The effect of the Government's freshwater reforms on slashing greenhouse gas emissions will be "minimal" unless they result in a large amount of trees being planted.

That's the finding of two separate government-commissioned studies, which estimated that land-based gross greenhouse gas (GHG) emissions would be reduced by just 2 to 4 per cent after the freshwater reforms.

The papers, led by AgResearch and Wellington-based institute Motu Economic and Public Policy Research, analysed the National Policy Statement for Freshwater Management, which directs regional councils to establish objectives and set catchment-by-catchment limits for freshwater in their regional plans.

"Across New Zealand, planned reductions in contaminants varies widely from catchment to catchment but the average reduction in contaminants expected is less than 10 per cent from current levels," said Dr Suzi Kerr, a senior fellow at Motu and co-author of the report synthesising the research.

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"Nitrogen will be reduced between 4 to 6 per cent, phosphorous by 5 per cent, sediment by 18 per cent and E.coli by 10 per cent.

"The small scale of the GHG reductions are because reductions in contaminants are targeted at a relatively small areas, the required reductions in contaminants are not likely to be large overall, and the likely low cost mitigation options outside of reforestation may have only small effects on gross GHGs."

The studies found that in many cases it was possible to achieve the nitrogen mitigation targets without changes in animal numbers.

"This consequently limits effects on GHG emissions, because it is animal numbers that really matter when it comes to GHGs."

For each one per cent of nitrogen reduced, gross GHG emissions are on average reduced by about 0.3 to 0.7 per cent.

Adding trees as a part of the mitigation mix increases this to closer to 1:1.
Both studies primarily focused on options that maintained land use and productivity.

"However, we did some additional analysis that found if farmers were willing to afforest some of their less productive bits of land, a high proportion of the mitigation could be achieved in that way," Kerr said.

They found that no single measure would substantially mitigate the effects of farming activities on the environment, with improvements in freshwater quality instead being gained by a range of approaches, such as keeping stock away from streams and wetlands, controlling hill-country erosion, improved stock and effluent and fertiliser management.

The greatest emission benefits from the freshwater reform were likely to come from activities on sheep and beef farms, especially hill country.

"On land used for sheep and beef farming, afforestation would provide the greatest reductions in sediments and phosphorous and the greatest GHG co-benefits," Kerr said.

"There's also a good alternative if farmers don't want to plant forest. Spaced pole planting allows landowners to maintain stock production, although the added benefit of carbon sequestration would be lower."

This was the first national-level research assessing the indirect effects of the water-quality component of the freshwater reforms on New Zealand's greenhouse gas emissions.

The independent teams included one led by AgResearch with assistance from Scion and Plant and Food. The other was led by Motu Economic Public Policy Research with Landcare Research and assistance from Niwa and AgResearch.

The two teams took quite different approaches, but some of their results are directly comparable.

The studies follow another recent Motu paper that suggested how establishing
native forests was an environmentally and economically attractive way to decrease the risk for high-emitting companies who could face a carbon market where prices soar as high as $270 per unit.

New Zealand and climate change

• Under present projections, the sea level around New Zealand is expected to rise between 50cm and 100cm this century, while temperatures could also increase by several degrees by 2100.

• Climate change would bring more floods (about two-thirds of Kiwis live in areas prone to flooding); make our freshwater problems worse and put more pressure on rivers and lakes; acidify our oceans; put even more species at risk and bring problems from the rest of the world.

• Climate change is also expected to result in more large storms compounding the effects of sea-level rise.

• New Zealand, which reported a 23 per cent increase in greenhouse gas emissions between 1990 and 2014, has pledged to slash its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030.