Analysis: As New Zealand gears up to fight climate change, experts warn that we need to actually reduce emissions, not just plant trees to offset our greenhouse gases, Marc Daalder reports
This year is shaping up to be a major one for climate policy. Between the Climate Change Commission releasing its recommendations around our Paris target and emissions budgets and a major climate summit in Glasgow in November, 2021 is the year the New Zealand Government will finally lay out in detail its plans to fight climate change.
Ahead of February 1, when the Commission will release drafts of its advice for consultation, experts warn that we shouldn’t be taken in by the allure of trees as a silver bullet. It’s true that major reforestation will be crucial to slowing global warming (and has added biodiversity benefits as well), because all plants sequester carbon breathed in from the atmosphere.
However, the benefit of any individual hectare of planted trees is limited and is no replacement for reducing gross emissions, which in New Zealand have flatlined for the better part of two decades. Even today, our emissions are nearly a quarter higher than they were in 1990, while many other countries have managed to reduce emissions.
Commission chair Rod Carr has made clear that he opposes planting our way out of climate change. Speaking to a business conference in November, he said the Commission wouldn’t accept a world in which New Zealand’s gross emissions continued to grow but it offset these by planting trees or purchasing overseas carbon credits.
“This is not a case of us buying our way into a high-emitting future,” he said.
Short-term solution
Dave Frame, the director of the New Zealand Climate Change Research Institute at Victoria University of Wellington, says new forests are a long-term commitment but not a long-term solution.
Why is that?
Any given hectare of forest eventually becomes saturated with carbon. Once a forest reaches maturity, the net amount of CO2 it sucks out of the atmosphere starts to decline.
“The capacity to sequester massively reduces once a forest becomes mature,” David Hall, a Senior Researcher at AUT’s The Policy Observatory who specialises in environmental policy, told Newsroom.
“As older trees start falling over, they’re releasing carbon dioxide as they decompose, and so forests become a source of greenhouse gas emissions at the same time as they continue to sequester. You get a balance, it starts to reach an equilibrium between what it’s sequestering and what it’s emitting as the forest becomes mature.”
Plantation forests fare no better and in fact store, on average, a lower amount of carbon than permanent forests. As the trees are harvested and sold off, they may be used as fuels or end up in landfills where they re-release that stored carbon.
“Every time you cut it down and those harvested wood products decay – if they’re paper or something – then the carbon gets re-emitted to the atmosphere,” Frame said.
“It depends on the fraction of the wood that is permanent and the fraction that ends up in landfill. Most of it, I think, ends up in the atmosphere, but a small amount would be permanently sequestered, but fairly small.”
That small amount is permanently sequestered either in wood products that are used for a long period of time or in the soil of the original forest. Above that, however, the carbon storage fluctuates wildly with the plantation harvesting pattern. If you graph it, like the Ministry for Primary Industries (MPI) has done below, it creates a saw tooth pattern.
The red line shows the soil carbon which is (essentially) permanently sequestered, the green line shows the average amount of carbon in the forest over the long run and the blue line traces the actual carbon in the forest at each given point in time.
For both plantation and permanent forest, the land they are grown on must remain in forest in perpetuity. If the plantation is one day not replanted, then the “average” plummets down to just what’s left in the soil. At the same time, there are diminishing returns in terms of actual emissions sinks.
As the graph above demonstrates, a given hectare of plantation forest might hold 600 tonnes of carbon dioxide, but this would all be soaked up around 18 years after planting. That means that hectare could offset about 33 tonnes of CO2 emissions per year for that period – enough to cover the emissions from four New Zealanders. As that hectare of forest’s ability to sequester wanes, however, another hectare would have to be planted to offset the same four people if they hadn’t reduced their emissions in the meantime.
A spokesperson for MPI says the figures for carbon storage in this graph are purely illustrative, but that the overall pattern still holds.
Not enough land
“I don’t think people realise the intergenerational commitment they’re making, in terms of totally removing choice over the land in the future, unless some other magic [CO2 sequestration] technology springs up,” Frame said.
“From the New Zealand perspective, there is not enough land to plant forests that would be equivalent to the amount of emissions that we’re emitting,” Hall agreed.
“Currently, forests offset about a third of what we emit and that could certainly be increased. The problem is, we’re just immediately in a trade off with land use and the more you rely on forests, the more you’re trading off with other land uses. Obviously that undercuts, first and foremost, sheep and beef farming, but to actually achieve the scale of offsetting all of our emissions, it would probably gobble up all of our dairy land.”
Hall also says there are different risk profiles to take into account. That is, carbon stored in a forest can be released through human intervention, through wildfires, through pests and through a number of other risks. The carbon in the atmosphere, meanwhile, is much more difficult to get out and has a long-term impact on warming as it decays over thousands or even millions of years. Therefore, from a risk averse perspective, it makes more sense to avoid emitting a tonne of carbon than to plant enough trees to soak that tonne back up.
Another risk to forest carbon is global warming itself. New research from scientists in New Zealand and the United States has found that, as temperatures rise, plants begin to expel carbon dioxide faster and absorb it more slowly. In a quickly warming world, some forests may soon become net sources of CO2.
Trees can only serve as a temporary aid against climate change. They’re a good single use item to reduce emissions in the near-term, particularly as we struggle to slow growth in gross emissions (let alone actually cut greenhouse gases). They may help us blunt the peak of our emissions curve, but they’re not a long-term solution.
Globally, we can’t realistically plant enough trees to offset all of our emissions for decades and centuries to come.
If carbon dioxide from fossil fuels is driving the problem, then we’re talking about [burning] multiple generations of forests going over millions of years that have been compressed into fossil fuels,” Hall said.
“One generation’s worth of global forests and global ecosystems could never be equivalent to the multiple generations, the millions of years worth of carbon that we’re thrusting up into the atmosphere by removing that from underground and powering our economies with it. It just doesn’t work at that level.”
If we just want to hit net zero in a single year (say, 2050), trees can do that, Frame says. Just start planting enough Pinus radiata around the country now and you might do it. But after three decades years, Pinus radiata starts to slow down in how much carbon it can absorb. If you’ve kept gross emissions flat the entire time, then your net emissions will start to creep back up.
“There’s a very cynical interpretation of net zero which is that if we say we’ll be net zero in 2050 and then we just simply make sure that we’re planting enough forests in that year and that we make some steps like that, then we can mathematically be at net zero and say we’ve discharged our responsibilities,” Frame said.
“But I think, as climate change gets taken more and more seriously, people will need to actually add a bit of nuance into it and think about sustainable net zero. Unless you’re cutting gross emissions of carbon dioxide, you’re really fiddling around the edges. Because that will continually increase your forcing [warming] effects on the world. And everything else you do – killing cows and planting trees – is just taking slivers out of that ongoing growth in forcing.”
