Does Composting Contribute to Climate Change?

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Robert Pavlis

There is concern that composting produces carbon dioxide and we all know that CO2 contributes to climate change. Does this mean that backyard composting by gardeners is contributing to climate change? Is this a practice we should stop in an effort to control rising temperatures?

It is a real dilemma for gardeners and especially organic gardeners. Just when you thought you were doing something good for the planet you find out that you may be causing harm.

Does composting contribute to climate change, and is there something gardeners should do different?

Does Composting Contribute to Climate Change?
Does Composting Contribute to Climate Change?

Does Composting Produce Carbon Dioxide CO2?

Yes — but that does not mean it is a bad choice for the environment.

Does Composting Contribute to Climate Change?

The answer to this question is far more complex and really depends on how you ask the question.

In simple terms, a compost pile will produce CO2, which increases the CO2 in the air, which in turn results in the warming of our planet.

That only looks at a specific part of the whole picture. Humans produce organic waste. We grow corn and only eat the kernels. The rest of the plant is organic waste and in fact  even a good part of the kernel exits our body as waste.

Compost Science for Gardeners by Robert Pavlis

Another significant source of organic matter is edible food that is discarded. That half-eaten apple ends up as food waste that we affectionately call kitchen scraps. Roughly 1/3 of the food produced globally, is wasted. Reducing wasted food can have a much bigger impact on climate change than anything you can do about composting.

As long as we don’t get rid of humans, we have to accept the fact that we create organic waste, and that waste decomposes at some point, contributing to global warming. We can’t prevent decomposition from happening. Therefore it is important to reword the original question.

As a gardener, which method of disposal has the least impact on global warming? 

The Decomposition Process

Organic matter contains a lot of carbon, hydrogen and oxygen, along with other minor molecules. This all becomes a food source for microbes that convert the large molecules into smaller and smaller molecules. This conversion requires energy and in the process of extracting that energy, the microbes produce CO2. This is exactly the same process that takes place when humans breathe out CO2.

The amount of CO2 produced depends very much on the method of decomposition. In general, a higher supply of oxygen results in more CO2 being produced.

Other greenhouse gases are also produced during decomposition, including methane and nitrous oxide.

Global Warming Potentials

To understand this topic it is critical to understand something called global warming potentials (GWP). GWP is essentially a measure of how much heat a gas absorbs. Carbon dioxide is used as a standard and has a relative value of 1.

Methane (CH4) has a value of about 30. This means that a molecule of methane contributes 30 times more heat to the air than a molecule of CO2. Or in simple terms, from a climate change perspective methane is 30 times worse than CO2.

Nitrous Oxide (N2O), commonly called laughing gas, has a GWP of 280. It is 10 times worse than methane, and 300 times worse than carbon dioxide.

Why are these numbers important?

Since composting has to happen in some way, it is best for climate change if it happens in such a way that we reduce the production of nitrous oxide and methane, even if it means we produce more CO2. A molecule of carbon can result in either CH4 or CO2, but not both.

Organic Matter in Landfill Sites

Since composting contributes to climate change, maybe there is a better way?

For homeowners, there are really only two other options; put the material in the garbage which ends up in landfill sites, or compost it. Putting it in the green bin for municipal composting is just another form of composting.

When organic matter is added to a landfill site, it gets mixed with all other types of garbage in very large piles. These piles exclude oxygen resulting in composting that produces a lot of methane.

Some landfill sites collect the methane and recycle it into energy. Only 650 sites of the current 2,000 US sites do this and the process is only 60 to 90% efficient.

20 percent of total US methane emissions come from landfills.

Virtually any composting you do at home is better than sending the material to a landfill site because methane is so much worse than carbon dioxide.

Composting Options

You can’t stop producing organic matter so the best option is to compost it in the most eco-friendly way. In this section I’ll look at some options to see which is best for reducing climate warming.

Municipal Composting and Climate Change

The effect of this process on the environment depends very much on how it is done.

Anaerobic digestion, carries out the decomposition in large digesters without oxygen and results in the production of methane. In some cases this methane is captured and recycled. In Toronto, Ontario for example, some of these biogases (mostly CH4 and CO2) are used to heat the facility and the digesters, but some of it is just burned off producing CO2.

Other facilities use aerobic composting which produces very little methane, but does produce CO2.

This seems like a good alternative, but you have to consider the production of greenhouse gases to transport your waste to the facility, run the facility, and then transport the compost to its final destination.

If and when municipalities recapture all of the biogas and use it efficiently, anaerobic digesters this might be a better alternative to home composting, as far as the production of greenhouse gases go.

Standard Hot Composting and Climate Change

Standard hot composting uses a traditional bin that creates an aerobic condition (with oxygen). These systems produce little or no methane but do produce CO2. Procedures that make hot compost help to keep the effect on climate change to a minimum; 3 x 3 ft size, chop organic matter into small pieces, use the right C/N ratio, keep it moist but not wet, and turn it regularly.

If the compost pile gets too wet, or if not enough air gets to the middle of the pile, it will start producing more methane. This study found that with enough air flow, no detectable methane was produced. Other studies did find some methane but the amounts are normally small.

Nitrous oxide is also produced during composting (see discussion below).

Some research has been done to determine the best hot composting method, however there are so many variables and very limited testing so far.

Cold Composting and Climate Change

The reality is that most gardeners do not get their compost piles hot enough to consider the process hot composting. It is really lukewarm composting. Others just pile up fall leaves and let them compost slowly.

It is not clear how this impacts climate change, but you can expect that most of these processes occur less aerobically than hot composting, which means they produce more methane, but less CO2. From an environmental point of view, this type of home composting is less desirable than hot composting.

Direct Composting and Climate Change

There are two other approaches to composting; spreading organic matter over the ground so nature can do its thing, or bury the kitchen scrapes underground.

Dropping organic matter as thin layers on top of the soil , what I like to call the cut and drop method, will produce very little methane because decomposition is very aerobic.

Burying the organic matter can be a problem. If it is buried too deep, it starts decomposing anaerobically, producing methane. The amount produced depends on the soil type. Coarse soil with good oxygen levels are less of a problem, but heavy clay soil with low oxygen could be a problem. Keeping the organic matter closer to the surface will help keep it more aerobic.

Bokashi and Climate Change

Bokashi is not a composting process. It is really a form of fermentation and at the end of the process the ferment produced still needs to be composted using one of the other methods.

Vermicomposting and Climate Change

Contrary to common belief, the process taking place inside earthworms is not a composting process. The earthworm’s gut grinds up the organic matter, and there is some preliminary composting taking place, but most of the composting takes place after the worm casts exit the worm. For this reason, the gases produced are mostly dependent on what you do with the casts.

In 2007 newspaper reports appeared with headlines such as ” Worms are Killing the Planet”. Some released research suggested that worms produced high levels of nitrous oxide. This research work has been reviewed and does not seem to have been well done. Worms are not high producers of greenhouse gases.

Another study compared three home composting systems and concluded that the total CO2 equivalents of nitrous oxide and methane produced was lower in vermicomposting than aerobic composting, which was lower than anaerobic composting. These results for vermicomposting and aerobic composting was confirmed by a study in Holland which also reported  that when CO2 production was included in the calculations, aerobic composting was better than vermicomposting.

These types of studies generally exclude the production of CO2 in calculating greenhouse gases because the amount produced is relative to the quantity of carbon in the starting waste material – measuring it, mostly measures how quickly it is produced. Vermicomposting produced more CO2 because it is a more complete process, (ie faster) during the test period.

The key point is that vermicomposting produces less nitrous oxide and methane, making it better for the environment.

The Elephant in the Room: Nitrous Oxide

Since nitrous oxide is so much worse for the environment than CO2, you would expect that this gets more attention, but that is not what I found. Few even mention it. I think in part this is due to the fact it is a more recent concern.

Nitrous oxide production is low during the hot composting process, but may become higher during the curing process (once the pile cools down). It may also be high once applied to soil.

In 2006, the Intergovernmental Panel on Climate Change (IPCC) suggested that N2O emissions during the composting process are 3% of the nitrogen in the waste but this is based mostly on manure systems, not yard waste. Recent reports suggest the number might be significantly lower than this, in which case it’s production from composting is not a major factor in climate change.

Compost piles tend to generate most of the nitrous oxide at the end of the process. Keeping nitrogen levels low may help keep emissions low. Adding manure, which is generally higher in nitrogen increases the production of nitrous oxide. Aeration keeps levels lower. This study found relatively low amounts of nitrous oxide produced in compost piles.

 The Carbon Cycle

Where does the CO2 produced by composting come from? It comes from the stored carbon in the organic material, which in turn  came from the air. It is important to understand that in order for composting to create CO2, the CO2 has to first be taken out of the air. Composting is simply returning carbon to the air, not adding additional amounts.

Not only that, but the CO2 released from composting is only a small part of the carbon in the compost. Most of the carbon remains in the compost and is added back to soil. This form of carbon is commonly called humus, but in my previous post I explained that humus does not really exist, but the carbon in soil does exist.

One of the main contributors to excess CO2 in the air is the loss of carbon from soil, as a result of traditional agriculture. Agricultural soils, and therefore much of the soil around recently built homes is very low on carbon. If we all grow plant material, compost it, and return it to the soil, there would be a net decrease of CO2 in the air and an increase in soil organic matter.

Recent studies show that backyards can be significant contributors to storing carbon in soil.

Reduced Fertilizer Production

Some claim that using your own compost reduces the use of fertilizer, thereby further reducing the impact on climate change due to manufacturing.

This may be true for a vegetable garden, but your landscape gardens don’t need any fertilizers in the first place. They will benefit from compost, but don’t need it in many cases.

Does Composting Contribute to Climate Change?

Composting does contribute to global warming because it produces CO2, but that is not really the important point. We have to look at the whole gardening process. Let’s reword this question to a more practical question. Does the process of growing things, followed by composting, contribute to climate change? Provided composting is done properly, the answer to that question is NO.

It is important that your composting process produces as little methane and nitrous oxide as possible. It is also important that you use as few purchased ingredients as possible – skip the amendments, fertilizer, and unnecessary gardening gear. Instead fertilize with your compost.

We can’t stop producing organic matter and so our goal is to find the best way to return its carbon back to the soil. Traditional hot composting is the best choice for most gardeners.

If you live near an anaerobic composting facility that completely recycles the produced biogases, it might be an eco-friendly option. Ideally, this facility returns compost to your garden once a year so that you can increase the carbon in your soil. But I suspect these facilities will never be as eco-friendly as backyard composting.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

15 thoughts on “Does Composting Contribute to Climate Change?”

  1. For the person going back and forth with news people, they probably aren’t the people to be conversing with. Go to the source which in this case your state university should have a climatologist PhD type person. One thing I learned by reading through the blog of the one we have in IL, is that most interestingly he pointed out that climate change had made our winters warmer than our summers for a variety of complex system reasons. I thought about it and he was def right! I haven’t been able to build a snow fort in like 15 years. Ugh fml.

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  2. very very interesting article, i walked away with significantly more knowledge then when i started. i would pose the other possible way to treat organic waste, the new idea is to salt and bury, this will actually trap the Carbon underground to be stored for thousands of years. This is the cheapest way to pull Carbon out of the air. Remove salt and bury the organics and plant replacement organics

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  3. Hi… What is missing from this is whether the impact of changes in home gardening behavior could have any notable impact on total CO2 emissions. The amount of energy being produced to keep this blog around and used in reading this post is certainly creating more co2 than would be saved of one gardener did this perfectly.

    There are much bigger levers to address. Messing around with stuff is so miniscule in total impact that the effort is unjustified.

    Especially when you consider the mixed positive effects of global warming on gardening I am not sure this whole topic has helped anything.

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    • This topic is fairly new and worth discussing so people start to think about and understand the effects of the larger scale factory farming, even organic. This might sound off topic, but there is a large movement back to smaller farms, using no till farming which reduce CO2 and sequesters carbon in the soil, which also reduced water usage because soils are more fertile and crops require less water to be productive, etc. I know this might sound like a lot of BS, but it takes new ideas, then trials and studies are showing it actually works. This might be off topic, but most dairy farmers did not use to cover their cow manure piles in the CA central valley until it was required. Buildings would have insulation in the walls and ceilings if it wasn’t required by code, etc, As one instructor explained, it no just one thing that makes the difference, there is no silver bullet, just a lot of silver Beebe’s. This is part of a holistic movement to improve agriculture, and make more money per acre, which is the trend, and required for small farms to survive. See Singing Frog Farm in Sabastopol CA and Ridgedale Regenerative Organic Farm, in Sweden. Its working . .and they need a lot of compost.

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      • Richard Perkins at Ridgedale in Sweden really knows his stuff. I follow him on IG and just bought his book “Regenerative Agriculture”.
        Personally I believe more CO2 in the atmosphere will be good for all plant life, including those we eat. I’ve read a book on the topic:
        “Food, Climate, and Carbon Dioxide” by Sylvan H. Witter. It was published in 1996 and represents a lifetime of research by the author. Highly recommended if you can find it, since it is out of print.
        CO2 saturation will naturally limit the ultimate warming from CO2 since these molecules can only absorb a small sliver of the infra-red energy being radiated away from the planet. Once all of this IR sliver is absorbed further CO2 concentrations can have no effect. This is commonly known in the scientific community but almost never discussed.

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  4. A very good attempt to explain a highly complex process without going into too much detail. I would add that all organic things die and end up “composting” somehow. Humans tend to pile it up and try to forget about it. Anything that biodegrades in an aerobic process is far better than the human high-volume landfill which is anaerobic and causing the GHG. I believe you are correctly trying to dispell paranoia about carbon dioxide in a cycle where co2 is mandatory and never goes away (nor is a negative).

    The bit I believe you are missing is that compost (aerobic) is not the final product and is not a fertilizer by itself. The presence of bacteria, fungi, microathropods in the compost are what bring nutrients to the soil using the correctly composted material as a food input.

    I live in Toronto and have used the so called compost which comes from composting after the Disco Road anaerobic digestion facility. I commend their ability to properly deal with so much volume of green waste is front running and positive. However their input material to the compost facility results in a bad product which is high in salts and is caustic in the garden even at small amounts. The end product was even delayed for many years while trying to solve the problem of using anaerobic material and I feel it still has not been solved. Their issue is how to deal with the volume of green waste and their selected anaerobic process is better suited to biogas production than an agricultural nutrient input. I think one has to choose the method based on the desired output which will then dictate the method to be used. Getting rid of massive amounts of city waste is not the same desired output as having a nutrient rich input to growing food or gardening. Alas the world is imperfect.

    Those that are interested in doing a correct and proper hot compost should be encouraged to do so and not worry about co2 contribution to global warming. That hot compost process is the same that mother nature utilizes in the best way. If a human can divert waste from landfill into compost on this manner, their carbon offset is greatly reduced, especially if that output goes into food production exactly as mother nature intends it to be.

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    • The one point I would disagree with is that “hot compost process is the same that mother nature utilizes” – nature uses cold composting.

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    • Interesting, but I’m not convinced. The statement “Glomalin eluded detection until 1996 because, “It requires an unusual effort to dislodge glomalin for study: a bath in citrate combined with heating at 250 F (121 C) for at least an hour.” sounds a lot like the discussions in the past about humus – can’t find it in soil until soil is treated in harsh ways.

      From Wiki: “The specific protein glomalin has not yet been isolated and described” and “The chemistry of glomalin-related soil protein (GRSP) is not yet fully understood, and the link between glomalin, GRSP, and arbuscular mycorrhizal fungi is not yet clear.[4][3] The physiological function of glomalin in fungi is also a topic of current research.[7]”

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  5. I watched a forum of Skeptics and Alarmists some time back. The panel mostly of well known Scientists. When Methane in the atmosphere was mentioned, one of the skeptics said that Methane in the atmosphere doesn’t last, that it is oxidized. None of the alarmists challenged this, so I drew the conclusion that it was fact. However, I never researched it.

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    • Yes, methane stays in the air about 10 yrs, CO2 for possibly hundreds. If we immediately stopped emitting both, that means we would soon not have to worry about added warming from methane, but only from CO2.
      Unfortunately, we aren’t stopping. So the fact that methane is so much more powerful than CO2 is a continuing cause for concern.
      Even if we did stop emitting, the methane already in the air would continue causing warming for most of a decade. And there are many types of ‘feedbacks,’ meaning that warming itself leads to more warming (e.g. warming causes permafrost to melt, and melting permafrost itself emits both methane and CO2, causing even more warming). So the added warming from methane can continue causing problems even after the methane itself is removed from the atmosphere.
      These feedbacks are actually why climate scientists aren’t alarmist, but the opposite. Feedback mechanisms are difficult to model exactly, and scientists don’t like to claim anything they can’t be accurate and certain about. So they actually tend to understate the likely future consequences of our continued emission of methane and CO2.
      The only safe path for humanity is to stop the emissions, so we don’t have to worry about the direct warming from them or the warming caused by the many feedbacks.

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      • ow many news outlets hype climate change. At the end of October 2019 here in Orlando news outlets were busy with headlines. The hottest October in history. To most consumers of news, I’m going to guess, that they assume this means the high temperatures for the total month is higher than the high temperatures for the month of any year in history. But if that were true it is all but impossible that October could have been the hottest month in history. We used to have records available for almost all significant cities online here in the US. I assumed NOAA would not allow this info to continue to be available to the public. So, I copied Orlando’s records to a spread sheet and yes the records are no longer available to the public. There are 16 records highs still in the records for Orlando for the month of October, all from the same year and that year was 100 years ago, i.e. 1919 and in 2019 there was only 1 record high recorded in Orlando. Yes, it was a hot October, but not that hot. What the news outlets almost always forget to do is to explain, possibly they don’t know, how their results was derived. Turns out what they were really reporting is the average high. Meaning they were also bringing in the low temperature for the day into the calculations. I assume this info comes from NOAA but I am not sure. I noticed a number of years ago the morning lows regularly run higher than the average low temperatures for the day, but the afternoon highs do not. In 1919 Orange County, FL had a population of approx 20,000, now it has a population, I believe, around 1,400,000. In 1919 most roads were still dirt, now we are a massive network of concrete and asphalt roads. I assumed that this combined with a massive number of sprinklers going off early in the morning, boosting the humidity in the air, and heat retention, would add to morning highs being higher today. At one point I got an opportunity to exchange emails with a local reporter from a major news outlet in our area. He said that Orlando and other large cities were referred to as giant Heat Islands, but he insisted that rural morning temperatures are higher today as well. I suspect he was at least partially correct, however there is little proof given that we don’t have any records for rural areas. On another point, I’m sure when news report on temperatures in Florida as a whole, they include multiple cities that have little history, meaning approx 50 plus years. When making absolute statements about record temperatures and one is mixing cities th this small amount of history about temperatures, in a said to be 4 ½ billion-year-old planet, it seems a bit suspicious. To use a few years and run a trend line most likely using regression and then reporting this as a complete truism without explanation seems a bit shaky to me. But at the same time, I realize that Orange County Florida is only one spot on the globe and one sample.

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