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Scientists agree the major driver behind the rise in global greenhouse gas emissions is
human activity. How does farming fit it and,what is the contribution of animal agriculture
and how are these values calculated?
The consuming public is more and more interested in where their food comes from, what’s the
carbon footprint. What’s the carbon footprint, or the what’s
the environmental footprint, of a gallon of milk, or a pound of beef, or a pound of chicken meat?
My name is David Schmidt and I’m an agricultural engineer at the University of Minnesota and
Regional Coordinator for the national project, Animal Agriculture in a Changing Climate.
There is a significant amount of miscommunication about the role of agriculture
in climate change. Some say that animal agriculture is the largest contributor to greenhouse gas
emissions while others deny any contribution from animals. The answer lies somewhere in
between. The objective of this video is to provide you with a solid foundation of how
emission estimates are calculated and the real contributions of animal agriculture to
US and global greenhouse gas(GHG) emissions.
Carbon is all around us. It is the fourth most abundant chemical element in the universe,
behind hydrogen, helium and oxygen. The biggest reservoir of carbon is stored
in rocks- approximately 66,000 gigatons with one gigaton is equal to 1 trillion kilograms.
The second biggest reservoir is the deep ocean, and the third largest reservoir is in fossil
fuels. The atmosphere and the surface ocean are the smallest carbon reservoirs but possibly
the most important. Carbon is moving between these reservoirs constantly because of a variety
of chemical and biological processes. This is known as the carbon cycle.
The total amount of carbon that cycles in and out of the atmosphere naturally each year
is about 210 gigatons. The arrows and yellow numbers indicate this movement, or cycling
of carbon. Plants and oceans are referred to as net carbon “sinks” because they
absorb more carbon from the atmosphere than they emit. These carbon emissions occur in
the form of plant respiration and chemical exchanges with the ocean.
The red numbers indicate the human influence in the cycle, also known as “anthropogenic
emissions.” They can be mostly be attributed to the burning of fossil fuels and changes
in land use. Human activities contribute nine gigatons of carbon emissions annually. About
two gigatons of that carbon gets taken up or absorbed by the ocean. Three gigatons of
that carbon gets absorbed by plants through photosynthesis and taken up in plant soil
system. All this movement results in an annual net increase of about four gigatons of carbon
going into the atmosphere each year.
As you can see in this diagram, the amount of carbon dioxide in the atmosphere was relatively
stable for hundreds of thousands of years, at an average of around 230 parts per million.
Then about 100 years ago, the CO2 concentration in the atmosphere began climbing to where
it is right now, about 400 parts per million.
This animated diagram more dramatically illustrates the rise in carbon dioxide levels in the earth’s
atmosphere in more recent years, since 1979. The numbers on the left and right indicate
the CO2 concentration in parts per million. Again this indicates the current CO2 level
reaching up to and even beyond 400 parts per million.
While we do not intend to focus on all of the greenhouse gases in this lesson, it is
important to note that carbon dioxide is not the only greenhouse gas. The most common greenhouse
gas is water vapor, followed by carbon dioxide , methane , nitrous oxide and fluorinated
gases. Excluding water vapor, the combined sources of carbon dioxide, primarily from
fossil fuel use and land use change, make up about 77% of the global greenhouse gases.
Because these other gases trap different amounts of energy per molecule of gas, scientists
have normalized the data into something called Carbon Dioxide Equivalents, or CO2 equivalents.
This “equivalent” refers to the equivalent heating potential of the gas. This is also
known as radiative forcing or global warming potential. For instance, a single molecule
of methane will trap approximately 25 times the amount of energy as will a single molecule
of carbon dioxide. So methane has a CO2 equivalent of 25. Nitrous Oxide has a CO2 equivalent
of 298. This use of CO2 equivalents allows us to evaluate the impact of the gases on
the environment - not just the amount of these gasses in the atmosphere.
Anthropogenic greenhouse gases are emitted by many sources and from every country. Together
these nations contribute a world total of 45 thousand million metric tons of CO2 Equivalents.
This graph shows percentages of greenhouse gas emissions by country in 2012. The United
States is currently the second highest emitter of these gases, contributing about 15% of
the world total. The highest emitting country is China. However, this same information can
be evaluated based on emissions per capita. This breakdown shows the US at about 19 tons
CO2e per year per person vs China at 7.5 tons CO2e per year per person.
Taking a closer look at the sources of greenhouse gas emissions in the United States alone by
economic sector, we see that agriculture contributes 9 percent of total emissions in
the US. Total emissions in the US add up to approximately 6,673 million metric tons of
CO2 Equivalents. Agriculture’s 9% represents about 515 Million Metric Tons of that amount.
Looking at the agricultural sector itself, we can see that agricultural soil management
is the biggest source, it accounts for about 50% of total agricultural emissions. This
is followed by enteric fermentation at about 32% and manure management at 15%.
Now looking at the type of gases emitted, about 55% of the agricultural emissions are
from nitrous oxide, which is produced naturally through the the microbial process of nitrification
and denitrification of mineral nitrogen in the soil. The remaining 45% is from enteric
methane or from methane formed during the microbial breakdown of manure. Note that these
emissions are only the direct emissions of greenhouse gases occurring on the farm. Other
emissions that would occur off farm - like emissions from fertilizer production or electricity
used on the farm are not included in these numbers.
We can also look more closely at emissions by animal species. In this chart you can
see the comparisons between beef cattle, dairy cattle, swine, poultry and all other livestock.
These differences are primarily a function of total animal numbers and the contribution
of enteric fermentation. Again these are direct emissions for animal production and do not
include emissions from the production of things like animal feed.
Overall if you look at all animals in the united states for example – the beef sector
would have the greatest impact on carbon footprint of this nation but that’s only because there
are so much more beef animals than dairy animals. We have 90 million beef animals and 9 million
dairy animals, so 10 times more beef animals.
However, a better way to think about greenhouse gas emissions is in terms of emissions per
unit of production. We can look at kilograms of CO2 equivalents per kilogram of product
produced or product consumed. This evaluation includes not only direct emissions from the
farm, but also the emissions that occur after the products leave the farm. We will discuss
this further a little later in the video.This graph compares the greenhouse gas emissions
of several products on per kilogram basis. Of all the products, lamb is the highest emitter
per kilogram of product consumed, and beef is the second highest emitter at 27 kilograms
of CO2 equivalents per kilogram of beef consumed. Dairy is much lower in emissions, with 1.9
kilograms of CO2 equivalents per kilogram of milk consumed.
Before getting further into attributing emissions to different sectors of animal agriculture
or to different sources on the farm, we’ll look at the system used to measure and calculate
these emissions. There is a way to quantify greenhouse gases.
This quantification method is called LCA, life cycle assessment. It has been done for
many years and it has been done by many different groups using different methodologies.
The Life Cycle Assessment, or LCA, is an accounting method that tracks all of the greenhouse gas
emissions produced by a given process, product or system. Often this is called a ‘cradle
to grave’ analysis, because it encompasses all of the emissions in the life cycle of
the process, product or system being analyzed. This includes anything from the extraction
of raw materials to the final disposal of the end product.
Animal scientists, engineers and others can further describe the scope and mission of
the LCA as it relates to animal agriculture.
Basically, the life cycle assessment looks at the entire life cycle associated with a
product. Let’s say if McDonalds or Walmart or some other chain were to ask me what’s
the carbon footprint or what’s the environmental footprint of a gallon of milk or a pound of
beef or a pound of chicken meat produced by your company.
Most producers would have no idea – but a life cycle assessment allows you to do just
It allows you to look at the entire life cycle impact of that product. For example, the carbon
footprint of a gallon of milk includes not just enteric gases that come out the front
end of the cow or methane or other gases that come off the manure, it includes everything
– the herbicides and other chemicals applied to crops, the crops themselves, the soils
where the crops are grown, the animals, whether it is enteric gases or manure gases, It includes
the cooling of the product, the transport of product and so on. Everything from cradle
to grave of this product. The true life cycle of this product.
Life Cycle Assessment is a systematic approach for primarily accounting for environmental
impacts. It is a systems scale analysis of any product or service really. In the dairy
industry. What it means is to divide the system into supply chain stages, typically. In each
of those stages we would have what we call unit processes that have material and energy
flows, inputs and outputs from other unit processes as well as, inputs or outputs from
nature. So emission to the soil, water, or air. And the process of LCA looks from cradle
to grave.
Dr. Thoma’s analysis in 2013 of greenhouse gas emissions from the production of milk
in the United States looked at the entire life cycle of the milk supply chain, starting
with the production of fertilizer to grow feed for cows through the consumption of milk
and disposal of milk packaging.
So if we are talking about just the dairy farm so that would be what we might consider
a gate to gate analysis and we would be interested in what happens just on the farm – that
would not be considered a full life cycle assessment. So, when we did the carbon footprint
for milk, we literally had to account for the coal, the transportation of the coal,
the construction of the power plant, the losses in the transmission lines to run the refrigeration
units at the retail. So all of that is accounted for.
This table from Thoma’s LCA shows the breakdown of greenhouse gas emissions across the milk
production supply chain. The colors represent the four different types of gas emitted by
each stage in the cycle, from feeding the cows, enteric fermentation, manure management
... all the way through the consumption of milk and disposal of packaging. The pie chart
further illustrates the percentage of each activity’s contribution to milk’s carbon
Thoma’s analysis found that the CO2 equivalents produced by each kilogram of milk consumed
ranged from 1.77 to 2.4. This is about 17.6 pounds of CO2 equivalents per gallon of milk
consumed. 72 percent of those emissions occurred before
the milk left the farm gate. So from the extraction of coal, say, for the
electricity that may be used anywhere in the supply chain all the way to the emissions
associated with wastewater treatment for wasted milk that goes down the drain or the plastic
container that ends up in the landfill and may generate methane. So all of those emissions
across the entire supply chain are, we attempted to account for – tally them up then say
this is the impact.
Thoma applied the same system to an analysis of greenhouse gas emissions from pork production.
This study took into account all of the activities in the pork supply chain that contribute to
emissions, from electricity and fuel to manure and waste, across all stages of production,
from the sow barn to the consumption of the pork products produced.
The LCA showed CO2 equivalents at an average of 8.8 to 11.6 kilograms of CO2 equivalents
per kilogram of pork, from production to consumption. This can also be calculated as 2.2 to 2.9
pounds of CO2 equivalents per 4 ounce serving of pork. Approximately 60% of the emissions
occurred before the product left the farm gate.
While the LCA is widely accepted as the most useful and accurate tool for estimating a
farm operation’s environmental impact, there is some interest in learning about farm specific
variables that might affect the results. Do differences in farm size, manure handling,
farm practices and technologies, soil conditions, regional climate systems and other farm factors
impact the emissions.
When we looked at this in various ways. . . . what wasn’t clear was – oh small farms are
not as good as big farms. We saw small farms that were down in the 0.8 0.9 range. We saw
large farms that were in the 1.7 1.8. Our conclusion from that was – it is not what
you are managing but how you are managing it so the implementation of best or beneficial
management practices and care of the animals, care in the ration formulation, all of these
things contribute to the better performing farms.
Dr. Thoma’s LCA for pork production found some effect from manure management. Farms
using anaerobic lagoons had slightly higher emissions than those using deep pit systems.
As noted in the Thoma report, a full LCA encompasses many variables, and with each variable there
are some assumptions to be made. How was the electricity used on the farm produced? Was
it coal based or nuclear? Was the corn grown for feed irrigated? If so, what energy source
powered the pumps? What was the animal diet? How many piglets per sow? How far away is
the slaughter plant? the consumer market? Was the meat cooked on a gas stove or electric
stove? How much of the final product was wasted - either in cooking or off the plate? All
of these variables must be assessed and generalized for this kind of study.
Scan level LCA’s are used to help pinpoint the main emission areas of a product or process.
For swine production, about 23% of the emissions are at the consumer level and 62% on the farm
level. These farm level emissions are split primarily between manure management and feed
production. This information helps the industry and individual producers target any emission
reduction strategies.
Helping farms perform better is the ultimate goal of the LCA. International standards have
been developed for conducting an LCA, which is important so that farmers, regulators and
others can get a clear picture of farm product emissions and identify what could be done
to reduce these emissions.
Once you know what the LCA impact is of your product then you really know where you are
– if you feel your too high then you can compare to what it would be if you were to
make changes – so that you can reduce.
But if you don’t know where you are, you have no idea whether changes would work. So
is why it is so critical to have good assessment methods, because they help you to know where
you are. The analogy is - you driving on the interstate – with a car that doesn’t have
an odometer. You have no clue how fast you are going but you see speed limits everywhere.
So you have no clue – am I going to fast? You know, what am I doing here? That’s where
the industry is right now. There are regulations, some of them very strict – for example in
California, yet the producers don’t know – am I complying am I not complying? Where
shall I go? Where is the goal pole? They don’t know, that’s a situation that is untenable.
And the public is exerting extensive pressure, the legislature is, regulatory agencies are.
There is nothing simple or straight forward about tracking greenhouse gas emissions and
global carbon cycling.
It is also clear that no process or product is entirely responsible for these emissions.
It is a combination of both natural and human activity that can be evaluated on a global
scale or on an individual product scale.
We are all aware of the value of agriculture as we look forward to the challenges of feeding
an every growing human population. However, we must also understand for our role in the
production of greenhouse gas emissions.
Thanks for learning about this important topic.


生物永續與農業帶來的溫室氣體排放 (Life Cycle Assessment and Greenhouse Gas Emissions from Animal Agriculture)

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April Lu 發佈於 2016 年 11 月 3 日    Ellen Hsiao 翻譯    Mandy Lin 審核
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