Doomsday Early Signs: The Science

The New York Times last week tried to highlight the dangers of climate change. On Friday, Alexander Burns opened his contribution with the following two paragraphs:

For years, climate change activists have faced a wrenching dilemma: how to persuade people to care about a grave but seemingly far-off problem and win their support for policies that might pinch them immediately in utility bills and at the pump.

But that calculus may be changing at a time when climatic chaos feels like a daily event rather than an airy abstraction, and storms powered by warming ocean waters wreak havoc on the mainland United States. Americans have spent weeks riveted by television footage of wrecked neighborhoods, displaced families, flattened Caribbean islands and submerged cities from Houston to Jacksonville.

On Thursday, Tom Friedman wrote an op-ed on the contradictory ways in which President Trump is handling two seemingly low-probability global doomsday events: North Korea’s use of nuclear weapons on the US or its allies, and climate change:

The other low-probability, high-impact threat is climate change fueled by increased human-caused carbon emissions. The truth is, if you simply trace the steady increase in costly extreme weather events — wildfires, floods, droughts and climate-related human migrations — the odds of human-driven global warming having a devastating impact on our planet are not low probability but high probability.

Friedman realized that the major difference between these two possible future events is that the latter has a much higher probability. For a business-as-usual scenario in which we continue with the same policies that we presently hold, the only contentious issue is the timing.

Estimating future events always comes with uncertainty about timing. In terms of climate change, the IPCC has addressed this via different scenarios (October 1, 2013 and October 28, 2014). Doomsday timing depends on our contributions to climate change (August 29, 2017 – dark orange region in Figure 2). A similar approach is to try to predict doomsday through its early signs. Some doomsday scenarios such as North Korea bombing the US with nuclear weapons would likely have few (if any) early warning signs; they would reflect totally irrational, suicidal thinking. Other doomsday scenarios would give us much more to work with. The largest computer file that I have on climate change is dedicated to the early signs that already exist.

A recently published paper in the Proceeding of the National Academy of Science (PNAS) deals directly with this issue. All three of its authors are distinguished scientists. One of them, Paul Ehrlich, is a member of the National Academy of Science and was one of the two original formulators of the IPAT identity that I use so often in this blog (November 26, 2012). I am including significant parts of the paper because the main differences between science and stories or opinions are details, methodology, and refutability based on observations. Readers should be able to distinguish between the conclusions of this paper and the New York Magazine article that I discussed last week:

Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines

  1. Gerardo Ceballosa,1, Paul R. Ehrlichb,1, and Rodolfo Dirzob


The strong focus on species extinctions, a critical aspect of the contemporary pulse of biological extinction, leads to a common misimpression that Earth’s biota is not immediately threatened, just slowly entering an episode of major biodiversity loss. This view overlooks the current trends of population declines and extinctions. Using a sample of 27,600 terrestrial vertebrate species, and a more detailed analysis of 177 mammal species, we show the extremely high degree of population decay in vertebrates, even in common “species of low concern.” Dwindling population sizes and range shrinkages amount to a massive anthropogenic erosion of biodiversity and of the ecosystem services essential to civilization. This “biological annihilation” underlines the seriousness for humanity of Earth’s ongoing sixth mass extinction event.


The population extinction pulse we describe here shows, from a quantitative viewpoint, that Earth’s sixth mass extinction is more severe than perceived when looking exclusively at species extinctions. Therefore, humanity needs to address anthropogenic population extirpation and decimation immediately. That conclusion is based on analyses of the numbers and degrees of range contraction (indicative of population shrinkage and/or population extinctions according to the International Union for Conservation of Nature) using a sample of 27,600 vertebrate species, and on a more detailed analysis documenting the population extinctions between 1900 and 2015 in 177 mammal species. We find that the rate of population loss in terrestrial vertebrates is extremely high—even in “species of low concern.” In our sample, comprising nearly half of known vertebrate species, 32% (8,851/27,600) are decreasing; that is, they have decreased in population size and range. In the 177 mammals for which we have detailed data, all have lost 30% or more of their geographic ranges and more than 40% of the species have experienced severe population declines (>80% range shrinkage). Our data indicate that beyond global species extinctions Earth is experiencing a huge episode of population declines and extirpations, which will have negative cascading consequences on ecosystem functioning and services vital to sustaining civilization. We describe this as a “biological annihilation” to highlight the current magnitude of Earth’s ongoing sixth major extinction event.

The loss of biological diversity is one of the most severe human-caused global environmental problems. Hundreds of species and myriad populations are being driven to extinction every year (1⇓⇓⇓⇓⇓⇓–8). From the perspective of geological time, Earth’s richest biota ever is already well into a sixth mass extinction episode (9⇓⇓⇓⇓–14). Mass extinction episodes detected in the fossil record have been measured in terms of rates of global extinctions of species or higher taxa (e.g., ref. 9). For example, conservatively almost 200 species of vertebrates have gone extinct in the last 100 y. These represent the loss of about 2 species per year. Few realize, however, that if subjected to the estimated “background” or “normal” extinction rate prevailing in the last 2 million years, the 200 vertebrate species losses would have taken not a century, but up to 10,000 y to disappear, depending on the animal group analyzed (11). Considering the marine realm, specifically, only 15 animal species have been recorded as globally extinct (15), likely an underestimate, given the difficulty of accurately recording marine extinctions. Regarding global extinction of invertebrates, available information is limited and largely focused on threat level. For example, it is estimated that 42% of 3,623 terrestrial invertebrate species, and 25% of 1,306 species of marine invertebrates assessed on the International Union for Conservation of Nature (IUCN) Red List are classified as threatened with extinction (16). However, from the perspective of a human lifetime it is difficult to appreciate the current magnitude of species extinctions. A rate of two vertebrate species extinctions per year does not generate enough public concern, especially because many of those species were obscure and had limited ranges, such as the Catarina pupfish (Megupsilon aporus, extinct in 2014), a tiny fish from Mexico, or the Christmas Island pipistrelle (Pipistrellus murrayi, extinct in 2009), a bat that vanished from its namesake volcanic remnant.

Species extinctions are obviously very important in the long run, because such losses are irreversible and may have profound effects ranging from the depletion of Earth’s inspirational and esthetic resources to deterioration of ecosystem function and services (e.g., refs. 17⇓⇓–20). The strong focus among scientists on species extinctions, however, conveys a common impression that Earth’s biota is not dramatically threatened, or is just slowly entering an episode of major biodiversity loss that need not generate deep concern now (e.g., ref. 21, but see also refs. 9, 11, 22). Thus, there might be sufficient time to address the decay of biodiversity later, or to develop technologies for “deextinction”—the possibility of the latter being an especially dangerous misimpression (see ref. 23). Specifically, this approach has led to the neglect of two critical aspects of the present extinction episode: (i) the disappearance of populations, which essentially always precedes species extinctions, and (ii) the rapid decrease in numbers of individuals within some of the remaining populations. A detailed analysis of the loss of individuals and populations makes the problem much clearer and more worrisome, and highlights a whole set of parameters that are increasingly critical in considering the Anthropocene’s biological extinction crisis.

In the last few decades, habitat loss, overexploitation, invasive organisms, pollution, toxification, and more recently climate disruption, as well as the interactions among these factors, have led to the catastrophic declines in both the numbers and sizes of populations of both common and rare vertebrate species (24⇓⇓⇓–28). For example, several species of mammals that were relatively safe one or two decades ago are now endangered. In 2016, there were only 7,000 cheetahs in existence (29) and less than 5,000 Borneo and Sumatran orangutans (Pongo pygmaeus and P. abelli, respectively) (28). Populations of African lion (Panthera leo) dropped 43% since 1993 (30), pangolin (Manis spp.) populations have been decimated (31), and populations of giraffes dropped from around 115,000 individuals thought to be conspecific in 1985, to around 97,000 representing what is now recognized to be four species (Giraffa giraffa, G. tippelskirchi, G. reticulata, and G. camelopardalis) in 2015 (32).

An important antecedent to our work (25) used the number of genetic populations per unit area and then estimated potential loss on the basis of deforestation estimates and the species–area relationship (SAR). Given the recognized limitations of the use of SAR to estimate extinctions, our work provides an approach based on reduction of species range as a proxy of population extirpation. The most recent Living Planet Index (LPI) has estimated that wildlife abundance on the planet decreased by as much as 58% between 1970 and 2012 (4). The present study is different from LPI and other related publications in several ways, including that here we use all decreasing species of vertebrates according to IUCN, mapping and comparing absolute and relative numbers of species, and focusing on population losses. Previous estimates seem validated by the data we present here on the loss of local populations and the severe decrease in the population size of many others (see also refs. 3, 4, 6⇓–8, 26). Here we examine the magnitude of losses of populations of land vertebrate species on a global system of 10,000-km2 quadrats (Methods). Species vary from common to rare, so that our analysis, which includes all land vertebrate species (amphibians, birds, reptiles, and mammals) deemed as “decreasing” by IUCN, provides a better estimate of population losses than using exclusively IUCN data on species at risk. Obviously, common species decreasing are not ordinarily classified as species at risk. IUCN criteria provide quantitative thresholds for population size, trend, and range size, to determine decreasing species (28, 33). We also evaluate shrinking ranges and population declines for 177 species of mammals for which data are available on geographic range shrinkage from ∼1900 to 2015. We specifically focus on local extinctions by addressing the following questions: (i) What are the numbers and geographic distributions of decreasing terrestrial vertebrate species (i.e., experiencing population losses)? (ii) What are the vertebrate groups and geographic regions that have the highest numbers and proportions of decreasing species? (iii) What is the scale of local population declines in mammals—a proxy for other vertebrates? By addressing these questions, we conclude that anthropogenic population extinctions amount to a massive erosion of the greatest biological diversity in the history of Earth and that population losses and declines are especially important, because it is populations of organisms that primarily supply the ecosystem services so critical to humanity at local and regional levels.


Population extinctions today are orders of magnitude more frequent than species extinctions. Population extinctions, however, are a prelude to species extinctions, so Earth’s sixth mass extinction episode has proceeded further than most assume. The massive loss of populations is already damaging the services ecosystems provide to civilization. When considering this frightening assault on the foundations of human civilization, one must never forget that Earth’s capacity to support life, including human life, has been shaped by life itself (47). When public mention is made of the extinction crisis, it usually focuses on a few animal species (hundreds out of millions) known to have gone extinct, and projecting many more extinctions in the future. But a glance at our maps presents a much more realistic picture: they suggest that as much as 50% of the number of animal individuals that once shared Earth with us are already gone, as are billions of populations. Furthermore, our analysis is conservative, given the increasing trajectories of the drivers of extinction and their synergistic effects. Future losses easily may amount to a further rapid defaunation of the globe and comparable losses in the diversity of plants (36), including the local (and eventually global) defaunation-driven coextinction of plants (3, 20). The likelihood of this rapid defaunation lies in the proximate causes of population extinctions: habitat conversion, climate disruption, overexploitation, toxification, species invasions, disease, and (potentially) large-scale nuclear war—all tied to one another in complex patterns and usually reinforcing each other’s impacts. Much less frequently mentioned are, however, the ultimate drivers of those immediate causes of biotic destruction, namely, human overpopulation and continued population growth, and overconsumption, especially by the rich. These drivers, all of which trace to the fiction that perpetual growth can occur on a finite planet, are themselves increasing rapidly. Thus, we emphasize that the sixth mass extinction is already here and the window for effective action is very short, probably two or three decades at most (11, 48). All signs point to ever more powerful assaults on biodiversity in the next two decades, painting a dismal picture of the future of life, including human life.

The doomsday scenario in this paper is the Sixth Extinction (February 3, 2015). The paper reaches the conclusion that not only it is not a new prediction but that we are already in the middle of the extinction process. The methodology was based on detailed investigation of databases that determined we are in a period of extinction both of entire species and also of individuals within non-extinct species. Additionally, the paper addresses the situations of the geographic areas surrounding these individuals and how they serve as proxies for further extinctions. The determination that this extinction is human-caused (anthropogenic) is based on its speed and global distribution. One common argument given the extinction of whole species is that the process is not the end of all life on Earth but a homogenization – a road to lower biodiversity (although presumably humans and domesticated animals, whether livestock or pets, would continue to survive). Examples of individuals dying out are putting this argument to rest.

Next week I will deal explicitly with the timing of the projected doomsday.

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About climatechangefork

Micha Tomkiewicz, Ph.D., is a professor of physics in the Department of Physics, Brooklyn College, the City University of New York. He is also a professor of physics and chemistry in the School for Graduate Studies of the City University of New York. In addition, he is the founding-director of the Environmental Studies Program at Brooklyn College as well as director of the Electrochemistry Institute at that same institution.
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