As Dave Frame attempts to explain the intricacies of how methane warms the planet, he seems exasperated. Frame is one of the country’s leading climate scientists. He’s been an author for the Intergovernmental Panel on Climate Change (IPCC), a director of Victoria University’s Climate Change Research Institute, and is now a physics professor at the University of Canterbury.

For more than a decade, he’s been trying to explain to New Zealanders that most of us are looking at our methane emissions all wrong. But trying to get New Zealand politicians, public servants, journalists and the public at large to understand the gas seems to be a never ending task.

“I feel like we’ve been through this so many times,” he says. “The logic keeps being the same and every time we have the conversation people go ‘yeah, I suppose that’s right,’ but boy is it painful.” To be fair to the public, trying to understand it can be painful too.

For Frame and many other climate scientists, methane isn’t exactly the global warming bogeyman that many believe — at least not when compared to carbon emissions. While it is an incredibly potent greenhouse gas, its short lifespan means its impact on the climate is vastly different to CO2.

Essentially, CO2 emissions last for a millennia or more and this means that ongoing CO2 emissions keep accumulating in the atmosphere indefinitely, heating the world more and more. Methane, however, lasts in the atmosphere for about a decade on average and is almost entirely gone from the atmosphere within about 50 years, although a small amount of warming persists for more than a century.

This short lifespan means methane emissions don’t accumulate indefinitely like CO2. On the other hand, methane is extremely powerful while it persists in the atmosphere, with a warming effect about 80 times greater than CO2 over its first 20 years.

To use an analogy, CO2 emissions are a bit like filling up the bath with the tap on low and the plug in. Every drop of water keeps slowly increasing the bath level (and the temperature). You can turn the tap up and down (or increase or reduce emissions) but every drop is still raising the level of the bath.

The effect of methane emissions is more like turning the tap on high with the plug out. The water level rises fast, but then stabilises as the amount going down the plughole matches the water from the tap (as methane breaks down). Turning up the tap (or increasing emissions) can still raise the bath level, but turning down the tap (or reducing emissions) results in the water level (the temperature) subsiding.

So while methane still warms the planet, ongoing emissions don’t accumulate and increase the temperature indefinitely like they do with CO2. The rate of decay means that methane emissions need to fall by about 0.3 per cent a year to no longer cause additional warming — for the water coming from the tap (ongoing emissions) to match what’s going down the plug. The emissions have still made the planet warmer (and the bath level higher), but they’re not increasing the temperature any further. This has the same temperature effect as CO2 emissions falling to net-zero, as the warming from past CO2 emissions is essentially locked in because it takes so long to break down.

If methane emissions fall faster than 0.3 per cent, however, then it will actually reduce the warming caused by the gas, like a cooling effect — with the same effect on the climate as planting trees to sequester carbon.

Also, if you stop emitting methane, then the warming caused by the gas soon drops and eventually disappears entirely. However, stopping CO2 emissions doesn’t stop the ongoing warming caused by earlier emissions, which are locked in for centuries.

Frame says this is why CO2 emissions must go to net-zero to halt global warming, whereas methane emissions can stop additional warming with a very gradual decline. He likens methane’s effect to the health impact of obesity and carbon emissions to terminal cancer. “You would want to address both,” he says. “But the really time-critical one is the cancer.”

This distinction, Frame believes, has been lost in public debate on methane emissions and, significantly, in how our greenhouse emissions are reported.

Other climate scientists, however, view methane as a more menacing presence. Andy Reisinger is probably Dave Frame’s main methane adversary. His bovine-burp credentials are also among the best in the country. He was deputy director of the New Zealand Agriculture Greenhouse Research Centre (NZAGRC) for a decade and has been an author on multiple IPCC reports. He is currently a climate change commissioner, advising the government on meeting its emissions targets.

Reisinger believes that just because stable methane emissions don’t cause the same increase in warming as CO2 that doesn’t mean that we shouldn’t do all we can to reduce the emissions as much as possible. Every tonne of methane is making the planet warmer than it otherwise would be and reducing it can avoid that additional warming. While there’s little we can do about the warming locked-in by past carbon emissions, actively putting methane into the atmosphere is within our control.

“Even if falling methane emissions don’t cause additional warming, that says nothing about if it’s something we ought to reduce,” Reisinger says. “To use a crude analogy, if a farmer has always dumped 10 bags of fertiliser into a river each year, the river will not get any more polluted. That doesn’t mean that I should continue to dump 10 bags of fertiliser into a river. There’s nothing positive about it.”

Measurement matters

A complicating factor is that the way we calculate greenhouse emissions doesn’t factor in how these two gases accumulate differently.

When the first global efforts to reduce greenhouse gases were beginning, a metric was developed so all of a country’s greenhouse emissions could be reported and compared using a single figure.

Because greenhouse gases all warm the atmosphere at different rates and break down over different time frames, you can’t simply add up all the gases to show how much each country is warming the atmosphere. So in the early 1990s a formula was developed, dubbed global warming potential 100 (GWP100), which has been the fundamental metric for expressing every country’s greenhouse emissions and targets ever since.

GWP100 is calculated by averaging the warming produced by each greenhouse gas over a century, and equating this with the warming from emitting a tonne of CO2. The figure is then reported as CO2- equivalent (CO2-e), or often just using the catch-all term “greenhouse gas”. Using GWP100, methane emissions are multiplied by 27 (the exact multiplier varies as science develops), so a tonne of methane equates to 27 tonnes of CO2-e, or 27 tonnes of greenhouse gas.

It’s a useful metric because it is simple to calculate and makes it easy to compare countries’ emissions. But it has some limitations.

GWP100 shows the warming caused by emissions as if they are one-off occurrences, comparing how those emissions will warm the planet compared to zero emissions. But if you try to add up these emissions over time, or calculate how emissions have increased or decreased over time, the figures don’t actually tell you how much warming these emissions have caused. This is because GWP100 essentially treats methane as if it accumulates indefinitely, like CO2, and so doesn’t account for the fact declining methane emissions can stabilise or reduce warming, while CO2 emissions always increase it.

This obscures how a country’s ongoing emissions will warm the atmosphere. For example, if a country is emitting a million tonnes of methane a year and no CO2, but the emissions were declining at 0.3 percent a year, then it wouldn’t be causing additional warming — the planet would be warmer than it would be without those emissions, but it wouldn’t warm further — the same effect as net-zero CO2. However, using GWP100, the country’s emissions would look the same as if it was emitting 27 million tonnes of CO2, which would be significantly adding to global temperature.

On the other hand, GWP100 underestimates the warming from rising methane emissions, which have a significant warming effect due to how powerful it is in the short-term.

With every country emitting different ratios of CO2 and methane, it becomes impossible to accurately show what a country’s emissions will do to the climate solely using GWP100.

“It’s like having a metric that combines the health impact of litres of alcohol per day with kilograms of arsenic,” Frame says. “It’s just a bad mix in terms of units.”

While it sounds controversial, the IPCC made the shortcomings of GWP100 clear in its latest report: “Expressing methane emissions as CO2 equivalent emissions using GWP100 overstates the effect of constant methane emissions on global surface temperature by a factor of 3 –4 while understating the effect of any new methane emission source by a factor of 4–5 over the 20 years following the introduction of the new source.”

For most developed countries this isn’t a significant issue as their methane emissions are a small proportion of their greenhouse profile. But for New Zealand, Frame says it’s significant.

“GWP100 is clear, simple and misleading,” Frame says. “And the clarity and simplicity is attractive to some people. But for countries like us, it’s a big deal. And although high methane emissions are unusual for a developed country, most developing countries have profiles like ours. But we’re left with this relic from a bygone age.”

A rising star?

Other metrics have been developed which can accurately show the warming eff ect of ongoing emissions — but whether that’s a good idea or not, and whether the shortcomings of GWP100 matter, is fiercely debated.

About a decade ago, Frame worked with climate researchers from Oxford University to develop a new metric called GWP*. The project was led by Oxford professor Myles Allen, someone Frame calls “one of the leading climate researchers of his generation”.

Allen became one of the first scientists to highlight that global CO2 emissions must reach net-zero to halt global warming — work that Frame was intimately involved with while working at Oxford as a researcher.

“It was really Myles’ insight that temperatures change depending on the cumulative emissions of CO2,” Frame says. “He showed that you have to get to net-zero carbon emissions to stop warming. This was a significant reframing of climate policy, because up until then people had been talking about stabilisation and equilibration of temperatures. They thought if we reduced CO2 emissions by 75 per cent that it would be possible to stop warming by 2100. We showed this wasn’t really the case. But it was a bit of a fight.”

The BBC later dubbed Allen “the physicist behind net zero”, and the concept has become the foundation of countries’ eff orts to limit temperatures to below 2°C — and as close to 1.5°C as possible — under the historic Paris Agreement of 2015.

Allen, Frame and others later developed the GWP* metric to accurately show how changes in global emissions would affect temperatures. Unlike GWP100, the GWP* metric accounts for the rate of methane emissions to show how they will affect temperatures.

Using this metric, if a country’s methane emissions were falling at 0.3 per cent a year, then the gas’ contribution to further global warming would be zero, accurately reflecting how its emissions would affect temperatures.

On the other hand, if methane emissions were rising, then the warming effect is amplified. “It finally enabled us to treat short-lived gases in the same way that you treat CO2 in a carbon budget,” Frame says.

In effect, this would mean that if New Zealand’s methane emissions fell by 10 percent each decade, then using the GWP* metric, a third of our greenhouse emissions would effectively be wiped from our greenhouse account balance sheet.

Frame says given the Paris Agreement commits countries to achieving a temperature goal, it is essential to use a metric that shows how countries’ emissions affect temperatures.

However, efforts to replace GWP100 have failed to gain traction. Partly, Frame says this is because GWP100 has been used for emissions targets and reporting for 30 years.

“There’s been criticism of GWP100 for decades, ever since it was created really, but because it plays a role like a joint in a body — it’s a hinge between science and policy — it’s quite hard to replace. People get very uptight about it and it gets its own inertia. But we’re clearly right about the physics and no one disputes that.”