If fossil fuel users can take the easy option of planting forest to offset their carbon emissions there's a danger emissions won't be reduced enough, Parliamentary Commissioner for the Environment Simon Upton says.
Stopping carbon emissions completely is the top priority for halting global warming, he told the Middle Districts Farm Forestry Association in a speech at the Feilding Golf Club on July 9.
The warming effect of carbon dioxide in the atmosphere effectively lasts for ever.
"Global temperature will not stabilise at any level - two, three or four degrees - if there are ongoing emissions of carbon dioxide."
Government's Zero Carbon Bill, open to submissions until July 16, would allow burners of fossil fuels and emitters of nitrous oxide to offset their emissions by planting forests.
Forest could be the cheapest offset option available - and allowing it to be used is likely to begin a large scale and long-term change of land use - and put farmers in direct competition with fossil emitters for land, he said.
Upton's most recent report, Farms Forests and Fossil Fuels, recommends another approach. He calls it the landscape approach.
In it, carbon dioxide emissions are reduced to zero without the help of offsetting from forestry. Instead forests are used as an offset for methane and nitrous oxide emissions - most of which are a result of livestock farming.
The landscape approach would allow landowners to manage land in an integrated way, Upton said.
"It aims at addressing climate mitigation with efforts to improve soil erosion, biodiversity loss and water quality issues. This landscape-based approach would see the landscape as more than just a place for storing carbon."
He's all for more trees, but also for planting them where it makes sense. By burning and clearing New Zealand forests humans have already emitted seven times as much carbon dioxide as they have emitted by burning fossil fuels, he said.
Restoring some of that balance will be a good thing. But he warns that the carbon offsetting effect of forest should be "discounted" - against the increased damage likely to be done by the fire, pests and disease global warming will bring.