Scientists from NTU have recently discovered that rising global temperatures – due to global warming – are having a greater influence on increasing methane levels, which in turn lead to further climate warming.
In a study recently published in Nature Communications, lead author and Dean of NTU’s College of Science, Professor Simon Redfern, and first author and researcher at the NTU Asian School of the Environment (ASE), Cheng Chin-Hsien, found that the average value of methane-climate feedback sensitivity that was reported by the latest Intergovernmental Panel on Climate Change Sixth Assessment Report (IPCC AR6) is most likely an underestimate and might not fully represent the links between methane levels and climate change.
This means two things: one, an already warming Earth is warming at a greater pace than once understood; and two, current policies and plans to curb the effects of global warming are, in fact, woefully inadequate.
Intergovernmental Panel on Climate Change Assessment Report
Every few years, the United Nation’s Intergovernmental Panel on Climate Change (IPCC) releases a series of reports collectively known as Assessment Reports (AR). The ARs review the latest data on climate research, as well as associated technical and socio-economic data, and are prepared by teams of researchers that have been selected by the IPCC based on various governments’ nominations. The reports are then further analysed by other governmental reviewers and organisations.
The current series of reports, the Sixth Assessment Report (AR6), is being released over several stages from 2021 to 2022. Amongst the many climate change issues discussed are the rising levels and effects of greenhouse gases, one of which is methane.
Methane Gas
Methane gas is the second most impactful greenhouse gas in the atmosphere behind carbon dioxide. It was suggested in the IPCC AR6 that methane was the cause behind about one-third of global warming related to human activities.
Unlike carbon dioxide, which can persist in the atmosphere for a couple of centuries, methane gas only lasts for about a decade before it breaks down. However, despite that relatively short lifespan, methane is actually one of the most efficient greenhouse gases at trapping heat. Over a 20-year period, methane can be up to 84-86 times more potent at causing global warming than carbon dioxide.
In the long run, carbon dioxide has a longer lasting warming effect, but methane gas dominates global warming in the short term.
From the 1980s to the early 2000s, methane levels experienced a decrease in growth rate. However, they started on a steady climb from 2007, which accelerated immensely from 2014 onwards. Despite the COVID-19 pandemic in 2020 and 2021 putting a temporary halt to industries around the world, both years still saw record levels of methane gas emissions.
Methane’s potency as a trapper of heat, and its accelerating global levels, means that there is an increasing need to address its emission levels. In order to set effective emission targets, scientists have to be able to estimate two things as accurately as possible: the warming effects of methane gas in the atmosphere as well as how much atmospheric methane concentrations are affected by climate changes. However, new revelations in the study of methane-climate feedback sensitivity are going to show that urgent changes in these emission targets are much needed.
Methane-Climate Feedback Sensitivity
Methane comes from human activities, like agriculture and fossil fuel extraction, and natural sources, like wetlands, but the bulk of the gas originates from human activities.
When methane is released into the atmosphere, it is also concurrently oxidized by hydroxyl (OH) radicals, which are molecules made up of a negatively charged oxygen atom and a hydrogen atom. The unpaired electrons in these OH radicals constantly seek to transfer to other molecules, allowing these radicals to react with methane and other air pollutants. Described as the “detergent” of the atmosphere for their ability to remove air pollutants, OH radicals oxidize methane gas, producing water vapour and carbon dioxide in the process.
However, changes in climate conditions can drastically affect methane emissions and removal. For instance, more precipitation can cause changes in soil pH levels and temperature, both of which can influence the activities of micro-organisms in the soil, thus affecting methane emissions. OH radicals are highly reactive in nature and their lifespans are therefore very short. As such, their atmospheric concentrations are also highly susceptible to changes in climate conditions, complicating the methane removal process.
For scientists to measure just how sensitive the bi-directional influence is between global heating and methane levels, they need to know two values, and here we get rather technical. The first is how much additional methane is being introduced into the atmosphere in a net amount (where the amount of emissions is subtracted by the amount removed) per degree of warming. This is measured by ppb °C-1 (where ppb refers to “parts per billion”).
The second value is how much incremental heat the Earth will absorb due to this additional amount of methane. This is measured in watts of additional heat per square metre of Earth’s surface per ppb of incremental methane level, or more simply as W m-2 ppb-1. Combining the two measurements together, scientist thus measure “methane-climate feedback sensitivity” in the form of W m-2 C-1.
The recent IPCC AR6 estimated that Earth’s methane-climate sensitivity to be around 0.02 W m-2 C-1, which means that Earth is absorbing an additional 0.02 watts of additional heat per square metre of surface for every degree increase in global mean surface air temperature due to the additional climate-driven methane. However, the team from NTU have found that this figure is likely to be a severe underestimate. Based on their examination of over four decades of data, their calculations suggest that methane-climate sensitivity is about four times greater, with Earth absorbing about 0.08 watts per square metre instead.
Delayed Responses in Methane-Climate Feedback
According to the NTU scientists, this difference in climate sensitivity values can be attributed to non-linearly delayed responses present in nature. In an immensely complex system such as our climate, there can be innumerable interactions and effects between multiple different factors and feedbacks, and their relationships with each other are not as neat or simple as we might think.
While some effects might make themselves clear to us in the short term, some other effects might take years or decades to surface. As Mr Cheng puts it, “the recent sudden surge in methane emissions and the increase in warming could be a result of climate change years or decades ago.”
He further explains, “while the IPCC suggests an accelerated methane emissions and removal with global warming, we recommend a more complete story over various time scales: during hotter years, the effect of accelerated methane emissions is masked by the accelerated methane removal; but during subsequent cooler years, especially if there were severe wildfires in previous hotter years, methane removal is substantially slowed down, thus extending the lifespan of atmospheric methane.”
“This is like unmasking the additional methane emissions earlier. There are also similar oscillations over multidecadal timescales, which are driven by other underlying processes. After considering the unmasked and delayed emissions, we get a much higher methane-climate feedback sensitivity.”
Future Implications
This discovery of higher methane-climate feedback sensitivity has shown that we still have so much more to learn if we are to thoroughly understand our climate and its potential future course. We have yet to fully comprehend all the consequences of human activities, and there is a pressing need to develop a deeper understanding of them if we are to better project future climate change.
Not only does this new information continues to emphasise our constant need to address and limit our greenhouse gas emissions, but it also highlights the greater sense of urgency with which we need to act. We will have to revise plans and policies regarding emissions and climate change targets. Ultimately, this is a greater warning that our Earth is not only warming up but doing so at a much faster rate than we once believed.