PDF Summary:The Ends of the World, by Peter Brannen

Mass extinctions have repeatedly reshaped life on Earth, and each time, the culprit has been dramatic shifts in climate and ocean chemistry. In The Ends of the World, Peter Brannen examines the geological and biological forces behind history's five major extinction events, from massive volcanic eruptions to asteroid impacts. He explains how carbon dioxide disruptions—whether from volcanoes or weathering mountain ranges—have repeatedly pushed Earth's climate past tipping points, creating conditions that wiped out most life.

Brannen draws connections between these ancient catastrophes and modern human activity. Today's carbon emissions rival those of past extinction events, and the environmental changes we're causing—ocean acidification, warming, habitat destruction—mirror the conditions that preceded earlier die-offs. While recovery from past extinctions took millions of years, the choices humanity makes in the coming decades could shape Earth's climate and ecosystems for hundreds of thousands of years.

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Let’s explore some types of environmental stressors and the recovery timescales and prolonged impacts of extinction-level events.

Types of Stressors on Ecosystems

Brannen identifies ocean acidification and anoxia as environmental stressors. Ocean acidification refers to a process in which the ocean's acidity increases and carbonate decreases, making it difficult for animals to build their shells and skeletons. Anoxia is a process where the ocean becomes less oxygenated. These stressors contributed to the Permian-Triassic and Triassic-Jurassic mass extinctions, as well as the K-T extinction.

(Shortform note: In this context, “carbonate” refers to the dissolved CO3²⁻ ion in seawater. This is one of the main chemical forms that inorganic carbon takes in the ocean. The ocean’s natural buffering system involves a balance between carbon dioxide, bicarbonate, and carbonate ions. When carbon dioxide dissolves in seawater, it forms carbonic acid, which then dissociates into bicarbonate and carbonate ions.)

Extreme heat and volcanism are other environmental stressors, according to Brannen. The End-Permian extinction was caused by a combination of factors, including extreme heat, ocean acidification, ozone destruction, acid rain, and toxic gases from volcanic activity. The oceans became anoxic, lacking oxygen, and filled with H₂S, a poisonous gas created by bacteria when oxygen is absent. This anoxic condition was caused by extreme heat, as warmer water holds less oxygen, and animals need more oxygen as temperatures rise. Additionally, weathering on land poured nutrients such as phosphorus into the ocean, leading to rapid plankton growth that further suffocated the oceans.

(Shortform note: A 2018 study by Penn et al. used Earth system models to simulate the end-Permian oceans and found that the combination of warming and oxygen loss alone could explain the observed geographic pattern and ecological selectivity of marine extinctions. Their results suggest that temperature-dependent hypoxia by itself can account for both the pattern and overall severity of the end-Permian marine biodiversity collapse, without invoking additional global-scale kill mechanisms. This challenges the idea that H₂S-filled oceans were the sole or dominant kill mechanism, as Brannen describes.)

The End-Triassic extinction was caused by similar factors, with warmer, less oxygenated, and more acidic oceans due to massive injections of CO₂ from volcanic activity. This led to reef systems breaking down, and if current trends continue, coral reefs around the world will become eroding piles of rubble by midcentury. Corals are sensitive to temperature changes and undergo dangerous bleaching events in excessively warm water. The End-Cretaceous extinction was caused by a combination of factors, including the meteorite collision, volcanic activity, and climate change. The asteroid's collision released massive amounts of energy, causing wildfires, tsunamis, and a "nuclear winter" effect that blocked sunlight and disrupted photosynthesis. Volcanic activity in the Deccan region released large amounts of CO₂ and other gases, contributing to global warming and ocean acidification. These factors led to the disappearance of the non-avian dinosaurs and many other species.

Vincent Courtillot’s Account of the End-Triassic and End-Cretaceous Extinctions

Peter Brannen’s account of the end-Triassic and end-Cretaceous extinctions is similar to that of Vincent Courtillot, who published his book Evolutionary Catastrophes in 1999. Courtillot’s book was one of the first to propose a unifying theory for the major mass extinctions, focusing on the role of large igneous provinces (LIPs) and their associated volcanic activity. He explains that the end-Triassic extinction was caused by the Central Atlantic Magmatic Province (CAMP), which released massive amounts of CO₂ and other gases, leading to global warming, ocean acidification, and the breakdown of reef systems. He also explains that the end-Cretaceous extinction was caused by a combination of the Chicxulub impact and the Deccan Traps volcanic activity, which released large amounts of CO₂ and other gases, contributing to global warming and ocean acidification.

Recovery Timescales and Prolonged Impacts

Brannen explains that the recovery from the Permian extinction was prolonged due to extreme conditions. Specifically, it required nearly a 10-million-year period for life to bounce back. The slow recovery was due to persistent extreme conditions following the extinction event. At the onset of the Triassic era, temperatures were extremely high, reaching up to 140 degrees Fahrenheit on land and 104 degrees Fahrenheit in the oceans. These temperatures were too high for multicellular organisms to survive. Additionally, the oceans remained anoxic for 5 million years after the extinction, and there was another major extinction event among the few survivors just 2 million years after the initial event.

(Shortform note: In When Life Nearly Died, Michael J. Benton explains that it wasn’t until the late twentieth century that scientists realized the recovery from the Permian extinction was unusually prolonged. This was due to advancements in radiometric dating and global fossil correlation, which allowed for more precise dating of rock layers and a better understanding of the global distribution of fossils. Before these advancements, scientists believed that the lack of fossils in certain rock layers was due to gaps in the rock record. However, with improved dating techniques, they discovered that these layers represented a period of ecological failure, where only a few opportunistic species survived.)

Pangaea, the vast landmass, may have contributed to the extreme conditions by disrupting the planet’s ability to regulate atmospheric carbon dioxide. The vast dry inner regions of the supercontinent did not weather, which is the earth’s most reliable mechanism for drawing down CO₂. The lack of coastlines also limited how much shelf space was available for marine life to store carbon.

(Shortform note: Some researchers have argued that Pangaea may have increased the earth’s ability to draw down CO₂. John E. Kutzbach and Robert G. Gallimore, two paleoclimate modelers, argue that the supercontinent’s unique geography created “megamonsoons” that would have increased rainfall and runoff along the coasts, which would have increased the weathering of rocks and thus the drawdown of CO₂. This would have offset the lack of weathering in the interior of the continent.)

The early Triassic period was so hot that there were no large fish at the equator, and no animals existed near the tropics on land. The only life that existed was overrun by opportunistic invasive species like the clam Claraia, and trees didn't return for 10 million years. Plant life was so decimated that rivers, once restricted to meandering narrow paths, expanded into wide, interwoven sandy waterways. There was a temporary surge in fungi, potentially from global decay of deceased organisms. Tabulate and rugose corals, the Paleozoic's reef constructors, died out completely. After the Permian, reefs gave way to accumulations of microbial slime.

Claraia

Claraia was a small, thin-shelled bivalve mollusk that spread rapidly in the earliest Triassic oceans. It’s considered a “disaster” species that thrived in the aftermath of the end-Permian extinction. Claraia’s success is attributed to its ability to tolerate low-oxygen conditions and its opportunistic reproductive strategy. Its thin shell and small size allowed it to grow quickly and reproduce rapidly, giving it an advantage in the unstable post-extinction environment. Claraia’s dominance was short-lived, however, as more complex ecosystems gradually reestablished themselves in the Triassic.

Modern Relevance and Future Risks

Brannen argues that human activity is delaying the upcoming ice age, though it could also lead to catastrophic warming. It's anticipated that if people combust two trillion tons of carbon, the next glaciation will be postponed by 50,000 years. If they burn all their fossil fuels, the planet will need 400,000 years for natural processes to remove enough carbon to return to the icy course of the Pleistocene. Choices made by our civilization in the upcoming decades could impact the climate for twice as long into the future as the existence of our species in the past.

(Shortform note: The claim that burning two trillion tons of carbon could delay the next ice age by 50,000 years is based on climate models that simulate the Earth's response to increased greenhouse gas emissions. These models incorporate the effects of carbon dioxide on global temperatures and the timing of glacial cycles. They show that the additional heat trapped by this amount of carbon dioxide would prevent the formation of ice sheets, even during periods when Earth's orbit would normally favor glaciation. This is because the increased greenhouse effect would keep polar regions too warm for snow and ice to accumulate.)

Even if we elevate carbon dioxide to match the Eocene, relocate marlin and crocodiles to the Arctic, and raise sea levels by more than 200 feet, it's likely to collapse into an ice age. Whether this ice age reemerges in 130,000 years or 400,000 years, New Orleans, New York, and the Nile Delta will reappear as submerged ruins. The oceans will need an equally immense amount of time to recover. It will take a minimum of 100,000 years for the carbonate chemistry of the oceans to return to pre-human levels.

(Shortform note: In The Long Thaw, David Archer explains that the long-term fate of fossil-fuel CO2 is controlled not by the relatively quick exchange with the surface ocean and biosphere, but by the very slow neutralization of that CO2 by carbonate and silicate minerals. He explains that most of the added CO2 is taken up by the ocean on timescales of decades to centuries, but that a residual 20–30 percent remains in the atmosphere–ocean system for many tens to hundreds of thousands of years. That long-lived tail persists because it can only be removed as deep-sea calcium carbonate sediments gradually dissolve, altering seawater alkalinity, and as new carbonate minerals are slowly re-precipitated and silicate rocks are weathered on land. Until these slow geochemical processes have run their course, the ocean’s carbonate system and the climate will not fully relax back to something like their preindustrial baseline.)

The impact on Earth's ecosystems will persist much longer. Past mass extinctions demonstrate that the biological recovery period extends beyond the time it takes for ocean chemistry to resolve. If we transition from the icy Pleistocene to a temporary Eocene greenhouse and then return to the ice, the biosphere might not be able to handle it. If humanity hasn't brought it to fruition already, this could be the start of the sixth great extinction. Removing the carbon from Earth's systems takes several hundred millennia. Restoring ecosystems requires millions or even many millions of years. That's what will most impact the geologic archive, should a scientist revisit it a century from now. It will be the extinctions that we cause.

The Geological Legacy of Human Impacts

The idea that “it will be the extinctions that we cause” that most impact the geologic archive draws on two scientific traditions: conservation paleobiology and Anthropocene stratigraphy. Conservation paleobiology uses the fossil record to inform conservation efforts, while Anthropocene stratigraphy studies the geological evidence of human impacts on Earth’s systems. Jan Zalasiewicz’s The Earth After Us exemplifies this approach. He imagines how future geologists might interpret the “human layer” in Earth’s strata, analyzing how technofossils, altered sediments, and biotic changes would appear in the rock record. Zalasiewicz argues that the most enduring signature of our age, as it will appear in strata tens of millions of years hence, is likely to be a profound reorganization of life itself: a geologically sudden loss of many wild species, the worldwide domination of a small number of opportunists and domestic forms, and the resulting pattern of fossils that will mark our interval as a major biological turning point in Earth history, comparable in sharpness—if not in every detail—to the great extinction boundaries of the past. This perspective treats the extinctions that we cause as one strand within a broader, geologically diagnosable transformation of Earth’s surface systems. It reflects a growing scientific consensus that human activities have initiated a new chapter in Earth’s history, one that will be legible in the rocks for millions of years to come.

Brannen notes that the current carbon emission rate is unparalleled over the past 300 million years. Humans currently emit 40 billion tons of CO₂ annually. If we burn every fossil fuel on the planet, we'll emit 5,000 gigatons of carbon. This will render Earth uninhabitable for mammals and cause sea levels to rise by 200 feet. When carbon dioxide was previously at this level, sea levels were 50 feet higher than they are now. The previous instance of carbon dioxide being released at this rate was during the End-Permian mass extinction, when 10,000 to 48,000 gigatons of carbon were emitted. The temperature rose by 16°C, and 90% of ocean species went extinct.

(Shortform note: To justify the claim that the current carbon emission rate is unparalleled over the past 300 million years, scientists compare the rate of carbon emissions today to the rate of carbon emissions during past mass extinctions. To do this, they analyze the chemical composition of ancient marine fossils and sediments.)

Let's examine the role carbon disruption has played in past extinction mechanisms and the severity, timescales, and mitigation of major extinction events.

Carbon Disruption and Extinction Mechanisms

Brannen explains that CO2 levels have played a significant role in earlier global extinction events. For example, the extinction event during the conclusion of the Permian epoch was caused by a dramatic increase in carbon dioxide, which led to a super-greenhouse effect. The extinction event at the close of the Triassic period was caused by a sharp rise in carbon dioxide, which led to a super-greenhouse effect. The extinction event concluding the Triassic period was less severe than the End-Permian, but it might serve as a template for what could happen in the coming centuries. The timescale of the volcanic eruptions that caused the End-Triassic mass extinction is similar to that of today's warming climate and ocean acidification.

(Shortform note: A super-greenhouse effect is a climate state in which global temperatures are so high that even the polar regions have no permanent ice and experience subtropical conditions. This extreme warming is caused by a dramatic increase in greenhouse gases like carbon dioxide, which trap heat in the atmosphere. The result is a planet with much higher average temperatures, altered weather patterns, and significant changes to ecosystems.)

Severity, Timescales, and Mitigation

Brannen notes that the Earth has experienced many climate changes over millions of years, such as the formation of polar ice caps and the spread of grasslands and savannas. These changes have caused few extinctions, with animals altering their habitats to adjust to the changing climate. However, an unusual series of extinctions began many millennia ago, coinciding with the spread of Homo sapiens across the planet. These die-offs mainly targeted large terrestrial animals and avoided marine life and flora. The pattern of these extinctions suggests that they were caused by human activity rather than natural forces like climate change or meteor strikes.

The First Synthesis of the Unusual Extinctions

A 2004 review in the Annual Review of Ecology, Evolution, and Systematics was one of the first to synthesize the unusual die-offs of large terrestrial animals that paralleled the global spread of Homo sapiens. The authors note that these extinctions were unusual because they targeted large terrestrial animals and avoided marine life and flora. They also note that these extinctions were unusual because they occurred over a relatively short period of time, suggesting that they were caused by a single event or process.

Brannen continues that large land mammals started going extinct many millennia ago, following the migration of humans from Africa to Australia, Europe, Asia, and North America. The extinction of these animals was not the result of natural causes but of overhunting by humans. Over the last four centuries, about 800 species have been recorded as extinct, a figure that's less than 0.1% of the 1.9 million known species. Earth might be better equipped to withstand mass extinctions now than it ever has been before.

(Shortform note: Some researchers disagree with Brannen’s assertion that the extinction of large land mammals was not the result of natural causes but of overhunting by humans. In a research article, scientists argue that the extinction of large land mammals was not solely due to overhunting by humans. They base this on a continent-by-continent comparison of the timing of extinctions, the arrival of humans, and climate changes. The researchers suggest that the extinctions were likely caused by a combination of factors, including climate change, habitat loss, and human hunting.)

A crucial factor in the contemporary planet's endurance is the alteration of the oceans over the last several hundred million years, leading to their present levels of oxygenation. This happened because plankton has become larger and denser over time, letting it sink significantly deeper into the ocean before life consumes it once more. The oceans' biological snowfall consumes oxygen. If plankton are able to descend deeper into the ocean before being consumed, then the ocean's Oxygen Minimum Zone (OMZ)—the section where dissolved oxygen is scarcest—will do so as well. Currently, the oxygen minimum zone is approximately 600 meters beneath the surface.

(Shortform note: The depth of the ocean’s oxygen minimum zone may not be as fixed as Brannen suggests. In a 2018 article, Denise Breitburg, Lisa A. Levin, and Andreas Oschlies explain that the ocean has already lost measurable amounts of oxygen since the mid-20th century. This is based on a global synthesis of ship and float measurements.)

However, in earlier times on Earth—when plankton was smaller and descended more gradually—the OMZ may have been significantly less deep, causing catastrophic effects on life. The current position of the OMZ is secure, deeper than the submerged landforms that host the majority of marine organisms. But in the Paleozoic, when the shallower OMZ began to rise (from sea level rise, global warming, or nutrient pollution, for instance), it spilled onto the continental shelves, bringing oxygen-starved waters up to the shallows where it could smother sea life. The consequence was widespread extinction.

The Discovery of the Oxygen Minimum Zone

In Under a Green Sky, Peter Ward notes that scientists only began to suspect a link between oxygen minimum zones and Paleozoic crises in the late twentieth century. This was when electronic sensors revealed the full extent of modern mid-water oxygen minima, and geochemists recognized that Paleozoic shelf strata at several crisis intervals contain the same kind of finely laminated, organic-rich muds and trace-element patterns that accumulate today beneath low-oxygen waters. This discovery revealed that a major driver of several Paleozoic and Mesozoic ecological collapses was the expansion of these oxygen-depleted zones over shallow marine habitats.