Kurzgesagt – In a Nutshell
Sources – Last Human
We thank the following experts for their help with this script:
Toby Newberry
Future of Humanity Institute, University of Oxford
Anders Sandberg
Future of Humanity Institute, University of Oxford
Dr. Korbinian Rüger
Researcher in Practical Philosophy and Ethics at LMU Munich
Because of the potential size of the future, the most important thing about our actions today might be their impact on future generations. This simple-sounding idea has some surprising moral implications: https://80000hours.org/kurzgesagt-last-human (Note: Since we did our own calculations in our video, the numbers in the source may differ from ours.)
Every generation assumes they’re important enough to witness the apocalypse and then life just goes on.
There is a phenomenon, called societal pessimism (sometimes also called “social pessimism”), which describes why we as a society as a whole are rarely truly optimistic. The following investigation provides a possible explanatory approach: we look at processes that we often think are beyond our control.
#de Vries, C. E. & Hoffmann, I. (2020): The Optimism Gap - Personal Complacency versus Societal Pessimism in European Public Opinion. Bertelsmann Stiftung.
https://www.bertelsmann-stiftung.de/de/publikationen/publikation/did/the-optimism-gap-all
Quote: “Societal pessimism describes the concern that society is in decline and heading in the wrong direction. Interestingly, a rather substantial quantity of people think that their country is not doing well overall, while still being generally quite satisfied with and hopeful about their own lives. This gap between societal pessimism and personal optimism suggests that one source of the considerable anxiety felt by so many people might be the perception that many of the processes that go on outside the bounds of their daily lives and experiences are so complex that they cannot do much about them.”
Modern humans arose some 200 thousand years ago. They were uniquely good at making tools, telling stories, thinking abstractly, planning and working together in large groups beyond their close family.
Here we give only a rounded order of magnitude. The research varies approximately between 200,000 (regarding fossils named “Omo I”) and 300,000 years (hominin fossils from Jebel Irhoud). Quite current research assumes about 233,000 years (with deviations upward and downward of approx. 22,000 years). At the same time there are other results which assume about 315,000 years.
#Vidal, C. M. et al. (2022): Age of the oldest known Homo sapiens from eastern Africa. Nature
https://www.nature.com/articles/s41586-021-04275-8#citeas
Quote: “Here we report geochemical analyses that link the Kamoya’s Hominid Site (KHS) Tuff, which conclusively overlies the member of the Omo-Kibish Formation that contains Omo I, with a major explosive eruption of Shala volcano in the Main Ethiopian Rift. By dating the proximal deposits of this eruption, we obtain a new minimum age for the Omo fossils of 233 ± 22 kyr.”
#Richter, D. et al. (2017): The age of the hominin fossils from Jebel Irhoud, Morocco, and the origins of the Middle Stone Age. Nature 546
https://www.nature.com/articles/nature22335
Quote: “Here we report the ages, determined by thermoluminescence dating, of fire-heated flint artefacts obtained from new excavations at the Middle Stone Age site of Jebel Irhoud, Morocco, which are directly associated with newly discovered remains of H. sapiens. A weighted average age places these Middle Stone Age artefacts and fossils at 315 ± 34 thousand years ago.”
#AAAS (American Association for the Advancement of Science)(2022): University of Cambridge press release - Earliest human remains in eastern Africa dated to more than 230,000 years ago
https://www.eurekalert.org/news-releases/939551?
Quote: “‘Unlike other Middle Pleistocene fossils which are thought to belong to the early stages of the Homo sapiens lineage, Omo I possesses unequivocal modern human characteristics, such as a tall and globular cranial vault and a chin,’ said co-author Dr Aurélien Mounier from the Musée de l’Homme in Paris. ‘The new date estimate, de facto, makes it the oldest unchallenged Homo sapiens in Africa.’”
It took us 150,000 years to grow to a population of 2 million. Improvements were gradual and eventually led to the agricultural revolution, arguably the biggest change in our history. This was when our numbers really started growing. It took ten thousand more years to get to 300 million.
#Kaneda, T. & Haub, C. (2021): How Many People Have Ever Lived on Earth? Population Reference Bureau PRB
https://www.prb.org/articles/how-many-people-have-ever-lived-on-earth/
Here, too, there are only very rough estimates. If one looks e.g. still a few thousand years further, around the period of the end of the last ice age (around 10,000 B.C.), the population numbers vary strongly, from one million up to 20 million.
Not only are the individual estimates different, they are also constantly changing.
For example, there is a database called HYDE (History database of the Global Environment), which in its version 3.1 gives 2 million people for the year 10,000 BC, but in an updated version (on which OWID's chart is based) gives 4 million.
#Goldewijk, K. K. et al. (2010): Long-term dynamic modeling of global population and built-up area in a spatially explicit way: HYDE 3.1
#OWID (2019): World Population Growth
This revolution, also called the Neolithic Revolution, was particularly characterized by the cultivation of plants and the domestication of animals. It began about 12,000 years ago, probably triggered by climate change and population growth.
New survival strategies, such as stockpiling, meant that people no longer had to constantly change their location. Permanent settlements emerged, the environment was changed (e.g., arable land), new technologies emerged, and art and culture grew. At the same time, new diseases and conflicts emerged.
#Herrera, R. J. & Garcia-Bertrand, R. (2018): The Agricultural Revolutions. Ancestral DNA, Human Origins, and Migrations
https://www.sciencedirect.com/book/9780128041246/ancestral-dna-human-origins-and-migrations
Quote: “The agricultural revolution is the name given to a number of cultural transformations that initially allowed humans to change from a hunting and gathering subsistence to one of agriculture and animal domestications. Today, more than 80% of human worldwide diet is produced from less than a dozen crop species many of which were domesticated many years ago. Scientists study ancient remains, bone artifacts, and DNA to explore the past and present impact of plant and animal domestication and to make sense of the motivations behind early cultivation techniques. Archeological evidence illustrates that starting in the Holocene epoch approximately 12 thousand years ago (kya), the domestication of plants and animals developed in separate global locations most likely triggered by climate change and local population increases. This transition from hunting and gathering to agriculture occurred very slowly as humans selected crops for cultivation, animals for domestication, then continued to select plants and animals for desirable traits. The development of agriculture marks a major turning point in human history and evolution. In several independent domestication centers, cultivation of plants and animals flourished according to the particular environmental conditions of the region, whereas human migration and trade propelled the global spread of agriculture. This change in subsistence provided surplus plant food that accumulated during the summer and fall for storage and winter consumption, as well as domesticated animals that could be used for meat and dairy products throughout the year. Because these new survival strategies no longer required relocation and migration in search of food, humans were able to establish homesteads, towns, and communities, which, in turn, caused rapid increases in population densities and lead to the emergence of civilizations. This dependence on plant and animal domestication entailed a number of other environmental adaptations including deforestation, irrigation, and the allocation of land for specific crop cultivation. It also triggered various other innovations including new tool technologies, commerce, architecture, an intensified division of labor, defined socioeconomic roles, property ownership, and tiered political systems. This shift in subsistence mode provided a relatively safer existence and in general more leisure time for analytical and creative pursuits resulting in complex language development, and the accelerated evolution of art, religion, and science. However, increases in population density also correlated with the increased prevalence of diseases, interpersonal conflicts, and extreme social stratification. The rise of agriculture and the influence of genetics and culture (gene–culture coevolution) continue to affect modern humans through alterations in nutrition, predisposition to obesity, and exposure to new diseases.”
But that increase was dwarfed by the industrial revolution. In 1800 there were a billion of us. The human population doubled in just 120 years and then again in fifty. Today, we number around 8 billion.
We show charts from 1700 onwards here starting from the Industrial Revolution, which happened around the middle of the 18th century.
In 1805, the 1 billion mark was reached. By 1925, this number had doubled to 2 billion, before doubling again by 1974 (to 4 billion), but this time in a much shorter time frame.
#OWID (2022): 1700 to 2021
https://ourworldindata.org/grapher/population?time=1700..latest
In total, over the last two hundred thousand years about 117 billion humans were born and lived, and 109 billion also died. Which means that about 7% of all humans that ever lived are alive right now. As many as were born in the first 150,000 years of human history.
The further back we look into the past of modern man, Homo sapiens, the less precise the estimates become as to how many people there were. For example, it is estimated that between 125,000 and 352,000 Homo sapiens lived in Sub-Saharan Africa around 130,000 years BC.
#Sjödin, P. et al (2012): Resequencing data provide no evidence for a human bottleneck in Africa during the penultimate glacial period. Molecular Biology and Evolution, Volume 29 (7).
https://academic.oup.com/mbe/article/29/7/1851/1070885
Quote: “Nevertheless, based on published estimates of the ratio between effective and census population size, a comprehensive value on the order of 10% has been found by Frankham (1995). This 10% rule roughly predicts that 120,000 − 325,000 individuals (depending on the assumed mutation rate) lived in Sub-Saharan Africa some 130 kya.”
#Kaneda T. & Haub C. (2021): How Many People Have Ever Lived on Earth? Population Reference Bureau PRB https://www.prb.org/articles/how-many-people-have-ever-lived-on-earth/
Every minute, 270 babies join the party.
We rounded a bit here, to be precise it's 266 new babies. Here we refer to the UN World Population Prospects 2019, with the average of the medium fertility variant of the years 2020 to 2025.
697,668,000/5 years = 139,533,600 per year
139,533,600/364 days = 383,334 per day
383,334/24 hours = 15,972 per hour
15,972/60 minutes = 266 per minute
#UN (2019): Department of Economic and Social Affairs - Population Dynamics. World Population Prospects 2019
https://population.un.org/wpp/Graphs/DemographicProfiles/Line/900
But there are not just more people, never before have we been as healthy and well off, or lived longer. With growing living standards our birth rates collapsed. The UN estimates that around the year 2100 we will hit our population peak and there will be 125 million people born each year. It is pretty unlikely that birth rates will stay stable forever, but let’s pretend to make our thought experiment simpler.
Here we refer once again to the UN World Population Prospects 2019, with the average of the number of births (both sexes combined) of the medium fertility variant of the years 2095 to 2100
629,582,000/5 years = 125,916,400 per year
#UN (2019): Department of Economic and Social Affairs - Population Dynamics. World Population Prospects 2019
https://population.un.org/wpp/Graphs/DemographicProfiles/Line/900
According to the UN, the world population will peak at 11 billion people in 2100. If we look at the birth rates worldwide, we see that they are constantly falling, from about 2.5 live births per woman at present to just under 2 in 2100.
At the same time, global life expectancy is rising steadily. Today it is around 73 years and will be around 82 years in 2100.
In general, there is a correlation between economic development and birth rate. However, the factors are very complex. Just to give two examples: For example, the increasing education of women worldwide may make more of them consider the implications on their lives from having children. On the other hand, higher education levels also tend to increase knowledge about and acceptance of contraception. You can find many more reasons here:
#OWID (2017): Fertility Rate.
https://ourworldindata.org/fertility-rate#
Quote: “Women who are better educated have to turn down more opportunities than women who are less well educated and so the ‘prize’ they have to pay for having children is higher.
(...)
Better education of women matters again as research shows that better education can increase the understanding and acceptance of contraceptive methods and the ability to use contraception effectively.”
#UN (2019): Growing at a slower pace, world population is expected to reach 9.7 billion in 2050 and could peak at nearly 11 billion around 2100
https://www.un.org/development/desa/en/news/population/world-population-prospects-2019.html
Quote: “The world’s population is expected to increase by 2 billion persons in the next 30 years, from 7.7 billion currently to 9.7 billion in 2050, according to a new United Nations report launched today.
The World Population Prospects 2019: Highlights, which is published by the Population Division of the UN Department of Economic and Social Affairs, provides a comprehensive overview of global demographic patterns and prospects. The study concluded that the world’s population could reach its peak around the end of the current century, at a level of nearly 11 billion.”
#UN (2019): Department of Economic and Social Affairs - Population Dynamics. World Population Prospects 2019
https://population.un.org/wpp/Graphs/DemographicProfiles/Line/900
The average lifespan of mammal species is in the region of 1 million years, with some surviving up to 10 million years.
#Ord, T. (2020): The Precipice: A book that seems made for the present moment
Quote: ”The typical longevity of mammalian species has been estimated at around 1 million years, while species in the entire fossil record average 1 to 10 million years.”
Other authors give a similar order of magnitude, but are more precise. In the following source they describe the extinction rates per million species years (E/MSY) and per million genus years. That is, the rate at which a species or a genus goes extinct per 1 million years. For mammals they give a range from 1.8 E/MSY to 0.165 extinctions per million genus years.
#Snyder-Beattie, A. E. et al (2019): An upper bound for the background rate of human extinction. Scientific Reports 91(1054)
https://www.nature.com/articles/s41598-019-47540-7
Quote: “We first evaluate whether the upper bound is consistent with extinction rates for a typical mammalian species. Using fossil record data, median extinction rates for mammals have been estimated as high as 1.8 extinctions per million species years (E/MSY), or equivalently μ=1.8×10−6 . Other estimates using fossil record data range from 0.165 extinctions per million genus years to 0.4 E/MSY for Cenozoic mammals.”
Our close relative homo erectus survived for about 1.9 million years.
A few years ago, the oldest fossil, a child's skull, was found in a cave in South Africa and was dated to ∼2.04 million to 1.95 million years ago.
At about the same time, the youngest fossil was found on Java and dated to 117,000-108,00 years ago.
#Herries, A. I. R. et al. (2020): Contemporaneity of Australopithecus, Paranthropus, and early Homo erectus in South Africa. Science 368
https://www.science.org/doi/10.1126/science.aaw7293
Quote: “The DNH 134 cranium shares clear affinities with Homo erectus, whereas the DNH 152 cranium represents P. robustus. Stratigraphic analysis of the Drimolen Main Quarry deposits indicates that unlike many other South African sites, there was only one major phase of relatively short deposition between ~2.04 million years ago and ~1.95 million years ago.”
#Rizal, Y. et al. (2020): Last appearance of Homo erectus at Ngandong, Java, 117,000–108,000 years ago. Nature. Vol 577
https://www.nature.com/articles/s41586-019-1863-2
Quote: “An increasingly complex picture of hominin evolution in Pleistocene Island Southeast Asia is emerging from fossil and genomic evidence. The chronology of Ngandong H.erectus is critical for this narrative. We have approached the age of the Ngandong site in three increasingly precise contexts: the Kendeng Hills landscape, the Solo River terraces and the Ngandong bone bed. Our age estimates for the vertebrate fos-sils—including H.erectus—at Ngandong are, therefore, firmly anchored within their regional chronological and geomorphical contexts. With modelled ages of 117 to 108kyr, the Ngandong bone bed can finally assume its correct position in the hominin biostratigraphical sequence of Island Southeast Asia.”
In this scene, we show Homo Erectus hunting a mammoth. They lived alongside mammoths and there is evidence suggesting they hunted them too.
#Sharon, L. (2006): Clashing with Titans. BioScience, Volume 56, Issue 4
https://academic.oup.com/bioscience/article/56/4/292/228973?login=false
Quote: "The oldest evidence of people dining on elephant comes from the time of Homo erectus, 1.8 million years ago, in Tanzania's Olduvai Gorge. By 500,000 years before the present, modern Homo sapiens were hunting elephants in the Mediterranean, and people left a trail of dead proboscideans behind as they moved north through Europe and Asia. As glaciers retreated at the end of the ice age 11,000 years ago, our ancestors penetrated the high Arctic, and woolly mammoths died out except in a few remote refuges, such as Wrangel Island in the Arctic Ocean, that were bypassed by the human tide. Around the same time, Clovis culture spread quickly across North America, and mastodons and mammoths vanished."
#Agam, A. & Barkai, R. (2018): Elephant and Mammoth Hunting during the Paleolithic: A Review of the Relevant Archaeological, Ethnographic and Ethno-Historical Records. Quaternary, Vol. 1, No. 3
Quote: "The consumption of elephant fat and meat began with the emergence of Homo erectus in Africa some 2 million years ago, spread across the Old and New worlds by different human groups, and persisted up until the final stages of the Pleistocene with the extinction of proboscideans in Europe, America and most parts of Asia."
Let us be conservative and assume that humans will survive for a million years, which leaves us 800,000 more years to dawdle away. Assuming a stable birth rate of 125 million people each year, this means there are roughly 100 TRILLION humans waiting to be born. 850 times greater than the number of people that have ever lived. This would make everybody alive today only 0.008% of all people that will ever live.
800,000 more years * 125,000,000 newborns each year = 100,000,000,000,000 humans waiting to be born, which is 856.4 times greater than the number of people that have ever lived. This is the calculation:
100,000,000,000,000 humans waiting to be born / 116,761,402,413 people that have ever lived = 856.4
Around 7,800,000,000 people in 2022 / 100,000,000,000,000 humans waiting to be born * 100 = 0.0078%
And now consider that this may be an extremely pessimistic estimate. If we match the survival time of the most successful mammals, then our future numbers rise to 1.2 quadrillion people that have yet to be born.
The survival time of the most successful mammals is about 10 million years. If we assume that the modern human has already existed for about 250,000 years, another 9,750,000 years remain.
9,750,000 more years * 125,000,000 newborns each year = 1,218,750,000,000,000 humans waiting to be born.
And even this seems far from our potential: As the sun slowly gets hotter and brighter, Earth will remain habitable for about 500 million years, giving so many more potential people the chance to become actual people.
It is assumed that the aging Sun causes rising temperatures on Earth. As a result, the CO2 level in the atmosphere decreases and higher plants like trees die out. This destruction of the biosphere would also result in the demise of humans, as the entire food chain would be destroyed. Probably at some point only microbial life would be possible.
One assumes that this time point will be approximately in 500 million years (0.5 Gyr) and that there will be no higher animals or plants on earth in 1 billion years.
#O'Malley-James, J. T. et al. (2013): Swansong Biospheres II: The final signs of life on terrestrial planets near the end of their habitable lifetimes. International Journal of Astrobiology. 13 (10)
https://arxiv.org/abs/1310.4841
Quote: “The later stages of the Sun's main sequence evolution (2-3 Gyr from the present) will result in much higher surface temperatures on the future Earth and therefore, much more extreme environments for the last life able to grow and survive on the planet. This work builds upon earlier work in which potential refuge environments for life on the future Earth were identified, in order to evaluate what remotely detectable biosignatures the Earth would produce as the Sun's main sequence evolution moves the inner edge of the habitable zone (HZ) outwards driving extreme climate change. The habitable lifetime of any planet is limited, the exact duration depending on the type of star (or stars) hosting that planet; hence, knowing the likely biosignature evolution of the dying Earth can inform us about the potential remote signature appearances of Earth-like exoplanets whose biospheres are dying. A habitable planet can be considered to become uninhabitable when it crosses the inner edge of the HZ, which is defined by the runaway greenhouse limit - the point at which the stellar flux reaching the top of a planet's atmosphere crosses a threshold value that triggers runaway heating.
(...)
The death of the biosphere as we know it today begins with the extinction of higher plant species. Rising temperatures cause silicate weathering rates to increase, increasing CO2 draw-down, lowering CO2 levels in the atmosphere. This results in conditions that are increasingly unsuited to (higher) plant life.
(...)
The continual decrease in CO2 levels eventually renders photosynthesis impossible for higher plant species, bringing an end to the age of plants. A corresponding decrease in atmospheric oxygen levels, coupled with the loss of primary food sources, would lead to the concurrent, sequential extinction of animal species, from large vertebrates to smaller ones, with invertebrates having the longest stay of execution. Life on Earth will then once again become microbial. Initially, while there is still sufficient atmospheric oxygen and carbon dioxide to fuel microbial metabolisms, this global microbial biosphere would likely be diverse, with maximum productivity values similar to those of the pre-photosynthetic Earth; 180-560x1012 mol C yr-1.
(...)
Plants using the C3 pathway to fix carbon (the dominant pathway in higher plants) would be able to survive until atmospheric CO2 levels drop to 150 p.p.m.; 0.5 Gyr from now.”
Our Sun provides energy for billions of years and there is so much water and material floating in the asteroid and kuiper belt that we could sustain many times our current population.
Stars go through several life stages. In the case of the Sun, this means that it will inflate into a Red Giant after it has burnt up all its hydrogen.
Stars like the Sun have a hydrogen supply for about 10 billion years. The Sun has already used up half of this supply, leaving about another 5 billion years.
#Australia Telescope National Facility: Main Sequence Stars (accessed on January 2022)
https://www.atnf.csiro.au/outreach/education/senior/astrophysics/stellarevolution_mainsequence.html
Quote: “Main sequence stars are characterised by the source of their energy. They are all undergoing fusion of hydrogen into helium within their cores. The rate at which they do this and the amount of fuel available depends upon the mass of the star. Mass is the key factor in determining the lifespan of a main sequence star, its size and its luminosity. Stars on the main sequence also appear to be unchanging for long periods of time. Any model of such stars must be able to account for their stability.
(...)
The main sequence is the stage where a star spends most of its existence. Relative to other stages in a star's "life" it is extremely long; our Sun took about 20 million years to form but will spend about 10 billion years (1 × 1010 years) as a main sequence star before evolving into a red giant.”
This source shows that the Moon, Mars and near-Earth asteroids alone provide numerous resources such as water, building materials and metals for all conceivable aspects of human life, e.g. energy, food or housing).
#Abbud-Madrid, A. (2021): Space Resource Utilization. Oxford Research Encyclopedia of Planetary Science.
So aside from nearby supernovae or Gamma Rays bursts, humanity would be relatively safe from extinction, maybe for billions of years.
A supernova, an unimaginable explosion at the end of a star's life, emits a gigantic amount of X- and gamma radiation that could destroy the Earth's protective ozone layer.
Experts assume, however, that the supernova would have to take place within a radius of 50, possibly even only 30 light years. However, all stars that could go supernova are further away.
Another danger is posed by gamma ray bursts. These are far more frequent and can damage the Earth from a much greater distance. However, they must be pointed directly at the Earth.
#NASA (211): 2012: Fear No Supernova
https://www.nasa.gov/topics/earth/features/2012-supernova.html
Quote: “Given the incredible amounts of energy in a supernova explosion – as much as the sun creates during its entire lifetime – another erroneous doomsday theory is that such an explosion could happen in 2012 and harm life on Earth. However, given the vastness of space and the long times between supernovae, astronomers can say with certainty that there is no threatening star close enough to hurt Earth.
Astronomers estimate that, on average, about one or two supernovae explode each century in our galaxy. But for Earth's ozone layer to experience damage from a supernova, the blast must occur less than 50 light-years away. All of the nearby stars capable of going supernova are much farther than this.
Any planet with life on it near a star that goes supernova would indeed experience problems. X- and gamma-ray radiation from the supernova could damage the ozone layer, which protects us from harmful ultraviolet light in the sun's rays. The less ozone there is, the more UV light reaches the surface. At some wavelengths, just a 10 percent increase in ground-level UV can be lethal to some organisms, including phytoplankton near the ocean surface. Because these organisms form the basis of oxygen production on Earth and the marine food chain, any significant disruption to them could cascade into a planet-wide problem.
Another explosive event, called a gamma-ray burst (GRB), is often associated with supernovae. When a massive star collapses on itself - or, less frequently, when two compact neutron stars collide - the result is the birth of a black hole. As matter falls toward a nascent black hole, some of it becomes accelerated into a particle jet so powerful that it can drill its way completely through the star before the star's outermost layers even have begun to collapse. If one of the jets happens to be directed toward Earth, orbiting satellites detect a burst of highly energetic gamma rays somewhere in the sky. These bursts occur almost daily and are so powerful that they can be seen across billions of light-years.
A gamma-ray burst could affect Earth in much the same way as a supernova - and at much greater distance - but only if its jet is directly pointed our way. Astronomers estimate that a gamma-ray burst could affect Earth from up to 10,000 light-years away with each separated by about 15 million years, on average. So far, the closest burst on record, known as GRB 031203, was 1.3 billion light-years away.
As with impacts, our planet likely has already experienced such events over its long history, but there's no reason to expect a gamma-ray burst in our galaxy to occur in the near future, much less in December 2012.”
As enormous as the solar system is, it is just one star system among billions in the milky way. If future people can colonize, say, 100 billion stars and live there for 10 billion years, while each generating 100 million births per year, then we can expect something like a hundred Octillion lives to be lived in the future. This is a 1 with 29 zeros, a hundred thousand trillion, trillion.
Just to show how many zeros there are:
100,000,000,000 stars * 10,000,000,000 years * 100,000,000 births per year = 100,000,000,000,000,000,000,000,000,000 people.
The Andromeda Galaxy will merge with the Milky way, adding another trillion stars for us to settle.
In the quote below, “MW” is for “Milky Way”, “M 31” is for “Messier 31” or “Andromeda Galaxy” and “4-5 Gyr” is for “4-5 billion years”.
#Schiavi, R. et al. (2020): Future merger of the Milky Way with the Andromeda galaxy and the fate of their supermassive black holes. Astronomy & Astrophysics 642
https://arxiv.org/abs/2102.10938
Quote: “The time evolution of the MW and M 31 orbits is such that the first close approach of the two galaxies will occur in 4−5 Gyr, with a weak dependence on the characteristics assumed for the background density, the dimension of the halos, and the initial velocity.”
#NASA (2020): Hubble Maps Giant Halo Around Andromeda Galaxy
https://www.nasa.gov/feature/goddard/2020/hubble-maps-giant-halo-around-andromeda-galaxy
Quote: “The Andromeda galaxy, also known as M31, is a majestic spiral of perhaps as many as 1 trillion stars and comparable in size to our Milky Way. At a distance of 2.5 million light-years, it is so close to us that the galaxy appears as a cigar-shaped smudge of light high in the autumn sky. If its gaseous halo could be viewed with the naked eye, it would be about three times the width of the Big Dipper. This would easily be the biggest feature on the nighttime sky.”
If we divide the total energy available in a galaxy by the average energy needs of a single person, then we get a tredecillion potential lives. A million, trillion, trillion, trillion potential people.
Here we refer to a paper in which the author calculates how many kilograms of biomass would be possible within the Solar System, Galaxy, or Universe based on the resources in asteroids. For the Galaxy he calculates 10^48 kg-years of time-integrated biomass. “Time-integrated biomass” is basically one kg of biomass over one year.
#Mautner, M. N. (2005): Life in the cosmological future: Resources, biomass and populations. Journal of the British Interplanetary Society 58 (5)
Quote: “The amounts of life that can be realized in any ecosystem are determined by the resources of materials and energy, the requirements of the biomass, the rates of usage or wastage, and the life span of the habitat. In the Solar System, carbonaceous asteroids and comets are accessible resources, and meteorite-based microcosms showed that these materials could support microbial and plant life. Based on the measured nutrients, bioavailable materials in the carbonaceous asteroids can yield a biomass of 10^18 kg, and the total materials of the comets can yield a biomass of 10^25 kg. The total amount of life in a habitat of finite duration, such as the Solar System, may be measured in terms of time-integrated biomass. In these terms, the potential amount of future life about the Main Sequence Sun can be 10^34 kg-years, largely exceeding the 10^24 kg-years of past terrestrial life. Life about brown, red and white dwarf stars may be energy-limited and contribute 10^46 kg-years in the future. The upper limits of life would in the universe would be obtained by converting all baryonic matter to biomass, and gradually converting the biomass to supporting energy. These projections of cosmo-ecology allow an immense 10^48 kg-years of time-integrated biomass in the galaxy and 10^59 kg-years in the universe.”
#Mautner, M. N. (2014): Astroecology, cosmo-ecology, and the future of life. Acta Societatis Botanicorum Poloniae 83 (4)
https://www.researchgate.net/publication/270583451_Astroecology_Cosmo-ecology_and_the_Future_of_Life
Quote: “For a scientific study of astroecology, life has to be quantified. A quantitative measure of life in an ecosystem can be formulated using the total amount of active biomass and its duration. This can be expressed in Equation (1) in terms of time-integrated biomass (biomass integrated over times available, BIOTAint) measured in kg-years (similar to labor measured in men-years).
Here mbiomass,t is the amount of biomass at time t and integration is from time when life starts in the ecosystem to any given time t. Integration to the final inhabited time of the ecosystem, tf , yields the total amount of life in the ecosystem. This BIOTAint may be measured in kg-years. For a constant steady-state biomass BIOTAsteady-state (i.e., a constant biomass maintained by a balance of formation and destruction) lasting for time t in an ecosystem, the time-integrated BIOTAint is then given simply by Equation (2).
For example, assuming that the present amount of life on Earth, on the order of 10^15 kg has been constant for the last billion years then BIOTAint on Earth has been 10^15 kg × 10^9 years = 10^24 kg-years.”
So far they are still large in kg-years (or kg of biomass over one year). But we wanted to relate them to an "average person" with a life span of 100 years.
In addition, we have integrated a number that gives a very rough estimate of how much of the potential biomass humans need for food and other things such as medicine, clothing, etc..
As far as food is concerned, we have used the concept of "trophic level" as a guide. In very simplified terms, we could also talk about individual stages of the food chain. From level to level, only about 10% of the energy can be used. This means, for example, that a rabbit can only use 1,000 kcal of 10,000 kcal of plant food. This means that from stage to stage ten times more food is needed.
Applied to humans, we would need roughly 10^4 times more "supporting" biomass than our "human" biomass.
Let's assume that humans are roughly at level 4 (level 1 would be plants, for example, and level 3 would be carnivores eating the herbivores at level 2).
Actually, we assume that humans are roughly on a level between 2 and 3. However, we would like to include in the calculation of the biomass that we use many animal products for other purposes than nutrition (e.g. medicine, materials). Therefore level 4 and 10^4 = 10,000.
#Encyclopædia Britannica (2018): trophic level
https://www.britannica.com/science/trophic-level
Quote: “trophic level, step in a nutritive series, or food chain, of an ecosystem. The organisms of a chain are classified into these levels on the basis of their feeding behaviour. The first and lowest level contains the producers, green plants. The plants or their products are consumed by the second-level organisms—the herbivores, or plant eaters. At the third level, primary carnivores, or meat eaters, eat the herbivores; and at the fourth level, secondary carnivores eat the primary carnivores. These categories are not strictly defined, as many organisms feed on several trophic levels; for example, some carnivores also consume plant materials or carrion and are called omnivores, and some herbivores occasionally consume animal matter.”
#Encyclopædia Britannica (2019): trophic pyramid
https://www.britannica.com/science/trophic-pyramid
Quote: “trophic pyramid, the basic structure of interaction in all biological communities characterized by the manner in which food energy is passed from one trophic level to the next along the food chain. The base of the pyramid is composed of species called autotrophs, the primary producers of the ecosystem. All other organisms in the ecosystem are consumers called heterotrophs, which either directly or indirectly depend on the primary producers for food energy.
Within all biological communities, energy at each trophic level is lost in the form of heat (as much as 80 to 90 percent), as organisms expend energy for metabolic processes such as staying warm and digesting food.”
All in all, these considerations result in the following calculation:
10^48 kg-years of time-integrated biomass in our Galaxy, customized by 10,000 kg * 100 years for a human (with 10,000 regarding the trophic level per year and 100 years life time) = 10^42 = a tredecillion or a million trillion trillion trillion.