Farmers will be expected work harder than most to reduce their emissions under the Zero Carbon Bill, Wanganui Federated Farmers provincial president Mike Cranstone says.
The Climate Change Response (Zero Carbon Amendment) Bill was introduced to Parliament on May 8.
He said the science behind it was not well explained. If it had been, people would realise farmers had not been "let off the hook".
Instead, he said, farmers would be shouldering an extra burden, expected to reduce methane emissions from animals by 10 per cent by 2030 while users of fossil fuels have until 2050 to get their carbon emissions to zero.
The bill splits the main greenhouse gases. It aims to get emissions of carbon and nitrous oxide to zero by 2050.
The other main greenhouse gas is methane, emitted mainly by sheep and cattle belching.
It is 30 times more potent than carbon dioxide as a greenhouse gas, but also decays out of the atmosphere in about 12 years.
If it can be reduced by 10 to 22 per cent it will not contribute to further warming. Instead the warming will remain stable.
"It should be promoted and celebrated by the country. If [those emissions] are stable there's no further warming of the globe. It reduces the whole country's greenhouse gas liability," he said.
After 2030 the yet-to-be-formed Climate Commission could decide farmers must reduce methane emissions another 24-47 per cent.
Cranstone said those numbers were "plucked out of the sky" and not based on science. Reducing methane emissions 10 to 22 per cent would be enough to stop further warming.
The likeliest way to reduce methane emissions is to reduce stock numbers, which will reduce productivity and hit the regions and the country's tax take.
"This is the industry that provides almost half of our exports. It's the money our country needs to buy medicines and pay teachers and maintain living standards," Cranstone said.
Farmers have agreed to limit nitrous oxide emissions to zero by 2050, mainly by using less fertiliser. They will also have to limit the fossil fuel used to farm, transport stock and process meat.
They have been told they cannot plant trees to offset their own emissions. Cranstone had planted 50ha of pines and retired 50ha of native bush - but can't use them.
Victoria University Climate Change Professor David Frame backs up Cranstone's analysis.
He said while farmers are not allowed to cause any more warming, motorists can carry on warming the planet by burning fossil fuels.
"It does raise a question about burden sharing and climate policy."
Most farmers have a "pretty sophisticated" understanding of the climate issue, he said.
"They are saying 'We will work to get to zero with nitrous [oxide]'. That ought to be welcomed."