Civilization as asteroid: humans, livestock, and extinctions

Graph of biomass of humans, livestock, and wild animals
Mass of humans, livestock, and wild animals (terrestrial mammals and birds)

Humans and our livestock now make up 97 percent of all animals on land.  Wild animals (mammals and birds) have been reduced to a mere remnant: just 3 percent.  This is based on mass.  Humans and our domesticated animals outweigh all terrestrial wild mammals and birds 32-to-1.

To clarify, if we add up the weights of all the people, cows, sheep, pigs, horses, dogs, chickens, turkeys, etc., that total is 32 times greater than the weight of all the wild terrestrial mammals and birds: all the elephants, mice, kangaroos, lions, raccoons, bats, bears, deer, wolves, moose, chickadees, herons, eagles, etc.  A specific example is illuminating: the biomass of chickens is more than double the total mass of all other birds combined.

Before the advent of agriculture and human civilizations, however, the opposite was the case: wild animals and birds dominated, and their numbers and mass were several times greater than their numbers and mass today. Before the advent of agriculture, about 11,000 years ago, humans made up just a tiny fraction of animal biomass, and domesticated livestock did not exist.  The current situation—the domination of the Earth by humans and our food animals—is a relatively recent development.

The preceding observations are based on a May 2018 report by Yinon Bar-On, Rob Phillips, and Ron Milo published in the academic journal Proceedings of the National Academy of Sciences.  Bar-On and his coauthors use a variety of sources to construct a “census of the biomass of Earth”; they estimate the mass of all the plants, animals, insects, bacteria, and other living things on our planet.

The graph above is based on data from that report (supplemented with estimates based on work by Vaclav Smil).  The graph shows the mass of humans, our domesticated livestock, and “wild animals”: terrestrial mammals and birds.  The units are millions of tonnes of carbon.*  Three time periods are listed.  The first, 50,000 years ago, is the time before the Quaternary Megafauna Extinction.  The Megafauna Extinction was a period when Homo sapiens radiated outward into Eurasia, Australia, and the Americas and contributed to the extinction of about half the planet’s large animal species (>44 kgs).  (Climate change also played a role in that extinction.)  In the middle of the graph we see the period around 11,000 years ago—before humans began practicing agriculture.  At the right-hand side we see the situation today.  Note how the first two periods are dominated by wild animals.  The mass of humans in those periods is so small that the blue bar representing human biomass is not even visible in the graph.**

This graph highlights three points:
1. wild animal numbers and biomass have been catastrophically reduced, especially over the past 11,000 years;
2. human numbers and livestock numbers have skyrocketed, to unnatural, abnormal levels; and
3. The downward trendline for wild animals visible in this graph is gravely concerning; this graph suggests accelerating extinctions.

Indeed, we are today well into the fastest extinction event in the past 65 million years.  According to the 2005 Millennium Ecosystem Assessment “the rate of known extinctions of species in the past century is roughly 50–500 times greater than the extinction rate calculated from the fossil record….”

The extinction rate that humans are now causing has not been seen since the Cretaceous–Paleogene extinction event 65 million years ago—the asteroid-impact-triggered extinction that wiped out the dinosaurs.  Unless we reduce the scale and impacts of human societies and economies, and unless we more equitably share the Earth with wild species, we will enter fully a major global extinction event—only the sixth in 500 million years.  To the other species of the Earth, and to the fossil record, human impacts increasingly resemble an asteroid impact.

In addition to the rapid decline in the mass and number of wild animals it is also worth contemplating the converse: the huge increase in human and livestock biomass.  Above, I called this increase “unnatural,” and I did so advisedly.  The mass of humans and our food animals is now 7 times larger than the mass of animals on Earth 11,000 or 50,000 years ago—7 times larger than what is normal or natural.  For millions of years the Earth sustained a certain range of animal biomass; in recent millennia humans have multiplied that mass roughly sevenfold.

How?  Fossil fuels.  Via fertilizers, petro-chemical pesticides, and other inputs we are pushing hundreds of millions of tonnes of fossil fuels into our food system, and thereby pushing out billions of tonnes of additional food and livestock feed.  We are turning fossil fuel Calories from the ground into food Calories on our plates and in livestock feed-troughs.   For example, huge amounts of fossil-fuel energy go into growing the corn and soybeans that are the feedstocks for the tens-of-billions of livestock animals that populate the planet.

Dr. Anthony Barnosky has studied human-induced extinctions and the growing dominance of humans and their livestock.  In a 2008 journal article he writes that “as soon as we began to augment the global energy budget, megafauna biomass skyrocketed, such that we are orders of magnitude above the normal baseline today.”  According to Barnosky “the normal biomass baseline was exceeded only after the Industrial Revolution” and this indicates that “the current abnormally high level of megafauna biomass is sustained solely by fossil fuels.”

Only a limited number of animals can be fed from leaves and grass energized by current sunshine.  But by tapping a vast reservoir of fossil sunshine we’ve multiplied the number of animals that can be fed.  We and our livestock are petroleum products.

There is no simple list of solutions to mega-problems like accelerating extinctions, fossil-fuel over-dependence, and human and livestock overpopulation.  But certain common sense solutions seem to present themselves.  I’ll suggest just one: we need to eat less meat and fewer dairy products and we need to reduce the mass and number of livestock on Earth.  Who can look at the graph above and come to any other conclusion?  We need not eliminate meat or dairy products (grazing animals are integral parts of many ecosystems) but we certainly need to cut the number of livestock animals by half or more.  Most importantly, we must not try to proliferate the Big Mac model of meat consumption to 8 or 9 or 10 billion people.  The graph above suggests a stark choice: cut the number of livestock animals, or preside over the demise of most of the Earth’s wild species.


* Using carbon content allows us to compare the mass of plants, animals, bacteria, viruses, etc.  Very roughly, humans and other animals are about half to two-thirds water.  The remaining “dry mass” is about 50 percent carbon.  Thus, to convert from tonnes of carbon to dry mass, a good approximation is to multiply by 2.

** There is significant uncertainty regarding animal biomass in the present, and much more so in the past.  Thus, the biomass values for wild animals in the graph must be considered as representing a range of possible values.  That said, the overall picture revealed in the graph is not subject to any uncertainty.  The overall conclusions are robust: the mass of humans and our livestock today is several times larger than wild animal biomass today or in the past; and wild animal biomass today is a fraction of its pre-agricultural value.

Graph sources:
– Yinon M. Bar-On, Rob Phillips, and Ron Milo, “The Biomass Distribution on Earth,” Proceedings of the National Academy of Sciences, May 17, 2018.
– Anthony Barnosky, “Megafauna Biomass Tradeoff as a Driver of Quaternary and Future Extinctions,” Proceedings of the National Academy of Sciences 105 (August 2008).
– Vaclav Smil, Harvesting the Biosphere: What We Have Taken from Nature (Cambridge, MA: MIT Press, 2013).

Home grown: 67 years of US and Canadian house size data

Graph of the average size of new single-family homes, Canada and the US, 1950-2017
Average size of new single-family homes, Canada and the US, 1950-2017

I was an impressionable young boy back in 1971 when my parents were considering building a new home.  I remember discussions about house size.  1,200 square feet was normal back then.  1,600 square feet, the size of the house they eventually built, was considered extravagant—especially in rural Saskatchewan.  And only doctors and lawyers built houses as large as 2,000 square feet.

So much has changed.

New homes in Canada and the US are big and getting bigger.  The average size of a newly constructed single-family detached home is now 2,600 square feet in the US and probably 2,200 in Canada.  The average size of a new house in the US has doubled since 1960.  Though data is sparse for Canada, it appears that the average size of a new house has doubled since the 1970s.

We like our personal space.  A lot.  Indeed, space per person has been growing even faster then house size.  Because as our houses have been growing, our families have been shrinking, and this means that per-capita space has increased dramatically.  The graph below, from, shows that, along with Australia, Canadians and Americans enjoy the greatest per-capita floorspace in the world.  The average Canadian or American each has double the residential space of the average UK, Spanish, or Italian resident.

Those of us fortunate enough to have houses are living in the biggest houses in the world and the biggest in history.  And our houses continue to get bigger.  This is bad for the environment, and our finances.

Big houses require more energy and materials to construct.  Big houses hold more furniture and stuff—they are integral parts of high-consumption lifestyles.  Big houses contribute to lower population densities and, thus, more sprawl and driving.  And, all things being equal, big houses require more energy to heat and cool.  In Canada and the US we are compounding our errors: making our houses bigger, and making them energy-inefficient.  A 2,600 square foot home with leading edge ‘passiv haus’ construction and net-zero energy requirements is one thing, but a house that size that runs its furnace half the year and its air conditioner the other half is something else.  And multiply that kind of house times millions and we create a ‘built in’ greenhouse gas emissions problem.

Then there are the issues of cost and debt.  We continually hear that houses are unaffordable.  Not surprising if we’re making them twice as large.  What if, over the past decade, we would have made our new houses half as big, but made twice as many?  Might that have reduced prices?

And how are large houses connected to large debt-loads?  Canadian debt now stands at a record $1.8 trillion.  Much of that is mortgage debt.  Even at low interest rates of 3.5 percent, the interest on that debt is $7,000 per year for a hypothetical family of four.  And that’s just the average.  Many families are paying a multiple of that amount, just in interest.  Then on top of that there are principle payments.  It’s not hard to see why so many families struggle to save for retirement or pay off debt.

Our ever-larger houses are filling the air with emissions; emptying our pockets of saving; filling up with consumer-economy clutter; and creating car-mandatory unwalkable, unbikable, unlovely neighborhoods.

The solutions are several fold.  First, new houses must stop getting bigger.  And they must start getting smaller.  There is no reason that Canadian and US residential spaces must be twice as large, per person, as European homes.  Second, building standards must get a lot better, fast.  Greenhouse gas emissions must fall by 50 to 80 percent by mid-century.  It is critical that the houses we build in 2020 are designed with energy efficient walls, solar-heat harvesting glass, and engineered summer shading such that they require 50 to 80 percent less energy to heat and cool.  Third, we need to take advantage of smaller, more rational houses to build more compact, walkable, bikable, enjoyable neighborhoods.  Preventing sprawl starts at home.

Finally, we need to consider questions of equity, justice, and compassion.  What is our ethical position if we are, on the one hand, doubling the size of our houses and tripling our per-capita living space and, on the other hand, claiming that we “can’t afford” housing for the homeless.  Income inequality is not just a matter of abstract dollars.  This inequality is manifest when some of us have rooms in our homes we seldom visit while others sleep outside in the cold.

We often hear about the “triple bottom line”: making our societies ecologically, economically, and socially sustainable.  Building oversized homes moves us away from sustainability, on all three fronts.

Graph sources:
US Department of Commerce/US Census Bureau, “2016 Characteristics of New Housing”
US Department of Commerce/US Census Bureau, “Characteristics of New Housing: Construction Reports”
US Department of Commerce/US Census Bureau, “Construction Reports: Characteristics of New One-Family Homes: 1969”
US Department of Labour, Bureau of Labour Statistics, “New Housing and its Materials:1940-56”
Preet Bannerjee, “Our Love Affair with Home Ownership Might Be Doomed,” Globe and Mail, January 18, 2012 (updated February 20, 2018)