Scientists raise alarm over
‘dangerously fast’ growth in atmospheric methane
As global methane concentrations soar over 1,900 parts per billion,
some researchers fear
that global warming itself is behind the rapid rise.
Tropical wetlands, such as the Pantanal in
Brazil, are a major source of methane emissions.
Credit: Carl De Souza/AFP via Getty
Methane concentrations in
the atmosphere raced past 1,900 parts per billion last year, nearly
triple preindustrial levels, according to
data released in January by the US National Oceanic and
Atmospheric Administration (NOAA). Scientists says the grim milestone
underscores the importance of a pledge made at last year’s COP26
climate summit to curb emissions of methane, a greenhouse gas at least
28 times as potent as CO2.
The growth of methane
emissions slowed around the turn of the millennium, but began a rapid
and mysterious uptick around 2007. The spike has caused many
researchers to worry that global warming is creating a feedback
mechanism that will cause ever more methane to be released, making it
even harder to rein in rising temperatures.
“Methane levels are growing
dangerously fast,” says Euan Nisbet, an Earth scientist at Royal
Holloway, University of London, in Egham, UK. The emissions, which
seem to have accelerated in the past few years, are a major threat to
the world’s goal of limiting global warming to 1.5–2 °C over
pre-industrial temperatures, he says.
Source:
NOAA
Enigmatic patterns
For more than a decade,
researchers have deployed aircraft, taken satellite measurements and
run models in an effort to understand the drivers of the increase (see
‘A worrying trend’)1,2.
Potential explanations range from the expanding exploitation of oil
and natural gas and rising emissions from landfill to growing
livestock herds and increasing activity by microbes in wetlands3.
“The causes of the methane
trends have indeed proved rather enigmatic,” says Alex Turner, an
atmospheric chemist at the University of Washington in Seattle. And
despite a flurry of research, Turner says he is yet to see any
conclusive answers emerge.
One clue is in the isotopic
signature of methane molecules. The majority of carbon is carbon-12,
but methane molecules sometimes also contain the heavier isotope
carbon-13. Methane generated by microbes — after they consume carbon
in the mud of a wetland or in the gut of a cow, for instance —
contains less 13C than does methane generated by heat and
pressure inside Earth, which is released during fossil-fuel
extraction.
Scientists have sought to
understand the source of the mystery methane by comparing this
knowledge about the production of the gas with what is observed in the
atmosphere.
By studying methane trapped
decades or centuries ago in ice cores and accumulated snow, as well as
gas in the atmosphere, they have been able to show that for two
centuries after the start of the Industrial Revolution the proportion
of methane containing 13C increased4.
But since 2007, when methane levels began to rise more rapidly again,
the proportion of methane containing 13C began to fall (see
‘The rise and fall of methane’). Some researchers believe that this
suggests that much of the increase in the past 15 years might be due
to microbial sources, rather than the extraction of fossil fuels.
Source: Sylvia
Michel, University of Colorado Institute of Arctic and Alpine Research
Back to the source
“It’s a powerful signal,”
says Xin Lan, an atmospheric scientist at NOAA’s Global Monitoring
Laboratory in Boulder, Colorado, and it suggests that human activities
alone are not responsible for the increase. Lan’s team has used the
atmospheric 13C data to estimate that microbes are
responsible for around 85% of the growth in emissions since 2007, with
fossil-fuel extraction accounting for the remainder5.
The next — and most
challenging — step is to try to pin down the relative contributions of
microbes from various systems, such as natural wetlands or
human-raised livestock and landfills. This may help determine whether
warming itself is contributing to the increase, potentially via
mechanisms such as increasing the productivity of tropical wetlands.
To provide answers, Lan and her team are running atmospheric models to
trace methane back to its source.
“Is warming feeding the
warming? It’s an incredibly important question,” says Nisbet. “As yet,
no answer, but it very much looks that way.”
Regardless of how this
mystery plays out, humans are not off the hook. Based on their latest
analysis of the isotopic trends, Lan’s team estimates that
anthropogenic sources such as livestock, agricultural waste, landfill
and fossil-fuel extraction accounted for about 62% of total methane
emissions since from 2007 to 2016 (see ‘Where is methane coming
from?’).
Global Methane Pledge
This means there is plenty
that can be done to reduce emissions. Despite NOAA’s worrying numbers
for 2021, scientists already have the knowledge to help governments
take action, says Riley Duren, who leads Carbon Mapper, a non-profit
consortium in Pasadena, California, that uses satellites to pinpoint
the source of methane emissions.
Last month, for instance,
Carbon Mapper and the Environmental Defense Fund, an advocacy group in
New York City, released data revealing that 30 oil and gas facilities
in the southwestern United States have collectively emitted about
100,000 tonnes of methane for at least the past three years,
equivalent to the annual warming impact of half a million cars. These
facilities could easily halt those emissions by preventing methane
from leaking out, the groups argue.
At COP26 in Glasgow, UK,
more than 100 countries signed the
Global Methane Pledge to cut emissions by 30% from 2020 levels by
2030, and Duren says the emphasis must now be on action, including in
low- and middle-income countries across the global south. “Tackling
methane is probably the best opportunity we have to buy some time”, he
says, to solve the much bigger challenge of reducing the world’s CO2
emissions.
