How much methane removal is required to avoid overshooting 1.5°C?

[Geoengineering News]

https://iopscience.iop.org/article/10.1088/1748-9326/ad5853/meta

Authors

Christopher J Smith and Camilla Mathison

Accepted Manuscript online 14 June 2024

DOI 10.1088/1748-9326/ad5853

Abstract

Methane is the second most important anthropogenic greenhouse gas after carbon dioxide. With an atmospheric lifetime of around a decade, methane mitigation starting immediately has the potential to avoid substantial levels of additional warming by mid-century. In addition to the methane emissions reductions that are necessary to limit warming, we address the question of whether technological methane removal can provide additional benefits by avoiding global mean surface temperatures exceeding 1.5°C above pre-industrial---the high-ambition Paris Agreement climate goal. Using an adaptive emissions methane removal routine in a simple climate model, we successfully limit peak warming to 1.5°C for overshoots of up to around 0.3°C. For substantially higher overshoots, methane removal alone is unable to limit warming to 1.5°C, but in an extreme scenario could limit peak warming by an ensemble median 0.7°C if all atmospheric methane was removed, requiring huge levels of net removal on the order of tens of petagrams cumulatively. The efficacy of methane removal depends on many emergent properties of the climate system, including climate sensitivity, aerosol forcing, and the committed warming after net zero CO$_2$ (zero emissions commitment). To avoid overshooting 1.5°C in the low-overshoot, strong-mitigation SSP1-1.9 scenario, a median cumulative methane removal of 1.2 Pg CH$_4$ is required, though this may be much higher if climate sensitivity is high or the zero emissions commitment is positive, and in these cases may require ongoing methane removal long after peak warming in order to stabilise warming below 1.5°C.

Source: IOP Science 

[H simmens]

Are there papers that explore whether there would be sufficient political, social, material and  economic stability in the world with an overshoot of .3° C to successfully remove massive amounts of methane or for that matter CO2? 

Or does the scenario and model developing community only look at the results of removal and not the nature of the inputs required to achieve a given removal goal? 

Herb

Herb Simmens
Author of A Climate Vocabulary of the Future

“A SciencePoem and an Inspiration.” Kim Stanley Robinson
@herbsimmens
HerbSimmens.com

On Jun 21, 2024, at 6:26 AM, Geoengineering News <geoengine...@gmail.com> wrote:



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[Clive Elsworth]

Herb

 

I find it hard to say how much the temperature would change to a change of atmospheric methane concentration. It depends on so much else, like other GHG concentrations and aerosols, which also provide a climate forcing. Also, Mann and Hansen’s disagreement adds further (long term) uncertainty.

 

It’s (something like) we’re in a car and atmospheric methane is a turbocharger. But we don’t know if we’ll take our foot off the gas, or how quickly we’ll slow down if we do, and that’s partly because we don’t know if we’re going downhill yet or not, or if there’s a cliff edge ahead that we’re about to drive off (cascading tipping points) – and if there is, how far off that is.

 

It’s easier to think in terms of climate forcings. Force produces acceleration (or deceleration by force of braking). Forster (and IPPC) say the additional methane since preindustrial produces a forcing of 0.56 W/m2. The current energy imbalance (according to Hansen) is about 1.3 W/m2. Halving methane by strengthening the atmosphere’s oxidative capacity (AOC) would reduce its forcing to around 0.28 W/m2. That’s a weak cooling compared to MCB, which according to Alan Gadian if done to extreme could induce another ice age. Given that the forcing since preindustrial is about 3 W/m2, Gadian’s extreme MCB would need to reverse the current energy imbalance to at least -1.7 W/m2. That would produce rapid cooling, but the oceans would still take decades to cool back down to preindustrial. Given that West Antarctic ‘passed its point of no return’ in 2014 we might need to go quite a bit cooler than preindustrial to stabilize it, and by inference, sea level.

 

SAI could also do it, but as you know Franz and I prefer MCB because it also strengthens the AOC, instead of weakening it. Franz thinks SAI would produce a significant AOC weakening, so methane would build up faster than it does today. Others have said SAI would not affect the AOC much.

 

That’s how I understand it.

 

Clive

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[Michael MacCracken]

Hi Herb--I'm afraid it is the latter of your two options for virtually every paper.

I would note that on the methane reduction approach, the iron salt aerosol proponents do offer a cost estimate of what it would take and there is the announcement a Swiss company that it is going to make a go at it (see https://amr.earth).  Note that they are a company, and they have the view they will have more latitude to proceed than other entities such as academic or international organizations.

Best, Mike

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