The Carboniferous Rainforest Collapse: What We Can Learn from Ancient Trees that Sowed their Own Extinction
Introduction
We live in an era of self-inflicted climate change, the direct effects of which are hard to ignore. Precipitation whipsaws between drought and flood, sunsets are turned blood red by the ash of forest fires, and extreme heat pushes our electrical grid to the breaking point. This historical moment easily inspires nihilism and despair- a sense that this crisis is immovable, cataclysmic, and unprecedented, and that we are powerless to change its outcome. However, we can contextualize our current situation by understanding the geological history of the Earth. The ecosystems of the deep past have a story to tell that serves as a warning about the effects of uncontrolled climate change, calling us to action. But they can also provide relief by demonstrating that in times of collapse and destruction, not all is lost. They show us how life reinvents itself and brings the world back into equilibrium. Earth’s history is pocked by climate crises and mass extinctions followed by periods of renewal and diversification. One such event, called the Carboniferous Rainforest Collapse, can help us understand the causes of climate change, how the global ecosystem responds, and it can even give insights into how and why we should fight climate change today.
Part 1: The Carboniferous World
The main characters in our story lived during the late Carboniferous Period, which ranged from approximately 323 to 299 million years ago. Any time travelers waylaid in the Carboniferous would have found themselves on a seemingly alien world that is not well known in the popular imagination. Much of the Earth was covered in immense swampland inhabited by massive tree-like plants known as arborescent lycopsids that have long since gone extinct. The first dinosaurs would not appear for tens of millions of years, during the Triassic Period. The first birds would not fly until 150 million years in the future. The first flowers would not bloom for the next 170 million years, and only become common in 220 million years. There were no fruits or vegetables to eat. One would have been struck by the silence, only punctuated by pigeon-sized dragonflies with 70-centimeter wingspans buzzing overhead like toy quadcopters. These insects were enabled to grow to enormous sizes due to the high-oxygen atmosphere. Shockingly large millipedes called Arthropleura scuttled across the forest floor, attaining lengths of up to 2.6 meters. Yet this world would eventually give rise to the ecosystems that we know today. The upheavals faced by the plants and animals in this distant past were necessary for the rise of our ancestors.
Although the planet was dominated by insects, vertebrates were beginning to colonize the land. Tetrapods, which include all animals descended from the first fish to leave the water, had begun to establish themselves 70 million years prior in the Devonian Period. They would eventually give rise to all reptiles, birds, and mammals. However, during the Carboniferous, most tetrapods were amphibians that laid their eggs in water, like salamanders and frogs today. The particularly moist climate suited their lifestyle well, and they evolved into diverse and occasionally large forms. These included the needle-toothed, fish-eating Baphetidae, and the elongated, short-limbed Colosteidae. Diplocaulus, a meter-long amphibian with horns and a boomerang-shaped skull, lurked at the bottom of lake beds, attacking small fish near the surface. The earliest reptiles, which were less tethered to the water because they laid eggs with shells on land- a radical adaptation- began to diverge from their amphibian cousins at this time.
The plants of the forests and swamps that played host to these animals were particularly fascinating, and extremely important in Earth’s history. The vast swamps, covering millions of square kilometers near the equator, were dominated by a set of species known as arborescent lycopsids that are only distantly related to the trees we know and love today. They lived in a huge, trackless wilderness that stretched from what is now the Midwestern USA, on to Eastern Canada, much of Europe, across Central Asia, and finally to China. Our lost time traveler, knee-deep in water and peering up to the forest canopy, may have described the lycopsids as being trees, but something about them would have felt different. They towered up to 50 meters high and had trunks up to two meters thick, but they supported their weight not through interior wood but with their tough exterior bark. A lycopsid began its life as an unbranched, leaf-covered pole, cutting a figure closer to that of a young saguaro cactus than a tropical rainforest plant. They grew in sunlit groves full of other young, unbranched lycopsids, quickly recolonizing environments that had been stripped of mature trees by floods or hurricanes. As the trees reached their full height, they finally branched near the very top, creating a comically small crown for such a tall plant. They perhaps most closely resembled the fictional Truffala trees dreamt up by Dr. Seuss in The Lorax. Most lines of evidence suggest that the lycopsids lived fast and died young, surviving only 10-15 years, although there is ongoing debate in the scientific community about their lifespan.
The lycopsids- the true masters of the Carboniferous- constituted over half of the biomass by volume of their habitats. This enormous footprint meant that they had an important effect on not just the local, but global environment. Their runaway growth was fueled by a voracious appetite for CO2, the key atmospheric component for photosynthesis. The lycopsids gave the Carboniferous Period its name: during their reign, more carbon-rich coal was laid down than at any other time in Earth’s history. The lycposids’ habitats have even been coined “coal swamps”. During their lives, the lycopsids removed CO2 from the air to fuel their fast growth. As they died, their bodies sank into the mire, first forming peat, which then became compacted into coal over millions of years.
Calculating how much carbon the lycopsid forests put underground, and how quickly this happened is essential for understanding the devastating series of climate change events that followed. Cleal and Thomas estimated that the lycopsid forests were prolific carbon sinks- they sequestered 65 to 234 tons of carbon per acre each year as part of their living biomass. Any increase in plant biomass means that more carbon is removed from the atmosphere. But what happens after plants die is critically important. Today, when plant litter decays, the carbon is quickly released back into the atmosphere in the form of CO2. However, during the Carboniferous Period, 70% of the carbon stored in plants remained in the ground when they died. Decay rates were slow due to low levels of fungal activity and acidic soils. Only 25% of carbon returned to the atmosphere from microbial activity, and another 5% returned via river runoff. Hence, about 43 to 158 tons of carbon were permanently sequestered per year in an area the size of a football field. Modern forests’ ability to sequester carbon pales in comparison, which permanently store only 0.8 to 4 tons of carbon per acre each year. This number is jaw-dropping when viewed with a global perspective: These ancient forests removed 93 billion tons of carbon from the atmosphere annually, compared to only 7.6 billion tons removed by forests worldwide today. Growing evidence suggests that the forests sucked so much CO2 out of the air that they had a destabilizing impact on the climate. There is increasing scientific consensus that plants- particularly the ubiquitous lycopsids- played a central role in triggering climate change by decreasing the ratio of CO2 to oxygen in the air. Feedback loops regulating surface temperatures, ocean ecology, and atmospheric composition amplified the effects that the lycopsids exerted on the ecosystem. Hence, their prolific carbon usage was not without consequences- the late Carboniferous Period featured repeated, large-scale climate change events which eventually dried out the coal swamps and resulted in the extinction of the lycopsid forest and many of its inhabitants.
Part 2: Climate Change and the Downfall of the Lycopsids
The Late Paleozoic Ice Age, which spanned from 360 to 260 million years ago, was next level. It reached its zenith in the Carboniferous Period and featured the most extreme global cooling and glaciation of the last half billion years. The Ice Age that our Stone Age ancestors survived tens of thousands of years ago was mild by comparison. Glaciers menaced mountainous parts of the tropics and evidence suggests that they descended to elevations as low as 4,000 feet near the equator. CO2 levels fell below 200 parts per million (ppm) while oxygen levels skyrocketed- by comparison, today’s CO2 levels stand at 414 ppm, showing that small fluctuations in this trace gas yield disproportionate consequences. Geological evidence from glaciers, plant fossils, and computer-based climate modeling converge to paint a picture of a world rocked by the formation of colossal glaciers interspersed with short intervals of rapid warming. Sea levels rose and fell by many meters with the changing temperatures, quickly altering coastal ecology and causing shorelines to move by hundreds of kilometers in some places.
The rapidly changing, see-saw climate was difficult for global ecosystems to adapt to, although the full effects were not immediately apparent. Just like climate change during the 20th century, it would have at first gone largely unnoticed as the rhythms of daily life unfolded. Generations of lycopsids, giant dragonflies, and monstrous millipedes would have lived and died without ever knowing that a global catastrophe was beginning to develop. The glaciers did not appear out of the blue, nor did they retreat in a single day. But the lives of the plants and animals of the coal swamps were eventually interrupted by more frequent weather disasters - droughts, floods, fires, and unseasonable temperature swings would have occurred with increasing ferocity and regularity. Eventually, the world hit a series of tipping points, changing the trajectory of history forever. The extinction that followed was not the result of a single hammer-blow, like the asteroid that killed the dinosaurs. Instead, the lycopsid forests and many of its denizens crept towards extinction, imperceptibly and over many generations.
The lycopsids, which had reigned supreme for 15 million years, suffered a series of stepwise collapses that first saw their territory diminish, fragment, and then vanish. Glaciers expanded 317 million years ago, before retreating 6 million years later when CO2 levels rose. While this did not destroy the forests, scientists have observed increased “turnover” of plant species where the composition of the forest changed rather subtly. Only 3 million years later- a blink in geological time- a period of even more intense glaciation coincided with a sharp decline in the lycopsids, which were replaced by tree ferns. The lycopsids needed a very wet climate year-round, but parts of the tropics began to experience distinct wet and dry seasons that favored drought-resistant species. As time went by, and the climate rapidly bounced between global warming and cooling, lycopsid spores became less and less common in the fossil record. The lycopsids received a final, crushing blow 2 million years later when the CO2 concentration plummeted to its lowest level, and glaciers continued to encroach as the ecosystem was pushed beyond a tipping point.
It is important to consider that the iconic lycopsids were not the only life to dwell in the coal swamps, and the fates of countless species were intertwined. There is abundant evidence that the Carboniferous Rainforest Collapse was also devastating for animal life. Climate, vegetation, and food webs are all interconnected. While many of the massive insects lived on for millions of years, amphibians did not fare as well. They were the dominant land-dwelling vertebrates throughout the Carboniferous until the rainforest collapse. At least nine families of amphibians went extinct in this short time span, and they never regained the prominence that they once had in the ecosystem. Drier climates were particularly punishing for amphibians, which must lay their eggs in water, and many species relied on fish as their main food source. But not all was lost. While ecosystem fragmentation is initially disastrous for life, populations that were isolated from one another quickly evolved new feeding strategies, and some animals benefitted from the drier climate. A new world was born from the ashes of the lycopsid forests. One obscure clade known as the amniotes benefitted enormously. While at the time, amniotes did not appear to be that much different than amphibians, they would eventually give rise to all reptiles, birds, and mammals. We can count these early amniotes as our own distant ancestors. Amniotes lay eggs that are protected by a membrane, and their young do not go through a larval stage, which liberated them from reproducing in the water. Hence, there was an explosion in early reptile species following the downfall of the lycopsids. Many new reptiles were not confined to eating just fish and insects- some became carnivores, eating amphibians and smaller reptiles. Others became herbivores, a first for land-dwelling vertebrates. The resulting food web began to resemble the modern food web not long after the Carboniferous Rainforest Collapse. This renaissance of reptiles shows that life can reinvent itself in new and profound ways after an ecological catastrophe, although this healing process takes millions of years and comes at a very high cost.
Part 3: What is to Be Done?
While the Carboniferous Period officially ended 299 million years ago, its legacy affects humankind every single day. Humans have been using coal to keep warm and smelt metal for thousands of years, dating as far back as Bronze Age China and Roman Britain, but pre-industrial mining operations remained small and did not affect the global environment. But large coal deposits dating to the Carboniferous have been used as a key energy source since the rise of capitalism, an economic system which, by its nature, demands the ever-increasing exploitation of resources. During the Industrial Revolution, exponentially greater amounts of coal were needed to feed the steam engines powering the economies of Europe and the United States. Today, nearly 8 billion metric tons of coal is extracted from the Earth each year, representing a disproportionately large fraction of our total carbon footprint. The global carbon footprint mirrored the use of coal to power our ever-growing markets. Yearly global CO2 emissions were estimated to be 10 million metric tonnes in 1750, increased to 100 million tonnes by 1836, a billion ton by 1886, and in 2021 stood at 36.3 billion tonnes, their highest level ever. This represents a 3,600-fold increase in CO2 emissions from the onset of the Industrial Revolution until today. This means that we are pumping CO2 into the atmosphere almost as fast as the lycopsids were able to put it underground during their golden age. The carbon that has been stored in the ground for the last 300 million years is now going back into the atmosphere, creating a perverse mirror image of the natural process that caused the most catastrophic climate change event in a half billion years. In the last 60 years alone, human activity has increased CO2 in the air from 320 to 414 ppm, according to US government data.
The lycopsids have pressing lessons to teach us about our place in the world and can give us insights about what to do in our age of climate crisis. They show us that we are not the first dominant lifeform with the power to change the global climate. And they illustrate that even the most abundant species can vanish after sowing the seeds of their own extinction. They tell us, in no uncertain terms, that the world will move on, evolve, and reinvent itself, with or without us. They warn us that if we seal our own fate for the wellbeing of the stock market, our legacy will be represented by a vanishingly thin layer of rock in the fossil record, our own remains trapped in the same strata as the plants and animals that we drove to extinction alongside us.
But unlike the lycopsids, which could never have predicted or stopped their overuse of the world’s resources, we are capable of foresight and planning to avert disaster. We must learn to take a long-term view of history, striving to build a better world not just for ourselves but for generations living thousands of years from now. This involves fighting against the short-term thinking of our economic and political system. The stability of our system relies on returning high quarterly profits for shareholders, no matter the cost to the environment or to humanity. The environmental philosopher Edward Abbey aptly wrote that “growth for the sake of growth is the philosophy of the cancer cell.” In other words, the infinite growth required to sustain capitalism will inevitably lead to our destruction, and we need to decisively break away from using a market-based economy for the sake of securing our collective future. The invisible hand of the free market- driven solely by its own desires- does not know or care about climate change, just like the lycopsids.
Switching to a green economy will require the mobilization of large swaths of society. Scientists and science communicators must see climate activism as being a fundamental part of our work. We should invest our time and energy into building a mass movement to stop climate change. We need to fight the misinformation spewed by right wing media and the fossil fuel industry, both in our writing and as activists in our communities. We must recognize that an effective movement will include a global coalition of Indigenous rights activists, trade unions, leftists, and young people in leading roles. Furthermore, we should only fight for reforms that are not paid for by the working class and poor of the neocolonial world. This stands in contrast to the popular “carbon credits” which were widely supported by large polluters at last year’s COP26 climate summit. In this vein, we should not shy away from calling for far-reaching reforms, including taking extractive industries like fossil fuels into public ownership, so that the enormous revenue they generate can be used to quickly build green infrastructure and make them obsolete, rather than line the pockets of politicians and investors. With coordinated and determined efforts, we will be able to keep the lycopsid’s carbon in the ground and achieve what they could not- a stabilized climate that never hits a disastrous ecological tipping point.
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