A proposal to treat agricultural greenhouse gases differently to fossil fuel emissions pleases farmer Mike Cranstone, but environmentalists are less sure.

Cranstone supports a proposal that uses forestry only to offset agricultural emissions, saying blanketing hills with forestry to soak up emissions from burning fossil fuels is "parking their waste on our landscape".

But environmentalist Nicola Patrick says that as both sets of gases have to be dealt with, planting forests is something that can be done now and will make a difference.

Parliamentary Commissioner for the Environment Simon Upton released his report Farms, Forests and Fossil Fuels: the next great landscape transformation? on Tuesday.

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In it, he proposed treating methane and nitrous oxide, greenhouse gases created by agriculture, differently from carbon dioxide emitted by burning coal, oil and gas.

Carbon dioxide goes on trapping heat in the atmosphere for thousands of years. Methane - mainly from sheep and cattle belching - is 25 times more potent, but decays after about 12 years.

Nitrous oxide gas is emitted from cows' urine patches and nitrogen fertiliser. It's 300 times more potent than carbon dioxide as a greenhouse gas, and lasts 114 years in the atmosphere.

Together, methane and nitrous oxide make up nearly half of New Zealand's greenhouse gas emissions.

New Zealand's one billion trees initiative is intended to offset greenhouse gas emissions - but Upton said trees were short-term storers of carbon and should only be used to offset the shorter term "biological" emissions from agriculture.

They would still have to deal to nitrous oxide, and pay higher prices for the fossil fuels they used.

Each sector should look after its own emissions, he said. Reducing methane emissions by 10 per cent over 30 years would be enough to stabilise warming from them, and there was no need to get them to zero, he said.

He would rely on new technology to deal to the nitrous oxide problem, and said extra time would help with that.

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Upton's longer-term approach would be better for maintaining rural life, and Cranstone will be very disappointed if the Government doesn't treat methane differently to carbon dioxide in the Zero Carbon Bill.

"Let's hope science and a bit of business common sense is used in making these policies. Our country and communities still have to survive in the meantime," he said.

Green Party member Nicola Patrick said she did not support the split gas approach. She said while methane didn't last as long as carbon dioxide, it was much more damaging to the atmosphere — and emissions of it have to be reduced.

There were good farming options available while reducing methane. An emissions neutral farm producing high-value niche organic products could make for a wonderful lifestyle, and less impact on the environment, she said.

Relying on trees, and in particular on plantation forestry to offset emissions, was "absolutely a sticking plaster", she said.

But she agrees with Green Party co-leader James Shaw that planting trees is one of the few available options for urgent action.

She favours longer-term native forestry over plantation forestry, because it would give better erosion management and higher-value products from selective logging.