a GWP of 85, TEN YEARS after it is gone?

    What we need is an agreed upon standard for the first five years a greenhouse gas is in the atmosphere. I was told that a good estimate would be 150 for the first five years compared to the GWP of CO2. I was a history major in college, so I will take what I am told. Unfortunately, not everybody can agree on the numbers.

  • stashgal

    If we don’t STOP POPULATION GROWTH & REVERSE IT, nothing else we do will matter.
    OVERPOPULATION is our greatest problem!

  • davidlaing

    It would be useful to know from actual data, not just from theoretical calculations, just how much warming, if any, is due to an increase in the concentration of methane in the atmosphere. Does anyone happen to know of such a data-based study?

  • neubarth

    How flacking stupid can these people be? All you have to do is look at the satellite data to see the massive emissions of Methane from the Arctic and from shallow waters elsewhere, such as the Bay of Bengal. The Megenta (pink) is in the 2000 + ppb range. Now, if agriculture is raising most of their cows in the Arctic and in the Bay of Bengal that might explain why all that methane is originating there, otherwise this research is bogus as long as those cows are in the picture. The methane in the Arctic is of Biological origin, but cows are not the source.