Recent trends in atmospheric methane.

A guest post from Jim

Over the last year, the global increases in methane emissions have begun to raise alarms.

Here's a brief sample:

• Sarah Kaplan's report in the Washington Post, reporting 10 Sept 2024, "Emissions of methane — a powerful greenhouse gas — are rising at the fastest rate in recorded history, scientists said Tuesday, defying global pledges to limit the gas and putting the Earth on a path toward perilous temperature rise."
• The European Space Agency's post, also on 10 Sept 2024, "The Global Methane Budget 2024 paints a troubling picture of the current state of global methane emissions. The new report reveals that human activities are now responsible for at least two-thirds of global methane emissions. This marks a significant increase in human-produced methane sources over the past two decades, with emissions rising by 20%, with the fastest rise occurring over the last five years."
• NOAA's summary of 5 Apr 2024, No sign of greenhouse gases increases slowing in 2023, reports that "Atmospheric methane, less abundant than CO2 but more potent at trapping heat in the atmosphere, rose to an average of 1922.6 parts per billion (ppb). The 2023 methane increase over 2022 was 10.9 ppb, lower than the record growth rates seen in 2020 (15.2 ppb), 2021(18 ppb)  and 2022 (13.2 ppb), but still the 5th highest since renewed methane growth started in 2007. Methane levels in the atmosphere are now more than 160% higher than their pre-industrial level,"

Climate bloggers have begun to notice. For example, here's David Appell at Quark Soup: 

"I haven't followed methane much because I couldn't find a good data source, but now I have, from NASA. And even better source is the Global Carbon Project, which is updated every 7.6 days. 

After that weird lull in the mid-aughts, methane is on the rise again and is the highest it's been in 800,000 years.   

Methane's radiative forcing has increased by about 0.4 W/m^2 since 1979, while CO2's has jumped about 1.6 W/m^2 in the same interval...

It's incredible that the world, despite all the rhetoric and (token) efforts, keeps allowing this to happen. Clearly, I think, these trends will only be taken seriously once some catastrophic effects happen, and by then it will be too late. So human and we can't even help ourselves."

The Global Carbon Project's methane budget links to recent publications:

• " The primary reference ...(pre-print) Global Methane Budget 2000–2020 Here's an excerpt from the abstract: "Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Emissions and atmospheric concentrations of CH4 continue to increase, maintaining CH4 as the second most important human-influenced greenhouse gas in terms of climate forcing after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 for temperature change is related to its shorter atmospheric lifetime, stronger radiative effect, and acceleration in atmospheric growth rate over the past decade, the causes of which are still debated. "
• Additional Analyses for Methane Budget 2024:Human activities now fuel two-thirds of global methane emissions. "Methane is rising faster in relative terms than any major greenhouse gas and is now 2.6-fold higher than in pre-industrial times. Global average methane concentrations reached 1931 parts per billion (ppb) in January of 2024 (Lan et al 2024). Annual increases in methane are also accelerating for reasons that are debated..." 

With these in mind, let's look at recent trends in methane from NOAA's Global Monitoring Laboratory

Here is a graph of global methane (as ppm CO₂ equivalent) since 2000.

We can remove the monthly variations to look at the underlying trend by doing a moving twelve-month average. Most of the variations have been averaged out. Here's the result:

 

A glimpse at the raw data reveals that the annual rate of change of global atmospheric methane has been monotonically positive since 2005. These data are well-approximated by a quadratic polynomial. The error in the quadratic fit to the twelve month moving average of the observed data for atmospheric methane (in ppm CO₂ eq) ranges from -0.140411 ppm to 0.113803 ppm with an rms error 0.0665136 ppm, a percent error less than 0.21%.

This graph shows the raw methane as ppm CO₂ equivalent (in black), its twelve-month moving average (in yellow), and the moving average's best fit quadratic (in blue). 

Removal of the seasonal variation in methane does not eliminate all of the short term departures from the best fit quadratic. The remaining fluctuations appear to have a period of about seven years. It is not clear what is the underlying physical processes responsible for this variation but it appears quite regular.

Seasonal variations, in methane, removed by the moving average, account for a difference of about 0.14 ppm CO₂ eq, less than 0.4% of the annual average. Long-term variations in the moving average methane, compared to the quadratic fit, account for another difference of about 0.067 ppm CO₂ eq, less than 0.2% of the annual average. Seasonal variations account for about 80% of the total variance between the quadratic model and the observed data.

In summary, the increase in methane is well approximated by a quadratic fit to the twelve-month moving average of emissions; we project that methane emissions will rise from 33.08 ppm CO₂ eq in 2005 to about 37.37 ppm CO₂ eq in 2025, an increase of 12.9% over only two decades. 

The International Energy Agency writes,"Methane has a much shorter atmospheric lifetime than CO2 (around 12 years compared with centuries for CO2), but it is a much more potent greenhouse gas, absorbing much more energy while it exists in the atmosphere...The Intergovernmental Panel on Climate Change (IPCC) has indicated a GWP for methane between 84-87 when considering its impact over a 20-year timeframe (GWP20) and between 28-36 when considering its impact over a 100-year timeframe (GWP100). This means that one tonne of methane can considered to be equivalent to 28 to 36 tonnes of CO2 if looking at its impact over 100 years."

Methane emissions are currently increasing annually by a relative rate of about 0.95%, clearly a serious situation.