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Because methane is a less persistent greenhouse gas, it has been regarded as a lesser evil. That calculation may need to be reassessed, Colin Campbell-Hunt writes.

We have a problem with methane. I am not talking about our well-known problem with cows and other ruminants, although we will come back to them. I am talking about the world’s emissions of methane, and the way in which we are currently including them in our tally of greenhouse gas (GHG) emissions.

The focus of the world’s efforts to control climate change is carbon dioxide, CO2.

It is the most prevalent GHG and it stays in the atmosphere, blocking the escape of heat, for centuries.

Methane (CH4) is of much shorter duration; its warming effect is nearly over within 100 years.

And we emit only a fraction of the tonnage of CH4 than we do of CO2. So far, methane doesn’t seem too bad.

But here’s the rub. While it is up there, methane is a far more powerful GHG than carbon dioxide.

How much more powerful?

If you wanted to add its effect to that of CO2, how much more dangerous is a tonne of CH4?

The conventional multiplier is now 25: a tonne of CH4 has 25 times the "global warming potential" as a tonne of CO2.

That is the number typically used by the Intergovernmental Panel on Climate Change and the UN Framework Convention on Climate Change when they add methane to other GHG in "CO2 equivalents".

The 25-times multiplier has in fact been creeping up over the years of IPCC reporting, starting at 21 in 1995, and now at 28 in the latest IPCC report. But the method used to come up with this number has not changed since 1995.

It is derived by estimating methane's global warming potential over its most dangerous first 100 years, and dividing it by that of carbon dioxide over the same period.

At the time, the IPCC scientists explained that there was nothing to argue for the 100-year period, and offered other multipliers for 20-year and 500-year periods.

It was the political process of the Kyoto Protocol that opted for the 100-year average.

Now let us remember why we are worrying about GHG emissions: to reduce as far as we can the probability of runaway, uncontrollable, irreversible changes to the planet’s climate systems.

There is mounting evidence that these are already being engaged: melting ice in Greenland, the Arctic and West Antarctic; increasing climatic extremes, both heat and cold; and all of these increasing at rates beyond what science was expecting just two-three years ago.

If you want to see what these processes look like, search online for "Siberian craters 2016".

Massive craters 30m wide are being ripped out of the Siberian tundra by exploding methane gas, released as the permafrost is melted by rising Arctic temperatures.

The warmer it gets, the more methane is released, the warmer it gets, the more methane is released ...

We do not have a century to allow methane to work out the worst of its effects; it is its effect over the next 20 years (or less) that will decide how fully these irreversible tipping points are engaged.

Over the shorter period of 20 years, methane’s global warming potential is not 25 times that of CO2, it is close to 80 times more dangerous.

When methane’s contribution to global warming is added at this higher rate, the latest IPCC report estimates that it would be responsible for 42% of all GHG warming, second only to the 52% from CO2.

Let’s come back to New Zealand’s ruminant cows, sheep and beef cattle.

Collectively they were responsible for nearly all of the 1.4 million tonnes of methane we put into the atmosphere in 2013.

Converted to CO2 equivalents at 25 tonnes each, that’s 35 million tonnes.

And averaged over our 4.5 million population, that’s eight tonnes of CO2 equivalents from methane per person.

New Zealand is in a class of its own on this statistic.

Among OECD countries, Australia comes next at a bit under five, Canada at three and the US at two.

If we factor in methane’s potency as a GHG over the 20-year horizon that is the only window we have to limit the probability of runaway climate change, our methane emissions in 2013 were the equivalent of putting 85 million tonnes of CO2 into the atmosphere, driving our total GHG emissions up to 131 million tonnes of CO2 equivalents.

That’s more than 29 tonnes per head of population.

Among OECD countries and using the same arithmetic, only Australia would match our total at 30 tonnes per head, Canada’s would be 25, and the US 23.

So here we have a more relevant measure of the importance we should be putting on eliminating our emissions of methane.

Over the 20-year horizon that is the period in which we have the best chance of limiting the radical changes that climate change will bring, our annual emissions of 1.4 million tonnes of methane will have the global warming potential of 85 million tonnes of carbon dioxide.

Imagining that we can keep the number of livestock that we have now, and have any impact on this volume of gas by changing the "enteric fermentation" in their digestions, goes well past my (generous) level of gullibility.

- Colin Campbell-Hunt is an emeritus professor at the CSAFE Centre for Sustainability, University of Otago. Each week in this column, one of a panel of writers addresses issues of sustainability.