Will Global Warming Cause Runaway Methane Emissions?
What’s the likelihood of a devastating climate feedback triggered by global warming and methane hydrate destabilisation?
In order to limit global mean temperature rise to well below 2°C, as recommended by the Paris Agreement, we must keep greenhouse gas emissions within a finite carbon budget.
However, calculating this budget is complex — in part due to the unknown impact of climate feedbacks: processes that amplify or dampen the effect of radiative forcing on surface temperatures (Loeb, Su and Kato, 2016).
One such feedback is the interaction between global warming and methane release from the destabilisation of methane hydrate. Methane hydrate is a naturally occurring form of ice that holds high concentrations of methane within its crystalline structure; 1 m³ of methane hydrate can sequester up to 180 m³ of methane (Ruppel and Kessler, 2017).
Palaeoceanographic evidence has attributed phases of rapid warming to the climate-triggered release of methane from hydrate destabilisation (de Garidel-Thoron et al, 2004). Though this attribution has been contested in more recent studies (Bock et al., 2010), scrutiny of methane hydrate reservoirs remains high, due to the extremely potent greenhouse effect of methane — 84 times more potent than CO₂ over a 20-year timeframe (IPCC, 2013).
Some projections of a methane hydrate destabilisation feedback forecast catastrophic runaway warming that could melt the Earth’s cryosphere within a generation (Kennedy et al, 2008). But what is the likelihood of a devastating methane hydrate destabilisation feedback?
Methane hydrate forms when gaseous methane — produced by the microbial decomposition of organic matter — is trapped by lattice-like water molecule structures (clathrates) under moderate pressure and low temperature conditions (Ruppel and Kessler, 2017).
Consequently, ~99% of methane hydrates are found underwater on continental slopes (where the deepening seafloor meets continental land); the remaining 1% occurs within northern high-latitude permafrost (frozen Arctic soil).
High temperatures and/or low pressure cause methane hydrates to destabilise and release methane gas.
Anthropogenic global warming increases ocean temperatures, increasing the risk that methane hydrates will destabilise and release large concentrations of methane into the atmosphere. But how much methane hydrate is there in the world, and how much methane could be released by anthropogenic oceanic warming?
According to the IPCC’s most recent assessment report, the total annual flux of methane is approximately 555 Tg yr^−1 (Kirschke et al, 2013). The global hydrate reservoir is estimated at 0.24–1.2 million cubic miles (Milkov, 2004), and thought to sequester around 15% of the world’s total exogenic carbon through an estimated 3 × 1015 m³ of methane (Boswell and Collett, 2011). Even if only ~0.1% of this gas was suddenly released into the atmosphere, atmospheric methane concentration would rise more than 60% (Ruppel, 2011).
Furthermore, the change in radiative forcing induced by increasing atmospheric methane concentrations would reduce the amount of infrared radiation escaping the Earth’s atmosphere, causing yet more warming and methane release through hydrate destabilisation. This amplifying effect on warming is the methane hydrate feedback.
Source: Andrew Melo, 2012.
It is clear that anthropogenic climate change is increasing ocean temperatures (IPCC, 2013). However, global warming simultaneously increases oceanic pressure, which stabilises methane hydrates. Oceanic thermal heat expansion and melting ice sheets cause sea levels to rise, and as sea levels rise by 1 m, pressure increases by ~0.01 MPa (Ruppel and Kessler, 2017). The stabilising effect of increasing pressure is likely to mitigate the destabilising effect of increased temperatures for many centuries before being outweighed (Ruppel, 2011).
Even when hydrates do destabilise and release methane, the gas bubbles have to travel long distances through marine sediments and water columns before reaching the atmosphere. Along the way, they can be broken down and sequestered by a number of biological, chemical and physical sinks.
Bacteria within marine sediments decompose methane through anaerobic oxidation, using seawater sulphates and calcium ions to form hydrogen sulphides and calcium carbonate, which is stable and does not risk releasing further methane (Knittel and Boetius, 2009).
Methane bubbles that pass through marine sediments and reach the water column unscathed are likely to be decomposed by microbial aerobic oxidation. Microbes use oxygen to break down methane, producing carbon dioxide which dissolves in seawater. While this prevents methane from reaching the atmosphere, it exacerbates ocean acidification, another impact of anthropogenic climate change, and depletes oceanic oxygen content for marine life. Nonetheless, 80–90% of released methane is prevented from reaching the atmosphere by these oxidation processes (Reeburgh, 2007).
These oceanographic properties (methane sinks and the pressure-temperature synergy) mean that runaway warming scenarios in response to methane hydrate destabilisation are likely overestimated (Ruppel and Kessler, 2017). Furthermore, Circone et. al. (2005) assert that methane hydrate destabilisation is self-regulating, due to the endothermic heat of reaction of methane, making runaway warming even less likely.
Though paleoclimatic evidence has attributed extreme temporary warming phases within glacial periods to a hydrate destabilisation feedback (de Garidel-Thoron et al, 2004), recent studies using climate models in tandem with ice core records indicate that warming was caused by a combination of emissions events, including methane emissions from wetlands (Carozza et al., 2011; Bock et al., 2010).
Catastrophic forecasts of a runaway methane hydrate destabilisation feedback may seem appropriate due to methane’s high-potency as a greenhouse gas, however, such scenarios may fail to account for mitigating effects which are likely to consistently prevent large quantities of hydrate-derived methane from reaching the atmosphere and significantly affecting carbon budgets.
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