As a climate scientist, I strongly rebut Tuesday's opinion piece by Robin Grieve who claimed he was relating facts from the Motu Research report, Cows, Sheep and Science: A Scientific Perspective on Biological Emissions from Agriculture, that I co-authored, and a report from the Parliamentary Commissioner for the Environment, both of which came out last year.

Both reports state methane very clearly contributes to global warming. In fact, the Intergovernmental Panel on Climate Change estimates that global methane emissions are responsible for more than 40 per cent of the total warming effect so far of all human activities.

Once a methane molecule is in the atmosphere, it doesn't matter whether it came from a New Zealand sheep, a Russian gas pipeline or a Chinese landfill. Our livestock sector is making the concentration of methane in the atmosphere higher than it would be otherwise, and this results in the world becoming warmer than it would be otherwise.

There is no scientific basis for Mr Grieve's assertion that somehow New Zealand's methane emissions are "mostly harmless".


Our report makes clear that methane does not last as long in the atmosphere as carbon dioxide. That means methane emissions do not have to be reduced to zero in order for the climate to stabilise, whereas carbon dioxide emissions do have to be reduced to zero.

Mr Grieve is right to pick up on that. But the conclusion he draws, that as long as methane emissions don't increase further they don't pose a problem, is badly misleading.

Imagine a car driving through a residential area at 100km/h and the driver arguing there is no risk because he wasn't accelerating any further. We'd have a problem with that - the greater the speed, the greater the damage. That's why we require people to slow down to 50 or even 30km/h. In the same way, reducing methane emissions would reduce the risk posed by global warming. They don't have to drop to zero, but the lower they can go, the lower the risk.

The Motu and Parliamentary Commissioner's reports brought together years of scientific research from a wide range of climate scientists, economists, earth scientists and other experts. There are differing views regarding how much effort should be put into reducing methane, and whether this effort should occur immediately or only after carbon dioxide emissions have begun to decline. But there is no dispute that continuing methane emissions at current levels will continue to keep the world warmer than it would otherwise be, and that reducing methane emissions would help reduce the impacts of climate change.

The goal to reduce emissions from all gases is fully consistent with the science. There is only one atmosphere. By necessity, if we allow the warming contribution from one gas to be greater, then the contribution from another gas must be less. Even 100 years into the future, the tonne of methane we emit today will have a significantly higher impact on climate change than one tonne of carbon dioxide.

If we insist methane emissions do not have to come down at all, this means carbon dioxide emissions will have to shrink impossibly rapidly if we want to limit the total warming from all gases to substantially below 2 degrees, as 195 countries agreed in Paris.

As Mr Grieve suggests, New Zealand could treat gases individually in our emissions trading system, to better recognise that methane and carbon dioxide have fundamental differences. But this would not change the need to reduce methane emissions as much as possible. Cutting carbon dioxide emissions to zero has to be the foremost goal, but the ways and costs of achieving this will depend on how much progress we can make in reducing emissions of other gases at the same time.

The question is: what is possible? Farmers have already shown that methane emissions can be reduced by increasing efficiency, and further gains are possible. Significant additional research is under way to further increase the options to reduce methane and make greater reductions feasible.

The atmosphere is a shared space, just like residential areas. If everybody slows down, it'll make for a much happier place to live in.

* Dr Andy Reisinger is deputy director of the New Zealand Agricultural Greenhouse Gas Research Centre and was a co-ordinating lead author for the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report.