7.5 Billion and Counting: How Many Humans Can the Earth Support?
By Andrew D. Hwang
Can the Earth support this many people indefinitely? What will happen if we do nothing to manage future population growth and total resource use? These complex questions are ecological, political, ethical—and urgent. Simple mathematics shows why, shedding light on our species' ecological footprint.
The Mathematics of Population Growth
In an environment with unlimited natural resources, population size grows exponentially. One characteristic feature of exponential growth is the time a population takes to double in size.
Exponential growth tends to start slowly, sneaking up before ballooning in just a few doublings.
To illustrate, suppose Jeff Bezos agreed to give you one penny on Jan. 1, 2019, two pennies on Feb. 1, four on March 1, and so forth, with the payment doubling each month. How long would his $100 billion fortune uphold the contract? Take a moment to ponder and guess.
After one year, or 12 payments, your total contract receipts come to US$40.95, equivalent to a night at the movies. After two years, $167,772.15—substantial, but paltry to a billionaire. After three years, $687,194,767.35, or about one week of Bezos' 2017 income.
The 43rd payment, on July 1, 2022, just short of $88 billion and equal to all the preceding payments together (plus one penny), breaks the bank.
Real Population Growth
For real populations, doubling time is not constant. Humans reached 1 billion around 1800, a doubling time of about 300 years; 2 billion in 1927, a doubling time of 127 years; and 4 billion in 1974, a doubling time of 47 years.
On the other hand, world numbers are projected to reach 8 billion around 2023, a doubling time of 49 years, and barring the unforeseen, expected to level off around 10 to 12 billion by 2100.
This anticipated leveling off signals a harsh biological reality: Human population is being curtailed by the Earth's carrying capacity, the population at which premature death by starvation and disease balances the birth rate.
Humans are consuming and polluting resources—aquifers and ice caps, fertile soil, forests, fisheries and oceans—accumulated over geological time, tens of thousands of years or longer.
Wealthy countries consume out of proportion to their populations. As a fiscal analogy, we live as if our savings account balance were steady income.
According to the Worldwatch Institute, an environmental think tank, the Earth has 1.9 hectares of land per person for growing food and textiles for clothing, supplying wood and absorbing waste. The average American uses about 9.7 hectares.
These data alone suggest the Earth can support at most one-fifth of the present population, 1.5 billion people, at an American standard of living.
Water is vital. Biologically, an adult human needs less than 1 gallon of water daily. In 2010, the U.S. used 355 billion gallons of freshwater, over 1,000 gallons (4,000 liters) per person per day. Half was used to generate electricity, one-third for irrigation, and roughly one-tenth for household use: flushing toilets, washing clothes and dishes, and watering lawns.
If 7.5 billion people consumed water at American levels, world usage would top 10,000 cubic kilometers per year. Total world supply—freshwater lakes and rivers—is about 91,000 cubic kilometers.
World Health Organization figures show 2.1 billion people lack ready access to safe drinking water, and 4.5 billion lack managed sanitation. Even in industrialized countries, water sources can be contaminated with pathogens, fertilizer and insecticide runoff, heavy metals and fracking effluent.
Freedom to Choose
Though the detailed future of the human species is impossible to predict, basic facts are certain. Water and food are immediate human necessities. Doubling food production would defer the problems of present-day birth rates by at most a few decades. The Earth supports industrialized standards of living only because we are drawing down the "savings account" of non-renewable resources, including fertile topsoil, drinkable water, forests, fisheries and petroleum.
The drive to reproduce is among the strongest desires, both for couples and for societies. How will humans reshape one of our most cherished expectations—"Be fruitful and multiply"—in the span of one generation? What will happen if present-day birth rates continue?
We cannot wish natural resources into existence. Couples, however, have the freedom to choose how many children to have. Improvements in women's rights, education and self-determination generally lead to lower birth rates.
As a mathematician, I believe reducing birth rates substantially is our best prospect for raising global standards of living. As a citizen, I believe nudging human behavior, by encouraging smaller families, is our most humane hope.
Reposted with permission from our media associate The Conversation.
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By Jacob L. Steenwyk and Antonis Rokas
From the mythical minotaur to the mule, creatures created from merging two or more distinct organisms – hybrids – have played defining roles in human history and culture. However, not all hybrids are as fantastic as the minotaur or as dependable as the mule; in fact, some of them cause human diseases.
When Looking Through a Microscope Isn’t Close Enough.<p>For the last few years, <a href="http://www.rokaslab.org/" target="_blank">our team at Vanderbilt University</a>, <a href="https://www.researchgate.net/lab/Gustavo-Goldman-Lab" target="_blank">Gustavo Goldman's team at São Paulo University in Brazil</a> and many other collaborators around the world have been collecting samples of fungi from patients infected with different species of <em>Aspergillus</em> molds. One of the species we are particularly interested in is <a href="https://doi.org/10.1006/rwgn.2001.0082" target="_blank"><em>Aspergillus nidulans</em>, a relatively common and generally harmless fungus</a>. Clinical laboratories typically identify the species of <em>Aspergillus</em> causing the infection by examining cultures of the fungi under the microscope. The problem with this approach is that very closely related species of <em>Aspergillus</em> tend to look very similar in their broad morphology or physical appearance when viewing them through a microscope.</p><p>Interested in examining the varying abilities of different <em>A. nidulans</em> strains to cause disease, we decided to analyze their total genetic content, or genomes. What we saw came as a total surprise. We had not collected <em>A. nidulans</em> but <em>Aspergillus latus</em>, a close relative of <em>A. nidulans</em> and, as we were to soon find out, <a href="https://doi.org/10.1016/j.cub.2020.04.071" target="_blank">a hybrid species that evolved through the fusion of the genomes</a> of two other <em>Aspergillus</em> species: <em>Aspergillus spinulosporus</em> and an unknown close relative of <em>Aspergillus quadrilineatus</em>. Thus, we realized not only that these patients harbored infections from an entirely different species than we thought they were, but also that this species was the first ever <em>Aspergillus</em> hybrid known to cause human infections.</p>
Several Different Fungal Hybrids Cause Human Disease.<p>Hybrid fungi that can cause infections in humans are well known to occur in several different lineages of single-celled fungi known as yeasts. Notable examples include multiple different species of <a href="https://doi.org/10.1002/yea.3242" target="_blank">yeast hybrids</a> that cause the human diseases <a href="https://rarediseases.info.nih.gov/diseases/6218/cryptococcosis" target="_blank">cryptococcosis</a> and <a href="https://www.cdc.gov/fungal/diseases/candidiasis/index.html" target="_blank">candidiasis</a>. Although pathogenic yeast hybrids are well known, our discovery that the <em>A. latus</em> pathogen is a hybrid is a first for molds that cause disease in humans.</p>
(Left) Candida yeasts live on parts of the human body. Imbalance of microbes on the body can allow these yeasts, some of which are hybrids, to grow and cause infection. (Right) Cryptococcus yeasts, including ones that are hybrids, can cause life-threatening infections in primarily immunocompromised people. Centers for Disease Control and Prevention<p><a href="https://doi.org/10.1371/journal.ppat.1008315" target="_blank">Why certain <em>Aspergillus</em> species are so deadly</a> while others are harmless remains unknown. This may in part be because <a href="https://doi.org/10.1016/j.fbr.2007.02.007" target="_blank">combinations of traits, rather than individual traits</a>, underlie organisms' ability to cause disease. So why then are hybrids frequently associated with human disease? Hybrids inherit genetic material from both parents, which may result in new combinations of traits. This may make them more similar to one parent in some of their characteristics, reflect both parents in others or may differ from both in the rest. It is precisely this mix and match of traits that hybrids have inherited from their parental species that <a href="https://www.nytimes.com/2010/09/14/science/14creatures.html" target="_blank">facilitates their evolutionary success</a>, including their ability to cause disease.</p>
The Evolutionary Origin of an Aspergillus Hybrid.<p>Multiple evolutionary paths can lead to the emergence of hybrids. One path is through mating, just as the horse and donkey mate to create a mule. Another path is through the merging or fusion of genetic material from cells of different species.</p><p>It is this second path that appears to have been taken by our fungus. <em>A. latus</em> appears to have two of almost everything compared to its parental species: twice the genome size, twice the total number of genes and so on. But unlike other hybrids, which are often sterile like the mule, we found that <em>A. latus</em> is capable of reproducing both asexually and sexually.</p><p>But how distinct were the parents of <em>A. latus</em>? By comparing the parts contributed by each parent in the <em>A. latus</em> genome, we estimate that its parents are approximately 93% genetically similar, which is about as related as we humans are with lemurs. In other words, <em>A. latus</em>, an agent of infectious disease, is the fungal equivalent of a human-lemur hybrid.</p>
How A. Latus Differs From its Parents.<p>Elucidating the identity of closely related fungal pathogens and how they differ from each other in infection-relevant characteristics is a key step toward reducing the burden of fungal disease. For example, we found that <em>A. latus</em> was three times more resistant than <em>A. nidulans</em>, the species it was originally identified as using microscopy-based methods, to one of the most common antifungal drugs, <a href="https://www.drugbank.ca/drugs/DB00520" target="_blank">caspofungin</a>. This result provides a clear example of the potential importance of accurate identification of the <em>Aspergillus</em> pathogen causing an infection.</p><p>We also examined how <em>A. latus</em> and <em>A. nidulans</em> interact with cells from our immune system. We found that immune cells were less efficient at combating <em>A. latus</em> compared to <em>A. nidulans</em>, suggesting the hybrid fungus may be trickier for our immune systems to identify and destroy.</p><p>In the midst of the COVID-19 pandemic, our quest to understand <em>Aspergillus</em> pathogens is becoming more urgent. Growing evidence suggests that <a href="https://doi.org/10.1111/myc.13096" target="_blank">a fraction of COVID-19 patients are also infected with <em>Aspergillus</em>.</a> More worrying is that these <a href="https://doi.org/10.3201/eid2607.201603" target="_blank">secondary <em>Aspergillus</em> infections</a> can worsen the clinical outcomes for those infected with the novel coronavirus. That being said, we stress that little is known about <em>Aspergillus</em> infections in COVID-19 patients due to a lack of systematic testing, and none of the infections identified so far appear to have been caused by hybrids.</p><p>So, when it comes to hybrids, some are fantastic (the minotaur), some are helpful (the mule) and some are dangerous (<em>Aspergillus latus</em>). Understanding more about the biology of <em>Aspergillus latus</em> may help in our understanding of how microbial pathogens arise and how to best prevent and combat their infections.</p>
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