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Humanity and its Impacts on Our Planet; Can We Develop Sustainably?

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We are said to be mere stardust.  Most of the elements that make up our bodies were initially created deep within stars and distributed through the cosmos during supernovas when stars collapsed and died.  Some people stress we are 60% water, which makes us more substantial than jellyfishes (95% water) but less dense than oak trees (~50% water).  We are formally the wise ape, Homo sapiens, having descended from earlier relatives about 200,000 years ago in Africa, and having since then spread throughout the world, becoming the top predator everywhere.  Sometimes we are just the naked ape with smaller than average canine teeth, larger than average penises, relatively modest strength for our size, and intelligent without question.  Some people think we are sufficiently superior to all other living things that we should be viewed as separate, created in God’s image.  Most people assume we are the Masters of this planet, its owners, sometimes even the beings for which the universe was created.

A different perspective sees us as the most destructive force now acting on our planet, a rival in power to that of the giant asteroid which slammed into the Yucatan Peninsula 66 million years ago.  It arrived with a destructive force sufficient to cause a nuclear winter lasting several years, disrupting photosynthesis, collapsing foodwebs, and leading to the extinction of over 75% of all species on earth including all dinosaurs other than birds.  The asteroid, about 12 km in diameter, destroyed in a matter of years while we have been slowly ramping up the intensity of our impacts over the last 50,000 years and especially during the last 50 years. 

Initially, we destroyed by over-hunting large species we discovered as we expanded our range beyond Africa.  Naïve to the capabilities of this puny, bipedal ape, the megafaunas outside Africa fell to us as we advanced across the world, first across Asia and into Australia, later into Europe, and finally into North and South America.  One by one the larger mammals, reptiles and birds disappeared from the fossil record shortly after humans arrived.  All might have ended there, except we developed culture and agriculture, and thus began the initially slow but inexorable increase in our numbers and the resulting growth in our technology and energy use which continues even today.

Today, with 8 billion of us living across this planet, we have captured ~25% of all terrestrial biological production to human uses, commandeered 80% of the land surface for our own needs, and correspondingly reduced the abundances of other species by over-harvesting and usurpation of habitat.  Our ingenious fishing methods, including the use of nets large enough to swallow jumbo jets whole which we drag destructively across the sea floor, mean our impacts are not limited to the land.  Overall, the extent of resulting change in distribution of biomass is enormous.  Evidence to support these claims is abundant.

Some Supporting Data

For example, Fridolin Krausmann, a professor in the Institute of Social Ecology at the University of Natural Resources and Life Sciences or BOKU (for  Universität für Bodenkultur) in Vienna, Austria, teamed with 8 colleagues in 2013, producing an article in PNAS (Proceedings of the National Academy USA) that dealt with the extent to which humanity has appropriated primary production globally. 

Net primary production is the total amount of organic matter created by plants via photosynthesis (measured as mass of carbon in organic products) minus that used in the plants’ own respiration.  NPP supports all other living organisms other than certain bacteria that metabolize ferrous compounds rather than organic foods in places like deep ocean thermal vents.  Krausmann and colleagues performed a detailed, global analysis starting with what NPP would have been over the past century in the absence of humans, their agriculture and economic activity.  This requires some assumptions concerning the ecosystems that would be present were we not farming or building infrastructure.  They then determined the actual NPP using global data sets on primary production and habitat type across the planet, and computed losses in NPP in places where forests had given way to farmland or to urban development.  Finally, for specific regions or the entire planetary land surface, they determined the percentage of NPP appropriated by humans as crops, livestock, forest products, and losses in primary production due to land use changes.  This total human-appropriated NPP was expressed as a percentage of potential NPP in the absence of our activities.  (I’ve gone into all these details to show the care with which they did their calculations.)

Potential global NPP has increased slightly since 1910 due to climate warming and the photosynthesis-enhancing effect of increased CO2 in the atmosphere.  Krausmann and colleagues estimate global NPP as 52 gigatonnes carbon (gtC) per year in 1910 and 59 gtC per year in 2005.  But human-appropriated NPP has increased 116% over that same time, to reach 14.8 gtC in 2005, or 25% of the potential NPP in our absence. 

Putting these results in context, our species has become so abundant that our activities to produce crops, meat and forest products for our own use are taking up a quarter of all the primary production the land surface of this planet is currently able to provide.  While the rest of nature is left with the other 75%, our appropriation of NPP has more than doubled since 1900.

This rather colorful map shows human-appropriated NPP from 1910 to 2005 for five world regions.  The yellow HANPPharv is the gigatonnes carbon we harvest each year, and the green HANPPluc is the gigatonnes carbon being lost because of our land-use changes.  The red line is the total HANPP as a percentage of potential total NPP.  Figure 3 © F. Krausmann & PNAS.

As their map reveals, there are differences among regions in the extent to which we have captured NPP to our own use, with Asia showing the most rapid growth while ‘western industrial’ countries show negligible change in the percentage since 1910.  These differences are largely attributable to differences in rates of population growth and urbanization in these regions.

More Data

That we have appropriated most of the planet’s land surface and reduced the abundances and biomass of other animals has been widely reported.  In 2019, Yinon Bar-on of the Weitzmann Institute, Israel, and two US colleagues reported, also in PNAS, on the distribution of biomass among components of the living world.  In doing so they also report on the changes in human biomass vs that of other organisms in the recent past.  If we were to weigh all the carbon in all animals, plants, fungi, algae, microorganisms and bacteria on the planet, we’d have an astounding 550 gtC.  The vast majority of this biomass is in plants (450 gtC), and because plants are almost entirely terrestrial, the bulk of biomass is on land.  Animal biomass, a mere 2 gC, is primarily marine, while bacteria (70 gtC) and other forms of life (7 gtC) are primarily in soils and permeable rocks.  Of that animal biomass, humans comprise just 0.06 gtC, a tiny fraction of the total.

And yet… we humans have managed to appropriate 25% of the net primary production of the planet to our own uses!  Our livestock account for 0.1 gtC, and the 0.16 gtC which is us and our livestock together dwarfs the total biomass of all other mammals on the planet (0.007 gtC).  Incidentally, 0.004 gtC of that total is marine mammals – the whales, sealions, dolphins and so on – meaning wild terrestrial mammals – all the squirrels, racoons, deer, moose, giraffes, rhinoceros, elephants and so on – comprise just one twentieth of the mass of humans or 0.003 gtC.

The Growth in Our Impacts

It did not use to be this way.  Beginning 50,000 years ago when we first emerged from Africa and continuing up to the present as we strain the last remaining fishes from the ocean, humans have been responsible for an enormous reduction in the abundance of large animals.  As well as causing the extinctions of most larger fauna on all continents outside Africa, we have reduced abundances of remaining species worldwide.  About half the land animals that were wolf-sized or larger became extinct, and the total biomass of wild land mammals is estimated to have been reduced from about 0.02 gtC before the megafaunal extinctions began to 0.003 gtC today.  That is a seven-fold reduction.  It is perhaps difficult to visualize the sheer abundance that used to be present and has now been lost.  In the oceans where our depredations have been much more recent, it is estimated that the biomass of fishes swimming about today is about one tenth of the amount as recently as the 1940s.

Our Built Environment

Let me close with one trivial item regarding our impacts on the planet.  It has to do with biomass in a sense.  In December 2020, an article appeared in Nature written by Emily Elhacham and colleagues, including Yinon Bar-On.  Their topic was the global mass of materials created by humans – all the cement, bricks, steel, plastic and so on, but not including our use of forest products (which is part of biomass).  They term this anthropogenic mass and calculated all such mass built including mass subsequently demolished or taken out of service which they term anthropogenic waste mass.  To standardize, they used total dry weight of products.  They state that global biomass, measured in total dry weight, rather than in mass of carbon is now approximately 1.1 tera tonnes mass (that is 1012 tonnes), and they show an astounding growth in anthropogenic mass from about 3% of global biomass in 1900 to equaling or surpassing global biomass in 2020.  We have not added 1.1 tera tonnes of mass to the planet, of course.  But through our mining and chemical industries we have recombined raw materials to make vast quantities of concrete, aggregate, bricks, asphalt, metals, plastics and other materials.  Yet another way in which we have been radically altering the planet we share with the rest of the biosphere.

To suppose that we are anywhere close to living sustainably on the planet requires that we assume the great majority of other species were orders of abundance too common before we started rearranging the abundances of species on the planet.  That is clearly unreasonable, but if we accept the enormous shifts that have taken place, we have to ask ourselves what we mean when we talk about sustainable development.  Mostly we don’t talk about this, but when we do we usually assume that keeping the world much as it is at the present, or maybe a decade or so ago would constitute sustainable living.

This graph examines the growth in what Elhacham refers to as anthropogenic mass over the last 120 years.  Global mass is plotted in teratonnes – 1012 tonnes — dry weight, and compared to global biomass also measured as teratonnes dry weight.  We’ve built a lot of stuff.  Image © E. Elhacham & PNAS.

So What Does This All Mean For Sustainable Development?

Reflect for a moment.  Think about this from the perspective of that other species trying its best to carve out a life in our shadow.  Over just a short number of years, say since 1950 – some snapping turtles in the lakes near where I live would have been young turtles back then – more and more habitat has been removed for human use, and numbers of your species have become greatly reduced.  Many local populations no longer exist.  Would you still consider humanity to have embarked on an exercise in sustainable living if there was no replenishment of your habitat, no opportunity for recovery of your former abundance?  And do we really suppose that the functioning of ecological systems has not been massively changed by these rearrangements of biomass and net primary production?  Humans have massively altered the planet. 

We seldom even wonder about whether these massive changes have created problems for the functioning of ecological systems (although some environmental scientists express real concern about the likelihood that we have brought the planetary system close to one or more tipping points that, once reached, will shift the system into a new domain with radically different features to what we have been accustomed to throughout human existence).  I think most of us are living in a fool’s paradise, unconcerned about the changes we have wrought, perhaps acknowledging the need for sustainable development, yet seldom (never) talking about serious efforts to repair what has been lost. 

In other words, both those of us who do not care about our impacts on the planet, and the great majority of those who do care, do not see any need to attempt even a modest, partial repair of the damage we have done.  It’s like we are flying a plane from which we have haphazardly removed most of the rivets.  Some of us figure we can go on removing rivets because ‘nothing bad has happened yet’.  Others think we should try to stop removing rivets.  Hardly anyone is saying, ‘wouldn’t it be a good idea to put some of those rivets back’?  Home sapiens, the wise ape indeed.