Why are we considering atmospheric methane removal?
Driven by anthropogenic climate change, Earth’s current average temperature has increased by 1.1°C since 1850. Already, we are witnessing severe ecosystem impacts, driving a sixth mass extinction event and increasingly severe and unpredictable weather events, threatening biodiversity, food security, and societal stability.
Temperatures will unfortunately continue to increase while we race to decarbonize, remove carbon dioxide, and rapidly cut methane and other greenhouse gas emissions. Temperatures will only stabilize if we achieve net-zero carbon dioxide emissions, and significantly lower short-lived climate pollutant (largely methane) emissions before natural systems are triggered to emit substantially more greenhouse gas. Current policies and action are projected to put us on a trajectory towards approximately 2.7°C of warming by 2100, but with uncertainty between 2.1°C and 3.5°C. Climate stabilization requires rapid, sustained action on all fronts; methane removal can only lower peak temperature if other mitigation measures are implemented such that a peak occurs.
Our understanding of climate feedbacks and tipping elements has evolved to highlight that there are higher earth system risks that we face at lower temperatures than previously understood — unfortunately, as our proximity to these tipping elements has also increased from ongoing emissions and warming. Increasing global and regional temperatures increase these risks; every fraction of a degree matters in lowering these risks. It’s important to do everything we can to prevent triggering these feedbacks, which will add to the irreversible damage caused, and make returning to safe temperatures an even larger challenge.
Atmospheric methane removal is a new field that explores whether there are ways to draw down methane once it is already in the atmosphere, mainly by enhancing methane’s natural removal processes. It could potentially play two important roles: (1) to address historical methane emissions in order to reduce near-term warming, lower peak temperature, and reduce natural feedback and tipping element risk, and (2) as a potential partial response to methane-emitting natural feedback loops and tipping elements to reduce how much these systems further accelerate warming. Atmospheric methane removal’s ability to accomplish these objectives will depend on what approaches prove feasible, how quickly and broadly they may scale, and their inherent impact tradeoffs.
Methane is a potent greenhouse gas, 43x stronger than CO2 molecule for molecule (pg 204, Table 7.15), with an atmospheric lifetime of roughly a decade. However, more methane is currently being emitted than its natural sinks can remove, so methane is accumulating in the atmosphere. This increase in atmospheric methane concentrations has caused an increase of 0.5°C in recent global average temperatures. Climate models depend on dramatic reductions in atmospheric methane levels—driven by reductions to methane emissions—in all scenarios where global warming stays below 2°C. Unfortunately, methane concentrations are not only rising, but accelerating, and there are early signs that part of this trend is driven by elevated natural methane emissions that are largely outside of our control, alongside the more addressable elevated anthropogenic emissions. Atmospheric methane removal approaches could be additional tools to deploy—in parallel with all other mitigation approaches—in order to address some of the 0.5°C and rising of methane-attributed warming from historical emissions, thereby helping to slow the rate of near-term warming, reduce peak temperatures, and lower the risk of natural feedbacks and tipping elements.
As the planet warms, certain climate feedbacks and tipping elements will result in increased greenhouse gas emissions. These include methane emitted from abrupt permafrost thaw and wetlands. Natural feedbacks and tipping elements have wide-ranging impacts beyond methane emissions, and should be avoided as much as possible for those reasons as well.
Abrupt permafrost thaw dynamics include rapid erosion, depressions from sinking land known as thermokarsts, and other similar phenomena. Abrupt thaw is expected to occur when global temperature passes a threshold around 1.5°C (likely between 1.0-2.3°C). While there’s wide variation in estimates of impacts, recent research suggests that methane emissions may increase by around 40 Mt/yr (with substantial uncertainty). This could add approximately 0.08°C to global average temperatures (and possibly more depending on the ratio of carbon dioxide to methane emitted).
Wetlands are expected to produce more methane due to higher temperatures and changing precipitation patterns which expand wetland areas and increase the activity of methane-producing microbes. There’s early evidence that this is already starting to happen. These rising natural emissions are currently under-represented in IPCC climate models, and could lead to 100+ Mt/yr of additional methane emissions within the next few decades, which would add approximately 0.2°C to global average temperatures.
Both wetlands and permafrost may emit far more methane, causing additional warming, human health, and ecosystem risks. Atmospheric methane removal may be able to reduce some of the warming , helping to lower global temperatures, reduce overall climate impacts, and lower the risk of additional tipping elements and natural feedbacks. Reducing overall temperatures with all available means remains absolutely crucial, including scaling up carbon dioxide removal in order to address elevated carbon dioxide emissions from other earth system feedbacks.