DrG's Medisense Feature Article
21102-Diet_Healthy_Planet
Diet for a Healthy Planet
by Ann Gerhardt, MD
October 2021
Print Version
Bottom
Line at the Top: Eat more
plant foods and less animal
foods, especially beef, to reduce greenhouse gases that cause climate
change. What we eat has a greater impact on emissions than
does where it was produced, though both contribute to the
problem. Read on for details about how food production
contributes to greenhouse gas emissions.
Growing, processing, packaging, transporting, and preparing food
generate greenhouse gases (GHG) that damage the atmosphere.
Just choosing locally grown food doesn’t come near to solving
the problem, because GHG emissions from food transportation are far
exceeded by emissions from animals (especially cows),
nitrogen-containing fertilizer, powering farm and food processing
machinery, deforestation and burning plant residue.
Various organizations estimate that agriculture, from origin to fork,
contributes between 10 and 26 percent of total global GHGs.
Regardless of the exact numbers, all agree that agriculture has a
significant impact on GHG production, with beef and dairy cattle
contributing far more greenhouse gases per kilogram (kg) of food than
any other food source.
The U.S. Environmental Protection Administration figures that beef
cattle accounts for 37%, dairy cattle for 11.5%, swine for 4.4% and
poultry for 0.6% of total food-related GHG
generation. The rest comes from the totality of
plant food production across the globe. Except for chocolate
and coffee, whose production generates almost as much GHG as does pig
meat, plant foods generate very little GHG per kg of food.
The only foods that come close to reducing GHG by pulling carbon
dioxide (CO2) out of the air are tree foods, like nuts and fruit, but
their farming, cooling, packaging and transport generate some net
GHG.
The people who do these calculations consider more than CO2 generation
in their calculations. CO2 comprises 76% of global GHG
emissions but is the least potent. Smaller amounts of nitrous
oxide, methane, hydrofluorocarbons, perfluorocarbons and sulfur
hexafluoride are much more potent and have an outsized global warming
potential, as shown in the table below. For that
reason, scientists report GHG in kilograms of CO2
“equivalents”, or CO2e, which takes into account
different GHGs’ potency.
Greenhouse Gas (GHG)
|
Percent of total GHG
|
Average atmospheric lifetime
|
100-year global warming potential compared to
CO2
|
CO2
|
76%
|
Long, but
variable, depending on transfer to land & sea
|
1
|
Methane
|
16%
|
12.4 years
|
28-39
|
Nitrous oxide
|
6%
|
121 years
|
265-298
|
Fluorinated
gases
|
2%
|
Weeks to 1000s of years
|
Varies up to 23,500
|
A large part of GHG production from agriculture is nitrous oxide and
methane from ruminant cow digestion and animal waste, and nitrous oxide
from fertilizers. Burning crop residues produces CO2, methane
and nitrous oxide. Using fossil fuels to power farm
machinery, cooling, transportation and processing emits CO2.
The OurWorldinData organization published a graph of greenhouse gas
emissions across the food supply chain (
https://ourworldindata.org/food-choice-vs-eating-local
).
They quantified the proportion of emissions caused by changing land use
(mostly deforestation), farming, producing animal feed, processing,
transport, retail and packaging. Cattle for beef consumption,
at 60 kg CO2e per kg of product, generates more than double the GHG as
other animal food sources, including dairy cows and sheep.
After those high-GHG emitting producers, in order of decreasing CO2e,
pig meat, poultry meat, fish and egg production generate much less GHG
(between 7 and 3 kg per kg of product.)
Cattle exhale CO2, generate and burp GHG during digestion, produce
GHG-producing manure and urine, and use fossil fuel for refrigeration
and transport to market. If they eat grain feed, add GHG
resulting from nitrogen-containing fertilizers, deforestation,
processing and transportation.
Cattle emit excess GHG during digestion because they don’t
have all the digestive enzymes necessary to break down grass and plant
roughage. To accomplish digestion, it takes bacteria to turn
plants into usable nutrients as well as four sequential stomachs, each
with a special purpose. After chewing food and
swallowing, undigestible metallic or heavy material drops into the
first stomach to bypass digestion and the rest passes to the second
stomach, the rumen. They regurgitate it back into their
mouths, chew the “cud” and swallow it again and
again, up to 40% of their day, depending on the food source.
The rumen churns the pulpy food, mixing it with fermenting bacteria,
which break down plant material to absorbable fatty acids they use for
calories. Any remaining material passes into the last two
stomachs and small intestine for further digestion and final nutrient
absorption.
In addition to nutrients, fermentation in both the rumen and colon
yields CO2 and methane, which cattle belch and fart into the
air. Manure and urine also contribute GHGs: Waste
products in each are fermented by soil bacteria, generating methane and
nitrous oxide.
Some companies have created novel methods to reduce cattle’s
GHG emissions. One makes cattle masks that capture exhaled
and burped GHGs and dispose of them safely. Another group has
trained cattle to pee into a confined space, so urine nitrogen
doesn’t combine with soil and air to make ammonia and nitric
oxide. Those plans seem unlikely to scale up to
thousands of cattle across the world but they get points for
ingenuity. A more practical plan is to use manure
as plant fertilizer, at times for animal feed on the same farm,
reducing the necessity for fossil fuel-intense fertilizer
production. However, how manure is stored and applied makes a
big difference in its own GHG generation.
No one can stop cattle from burping, breathing, farting, stooling and
urinating (they would blow up). So we need super-efficient
waste management and GHG capture and currently energy intensive
conversion to a non-gaseous substance, or we stop consuming beef and
dairy foods.
What about other animals? Fossil fuel utilization, powering
heated chicken coops, farm machinery and transport to market, makes up
the bulk of emissions in the poultry industry. Birds also
breath out CO2 and require food, the production of which generates GHGs
but, compared to cattle, GHG emission is small. If poultry
farmers were to satisfy their energy needs with electricity from
renewable sources instead of fossil fuels and produce feed locally
using the birds’ manure for fertilizer, poultry would be one
of the least climate-impactful foods.
Food production for a plant-based diet is better for the planet, since
it entails less GHG emission than does an omnivorous diet, especially
one full of beef. But it is not GHG emission free.
Crop land too often comes from deforestation, causing fewer trees pull
CO2 out of the air. Using fossil fuels for farm machinery,
cooling, processing and transport generates CO2. Stagnant or
burned crop residues generate CO2, methane and nitrous
oxide. One of the greatest sources of GHG in food
crop agriculture is nitrogen-containing fertilizer.
U.S. farmers use mega-tons of fertilizer each year, containing 11.5
million metric tons of nitrogen that plants need to grow.
Just producing it requires fossil fuel.
Fertilizer’s nitrogen generates potent GHG nitrous oxide upon
contact with soil and air, especially with excessive, inefficient
use. Farmers could reduce nitrous oxide emissions just by
using lighter applications more often. They also could
obviate the need for fertilizer by using manure generated
locally. A new company named Nitricity designs
solar-powered units for farms to make their own nitrogen-based
fertilizer and deliver it minimally and efficiently in an irrigation
system. These approaches would reduce agriculture’s
climate impact.
Switching the purpose of land from grazing and growing feed to growing
human food would feed billions more people with far less impact on
climate change than we currently do, assuming we can get it to
them. About ten percent of the world’s population
is, to some degree, vegetarian by choice. Many of them eat
occasional poultry, dairy and fish in addition to the plant protein
foods, legumes, seeds, nuts and whole grains. We
don’t need heavy animal protein diets. People in
many other countries consume far less animal foods than Americans and
live productive, healthy lives.╣