Fueled by increasing demand for meat and dairy, growth in livestock production could have serious and irreversible effects on the entire planet.
By 2050, growth in livestock production could generate as much as 70 percent of the greenhouse gas emissions considered to be a safe threshold for the planet.
That leaves little room for other sources of greenhouse gases, which account for over 80 percent of emissions.
In 2000, livestock production accounted for about 14 percent of global greenhouse emissions.
If demand for meat, poultry, eggs and dairy keeps pace with projections, by 2050 the environmental consequences of livestock production could be responsible for 70 percent of the total greenhouse gas emissions considered a safe threshold for the planet, new research says.
This leaves little room for all of the other sources of greenhouse gases, such as transportation and electricity, which now account for more than 80 percent of emissions.
Livestock could generate an even greater proportion of the sustainable threshold for other environmental indicators, the researchers report online today in Proceedings of the National Academy of Sciences.
"It's sobering," said study lead author Nathan Pelletier who did the work with Peter Tyedmers while at Dalhousie University in Halifax, Nova Scotia.
"We're not suggesting that everyone in the world become vegan or vegetarian," he emphasized. "We really stress the importance of policies aimed at production and consumption over time by changing not just how much we eat, but what we eat and how frequently we eat it."
The pair considered three aspects of global livestock production: greenhouse gas emissions, biomass consumption and nitrogen emissions. They looked at estimates of current and future levels for each of these and compared them with projections for the Earth's limits, beyond which these systems may become dangerously or irreversibly out of whack.
For greenhouse gases, a 2006 United Nations report estimated that livestock production in 2000 produced about 14 percent of global greenhouse gas emissions, largely via nitrous oxide emissions from fertilizer used to grow feed and from manure and via digestive methane emissions from cows and other ruminants.
Meanwhile, many scientists agree that a global average temperature increase of two degrees Celsius (3.6 degrees Fahrenheit) is the threshold past which serious and irreversible negative impacts of climate change are likely to occur.
Based on that threshold and U.N. projections for future global livestock demand, the team calculated that livestock would account for around 70 percent of the greenhouse gas emissions in 2050 that would keep the planet below the two degree threshold.
Similarly, Pelletier and Tyedmers determined that future livestock demand would consume 88 percent of all of the biomass that humans can safely harvest from the earth's lands.
Humans already are exceeding the third safety threshold, reactive nitrogen production, according to other researchers. The new study projects that this threshold would be surpassed by almost threefold.
Reactive nitrogen is nitrogen that is taken out of its inert form in our atmosphere and converted into ammonia or nitrate that plants can use -- either through fertilizer production or by soil microorganisms.
Exceedingly high levels of this type of nitrogen contribute to coastal dead zones, smog and acid rain, but "we can't produce food without it," Pelletier noted.
The team tested a few scenarios to determine how to reduce livestock's proportion of the future environmental burden. Switching all beef to more resource-efficient poultry only reduced impacts by 5 to 13 percent.
Technical improvements such as making fertilizer use and manure management more efficient will not be enough, the team said.
"If our models are even close and these sustainability thresholds are right, these technical measures are not going to be enough to allow us to remain within these sustainability thresholds," Pelletier said.
The only scenario the team tested that brought reactive nitrogen levels below the sustainability threshold by 2050 was one where humans' recommended protein consumption was provided entirely by vegetable sources. (The team used soy in their calculations.)
"We stressed that that's not realistic. We chose it in order to show the range of impacts," Pelletier said.
"Global livestock is an underappreciated point of the global sustainability problem," Helmut Haberl of the Institute of Social Ecology in Vienna told Discovery News.
"One of the most important sustainability goals is to feed all humans," he said. "We have to be sure of that first. At the same time, when we have one billion undernourished people, we probably have one billion people who eat too much and particularly too many animal products. I think we have to face the challenge of thinking about our lifestyle in terms of what we eat."