A major new study shows the amount of methane in the atmosphere has been rising since 2007 – and it's not slowing down. While methane emissions dropped at the start of the century, new air sampling data shows they've been ramping up, particularly in recent years, and at rates not seen since the 1980s. This increase in methane was unexpected, they say, so it wasn't included in the climate change projections used in Paris Agreement targets. Science reporter Jamie Morton spoke with one of its Kiwi co-authors, Professor Martin Manning, about what this means for New Zealand and its methane-heavy greenhouse gas inventory.

Your new paper found rising methane levels in the atmosphere could threaten the Paris Agreement's goal to keep temperature rise below 2C. Can you elaborate on what the study found and how you and your fellow authors came to this conclusion?

Governments agreed to try and keep global average warming below 2C, or as some say "well" below 2C.

The most detailed climate model analyses on how to achieve this were done for the UN Intergovernmental Panel on Climate Change (IPCC) report that came out in 2013 and used a scenario called RCP2.6 that had CO2 emissions going to zero or negative in the second half of this century.

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However, reducing CO2 emissions takes quite some time because it requires current energy production systems and their global infrastructure to be replaced with renewable energy and/or by developing another new technology that could capture the CO2 from fossil fuel use and put that where it would not get into the air.

The most detailed scenarios for achieving the 2C target also moved to biofuels from plants after 2050 and so this would result in net removal of CO2 from the atmosphere.

But to make enough time available for this major global transition in energy production to take place, the scenario also included rapid reduction in methane emissions on the grounds that a significant fraction is from waste or leakage of natural gas or poor management of coal mines.

Because methane has a much shorter lifetime in the air than CO2 the effect of reducing methane emissions would be seen more rapidly and so buy time needed for the slower reduction in CO2 emissions to become effective.

On that basis the climate model runs done for the 2013 report to governments used observed data to the end of 2011 when methane was increasing a little, but the RCP2.6 model runs all then had methane emissions decreasing from 2011 on, and its peak concentration in the atmosphere occurring rapidly by 2013 or 2014.

What has actually happened is that atmospheric methane concentrations kept increasing and started to do so more rapidly in 2014.

Instead of decreasing by about 3 per cent, they have increased by that amount and do not appear to be slowing down.

What has likely been driving these sharp rises in global methane emissions? Can much of it be attributed to the spread of intensive farming across Africa?

There is a subtle difference between heavy and light methane.

Methane from fossil fuels is heavy and from agriculture and wetlands is light.

The observed changes in methane show that it is the lighter sources that are driving the increase and that it is also predominantly in the tropics.

Those who have looked at agricultural production say that can be part of the reason for the observed increase but it would be wider than just in Africa and include increases in Asia and South America as well.

But the increases seen in livestock are not enough to explain all of the observed increase in methane.

So there is probably also more production of methane from wetlands across places like the Amazon basin, Congo basin and parts of Indonesia.

There is also some quite independent evidence that the lifetime of methane in the atmosphere may be increasing - that is its removal by atmospheric chemistry is a bit slower.

Methane - most of it from ruminant livestock like sheep and cattle - accounts for nearly half of New Zealand's greenhouse gas emissons. Photo / File
Methane - most of it from ruminant livestock like sheep and cattle - accounts for nearly half of New Zealand's greenhouse gas emissons. Photo / File

So this can be a third part of the cause.

There has been a lot of discussion about more methane coming from the tundra and coastal areas of Canada and Siberia, but so far that does not seem to be a major factor.

Do you feel that methane is the greenhouse gas the world isn't talking about enough? And why does this urgently need to change?

Do we need to talk about it? Yes.

Detailed economic analyses of reducing greenhouse gas emissions have been very clear for more than 20 years that reducing all major greenhouse gases can achieve a target with less cost than reducing just CO2.

What implications do you believe this new paper has for New Zealand, where methane accounts for 40 per cent of our emissions in terms of global warming potential. While scientists are actively searching for solutions to curbing emissions from ruminant livestock, does this study point to a need for greater policy action on methane? I note Parliamentary Commissioner for the Environment Simon Upton's recent report suggesting that to prevent global warming, methane emissions will need to be reduced by 10-22 percent below 2016 levels by 2050, with further cuts by the end of the century.

The relative reduction in these gases at a country level can be quite dependent on that country's economy and structure, so New Zealand can be different to the UK.

But there have been many detailed studies of the economic implications for New Zealand, some of which also deal with a global economic context as well, and all have shown that we can and should cut methane emissions.

Dr Andy Reisinger [deputy director of the New Zealand Agricultural Greenhouse Gas Research Centre] did the report for the parliamentary commissioner which led to their statement that we should cut methane emissions by 10 to 22 per cent by 2050.

Andy worked for me both in NIWA and then in Victoria University and we have been swapping emails recently, so I know that we agree on all the details.

It's also worth noting, however, that there's disagreement in the academic community over how we account for methane emissions. Your Victoria University colleague Professor Dave Frame and his fellow researchers recently argued the way methane is currently accounted for - by using the idea of CO2 equivalence - exaggerated the long-term effects of methane on the climate, and that we need a new system. There are also concerns from our agriculture community around whether current tools are capable of accurately accounting for farm emissions. Do you feel such issues need to be resolved before we can properly design policy interventions?

Do we wait for total agreement? No.

There are always differences in opinion on the details.

So this is no reason for New Zealand to delay setting up some generic form of a "zero carbon" target while the details of how rapidly and how much that applies to agricultural production should be subject to review every five years.

As technological options change, the way of achieving the target can be expected to evolve as well.

Also, nothing in a debate about the importance of methane alters the priority of reducing transport emissions rapidly.

Norway has a third of its new car sales being electric zero emission ones, so the most important question is how long will it take New Zealand to get there?

There are more than a dozen ways of comparing emissions of different greenhouse gases but many economic analyses at both international and country levels have shown that it makes very little difference which type of comparison you use.

The way that methane gets brought into a New Zealand policy framework does have to treat uncertainties in farm emissions carefully, even if it means discounting all farm emissions until there are better ways of determining them.

But accurate measurements of farm emissions is probably of more concern to New Zealand climate scientists than it is to our farmers.

When I was in NIWA, Keith Lassey in my group brought in from the US techniques for measuring methane coming from individual cows and sheep.

That has now been taken over by AgResearch and despite being a small country we probably have more of those measurements than any other country.