How many trees must we plant to offset warming from agricultural methane?

Greenpeace Aotearoa was quick to react to a new report by Simon Upton, the Parliamentary Commissioner for the Environment.  It seized on the document as ammunition for its lobbying to cull the country’s dairy herd.

Professor Keith Woodford, Principal Consultant at AgriFood Systems Ltd, drew a contrary conclusion.

The report – Simon Upton describes it as a “note” – is headed How much forestry would be needed to offset warming from agricultural methane?

Mr Upton writes:

“There are no recommendations in this note. My aim in publishing it is simply to lay out what can and cannot be credibly claimed with respect to offsetting livestock methane, in the hope that this will help foster a better-informed debate about New Zealand’s 2050 target for biogenic methane.”

But it is not a brief note.  It is 49 pages long and (as Professor Woodford observes) “it is not an easy read”.

A press statement from Greenpeace says the note shows New Zealand can’t plant its way out of climate pollution from intensive dairy. Instead, the Government has to act – and cut synthetic nitrogen fertiliser and reduce the dairy herd, for the sake of the climate and future generations.

The headline on the statement sums up Greenpeace’s position: Report Shows Tree Planting No Substitute For Cutting Dairy Emissions

Greenpeace lead agriculture campaigner Christine Rose says:

“The numbers speak for themselves. There are too many cows in this country, and the stats show intensive dairy polluters can’t plant their way out of dealing with the emissions they create.”

To reduce dairy emissions and protect the climate, Ms Rose contended, the dairy herd must be shrunk through phasing out synthetic nitrogen fertiliser and imported feed.

Professor Woodward, writing for, notes that Upton looks at the net-zero question using two different metrics, both focusing on 2050 as the initial target date.

The first is the GWP100 measure of emissions, the official international method used for comparing short and long-lived gases.

The second measure focuses on the amount of further warming, which is not the same as the level of emissions, to ensure that the land-based sector is not adding to warming after 2050. The second analysis uses the principles that underpin the alternative GWP* metric.

Using the GWP100 measure, Mr Upton reports that New Zealand would need to plant about four million hectares of new pine forests progressively over the next 100 years to balance current methane emissions.

Using a warming metric based on principles set out in the alternative GWP* metric, New Zealand would need to plant slightly less at 3.9 million hectares, but with all of this occurring by 2050 and none thereafter.

Professor Woodford writes.

“Not surprisingly, Upton then concludes in his final paragraph as follows: ‘This work shows that very large areas of forest would need to be planted to make any significant dent in the marginal warming effect of New Zealand’s livestock methane emissions. For that reason, if forest planting were to be used to offset livestock methane, it would have to be in addition to – not instead of – reducing national gross emissions of biogenic methane by 24–47% by 2050. We cannot simply plant our way out of this problem’.

Professor Woodford says his conclusion “is somewhat different”, because Mr Upton’s starting point has been to assume production forestry with approximately 28-year rotations, for which the average sequestration over multiple cycles is equivalent to only 16 years of initial growth.

Professor Woodford writes:

“This short-rotation assumption leads inevitably to his results and conclusion.

“The conclusion would be different using long-rotation forests or so-called permanent exotic forests, possibly transitioning eventually to native forests.  For example, with permanent forests the sequestered carbon can be five times that of the average under short-rotation forests. Accordingly, his conclusion should have been that we cannot plant our way out of this problem with short-rotation forests.”

A second conclusion by Professor Woodford reflects Mr Upton’s long-held perspective that there is a logic associated with using forestry to balance emissions within the land-based sector instead of as a get-out-of-jail card for the energy and transport sectors.

“If we accept that proposition, which has considerable merit and is worthy of consideration, then there is a lot more analysis needed as to alternative pathways forward and their effects on all sectors of the economy. I hope that Simon Upton takes up those challenges.

“In seeking out those alternative pathways, we should never forget that more than 80 percent of New Zealand’s merchandise exports derive from the primary industries, and approximately half of those total exports derive from methane-emitting ruminants. Without vibrant export industries, as a nation we are in big trouble.”

Sources: Parliamentary Commissioner for the Environment; Scoop’ 


Author: Bob Edlin

Editor of AgScience Magazine and Editor of the AgScience Blog