By Guest Author 15/07/2020 1

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I would like to know how much difference we could make to our commitment under the Paris Agreement and our total greenhouse gas emissions if we removed all cows and sheep from the country and grew plants in their place (hemp, wheat, oats etc). Surely we could easily become carbon neutral if we removed all livestock? How much more oxygen would be produced from plants growing instead? How would this offset our emissions? And what if we returned the land the animals were on to native forests or even pine plantations?

Sebastian Leuzinger, Auckland University of Technology

This is an interesting question and gives me the opportunity for some nice – albeit partly unrealistic – model calculations. Before I start, just two comments regarding the question itself.

Oxygen concentrations have been relatively stable at around 21% of the air we breathe for millions of years. This will not change markedly even if carbon dioxide emissions increase for years to come. Carbon dioxide concentrations, even in the most pessimistic emissions scenarios, will only get to around 0.1% of the atmosphere, hardly affecting the air’s oxygen content.

Secondly, grazing animals like cows and sheep emit methane — and that’s what harms the climate, not the grassland itself. Hemp or wheat plantations would have a similar capacity to take up carbon dioxide as grassland. But growing trees is what makes the difference.

Here’s a back-of-the-envelope calculation to work out how New Zealand’s carbon balance would change if all livestock were removed and all agricultural land converted to forest.

If New Zealand stopped farming cows and sheep, it would remove methane emissions. Heath Johnson/Shutterstock

Converting pasture to trees

This would remove all methane emissions from grazing animals (about 40 megatonnes of carbon dioxide equivalent per year).

New Zealand has about 10 million hectares of grassland. Let us assume that mature native bush or mature pine forests store the equivalent of roughly 1,000 tonnes of carbon dioxide per hectare.

If it takes 250 years to grow mature native forests on all former agricultural land, this would lock away 10 billion tonnes of carbon dioxide within that time span, offsetting our carbon dioxide emissions (energy, waste and other smaller sources) during the 250 years of regrowth. Because pine forest grows faster, we would overcompensate for our emissions until the forest matures (allow 50 years for this), creating a net carbon sink.

Note these calculations are based on extremely crude assumptions, such as linear growth, absence of fire and other disturbances, constant emissions (our population will increase, and so will emissions), ignorance of soil processes, and many more.

If agricultural land was used to grow crops, we would save the 40 megatonnes of carbon dioxide equivalent emitted by livestock in the form of methane, but we would not store a substantial amount of carbon per hectare.

Steps towards a carbon-neutral New Zealand

How should we interpret this rough estimate? First, we must acknowledge even with our best intentions, we still need to eat, and converting all agricultural land to forest would leave us importing food from overseas — certainly not great for the global carbon budget.

Second, it shows if livestock numbers were at least reduced, and we all turned to a more plant-based diet, we could reduce our emissions substantially. The effect would be similar to reforesting large parts of the country.

Third, this example also shows that eventually, be it after 250 years in the case of growing native forests, or after about 50 years in the case of pine forests, our net carbon emissions would be positive again. As the forests mature, carbon stores are gradually replenished and our emissions would no longer be compensated. Mature forests eventually become carbon neutral.

Even though the above calculations are coarse, this shows that a realistic (and quick) way to a carbon-neutral New Zealand will likely involve three steps: reduction of emissions (both in the agricultural and energy sectors), reforestation (both native bush and fast-growing exotics), and a move to a more plant-based diet.The Conversation

Sebastian Leuzinger, Professor, Auckland University of Technology

This article is republished from The Conversation under a Creative Commons license. Read the original article.

One Response to “What if we took all farm animals off the land and planted crops and trees instead?”

  • Professor Leuzinger, you have a basic part of your reasoning completely wrong. Biogenic methane is a flow gas. It is emitted by animals as part of the plant digestive process. Methane molecules degrade to Carbon Dioxide in approximately 12 years. Carbon Dioxide is then consumed by forage plants in the photosynthesis process and used to build proteins, carbohydrates, and cellulose in the plant. The plants are then eaten by animals completing a cycle. As long as animal numbers are stable the actual global warming from animals’ methane emissions is ZERO. In New Zealand sheep and beef cattle have reduced significantly since 1990, but dairy cow numbers have increased markedly leading to a small net warming effect. There are many reports from around the world that careful grazing of animals will actually increase the carbon stocks in the soil achieving the same result as tree planting. Widespread tree planting puts off the need in the short term to fundamentally change the use of fossil fuels. In addition New Zealand meat and dairy produce has the lowest carbon footprint globally for meat and dairy products. If New Zealand’s current animal production were to be produced elsewhere emissions from animals globally will increase. The net effect of covering New Zealand in pinetrees may well end up with a net global increase in warming effect.