New Zealand farmers have some hard decisions to make and they need to think long-term, influential environmentalist Guy Salmon says.

They are caught between the twin pincers of alternative proteins and higher environmental standards. And they need to act fast in response to climate change.

Salmon gives the last Whanganui Science Forum talk of the year in the Davis Lecture Theatre on December 6, at 7.30pm. It's titled The Coming Transformation of New Zealand Agriculture.

He heads the Ecologic thinktank, was a driving force behind the Land and Water Forum, and advises Paamu (formerly Landcorp), other businesses and government.

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Salmon wants to interact with farmers and anyone else who's interested in how farming responds to environmental challenges. His talk will cost $4 for forum members, and $5 for others.

Farmers have to keep four main environmental factors in mind - water quality, biodiversity, soil conservation and now climate change, he said.

"People are still wanting to avoid facing up to the realities of that."

Environmental advocate and researcher Guy Salmon. Photo / supplied
Environmental advocate and researcher Guy Salmon. Photo / supplied

Action is urgent though.

"There's only 30 years left to make it, and we have wasted the 26 years since 1992. We should be pretty well half way to zero but emissions are still going up."

Ruminant livestock - sheep and cattle - are like internal combustion engines. They are inefficient technologies, Salmon said, that produce a huge amount of greenhouse emissions.

Research has failed to find any major means to reduce those emissions, and Government will want to offset all of them. That will be expensive.

A farmer thinking of building a herd home for cattle needs to consider what offsetting their emissions will cost in future.

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"We have got to solve this climate problem within the lifetimes of the young agriculture graduates leaving Massey at the moment."

Government policy is needed quickly, and it could set rules or be flexible.

Salmon thinks farmers banding together across catchments could be part of a solution. The emissions of the whole group could be measured, with decisions made within the group about the best places to plant the trees to offset them.

Such catchment groups have worked well for water quality issues, he said.

Forestry and growing trees for biodiesel could be part of the answer. Better land could be used for horticulture rather than livestock.

"There are huge opportunities for expansion in horticulture. It's going to be really important for New Zealand."