Conservation Comment: We need to protect Whanganui's economy and productive land

With stable populations of sheep and cattle and areas in vegetation, the 'green' biogenic carbon cycle on farms is in equilibrium, with no contribution to additional global warming. Photo / Supplied

Whanganui District Council is writing a Climate Change Strategy, and the process recently involved a stakeholder workshop for the rural sector.

Agriculture is a significant contributor to New Zealand's greenhouse gas emissions, but mitigation actions could severely impact on Whanganui, whose economic prosperity is so reliant on farming, meat and milk processing and the related service industries.

A report from Horizons which has had recent media coverage and is providing local emissions data for the strategy shows that 57 per cent of Whanganui's emissions are from agriculture.

Much of the world's investment in scientific research during the last two decades has been focused on climate change. We must use this more thorough knowledge when making mitigation decisions, the impacts lasting much longer than one of the greenhouse gases (GHG) that they are focused on.

Eighty-two per cent of Whanganui's agricultural GHG emissions are biogenic methane, belched from the ruminant stomachs of sheep and cattle. Biogenic means that it is part of the carbon cycle that applies to all ecosystems. In agriculture, grass grows and by photosynthesis removes CO2 from the atmosphere, as ruminant animals digest grass methane is belched and there is also carbon in the meat or milk.

Methane behaves very differently to CO2 in the atmosphere, it only lasts for 15 years before breaking down to CO2 and water, in similar proportions that were photosynthesised by the grass that was eaten. With stable populations of sheep and cattle, the carbon cycle is in equilibrium, there is no additional methane in the atmosphere and no additional warming to the planet.

As you digest this knowledge, consider the CO2 that you have just breathed out. It is the return of CO2 to the atmosphere that was removed by the growth of the food you have eaten. Converting our grass pastures to pine trees is promoted as a mitigation, but pine trees should be regarded as a slow rotation grass pasture and is just part of the carbon cycle.

Rather than a 15-year cycle carbon is cycled every 25-45 years, depending on whether the forest is harvested or left to grow to maturity. The carbon is sequestered in the wood of the growing pine tree.

At harvest the carbon cycle continues, with needles and branches decomposing, and a log is removed of which 90 per cent are exported to China. What we currently ignore is that our radiata pine as a fast-grown softwood is primarily used in China for packaging or boxing concrete.

The carbon sequestered in the log is not locked up permanently, it is returned to the atmosphere when this packaging and boxing is either burnt as fuel or decomposes in a landfill.

With stable populations of sheep and cattle and areas in vegetation, the 'green' biogenic carbon cycle on farms is in equilibrium, with no contribution to additional global warming. Currently livestock emissions, that are just part of the biogenic carbon cycle, are treated the same as emissions from burning fossil fuel.

When fossil fuel is consumed carbon that has been stored for thousands of years is converted to CO2 which contributes to warming every year of the 1000 years that it persists in the atmosphere.

The 6.5 per cent increase in global CO2 concentration from 385 ppm in 2010 to 410 ppm in 2019 largely came from fossil fuels, not from methane belched from sheep and cattle as they digest grass grown on Whanganui's hill country.

When drafting legislation, it is simpler to treat 'green' biogenic carbon emissions similarly to fossil carbon emissions. This however incentivises the conversion of productive farmland to forestry, enabling fossil fuel users to offset their emissions rather than change their behaviour.

Planting pine trees on productive hill country does not permanently remove CO2 from the atmosphere, but the impact on Whanganui's economy and communities will be irreversible.

When councils in provisional New Zealand write their Climate Change Strategies, they should show leadership in promoting the importance of agriculture to their districts' prosperity and highlight how vulnerable we are on losing our productive farmland resource.