“As we put more greenhouse gases into the atmosphere and temperatures rise, we are quickly rewinding the climate clock to climate states not seen in human history,” wrote acclaimed University of Wisconsin paleo-climatologist Jack Williams in an email to The Daily Galaxy. “We can expect that over the next few decades, climates will most resemble those of the warm Pliocene, roughly three million years ago, or perhaps even the hothouse Eocene, 50 million years ago.”
“How can you really compare these great events in the geologic past, which occur over such vast timescales, to what’s going on today, which is centuries at the longest?” asks Daniel Rothman, professor of geophysics and co-director of MIT’s Lorenz Center “So I sat down one day and tried to think about how one might go about this systematically.”
“This is not saying that disaster occurs the next day,” says Rothman about his 2017 study, Thresholds of catastrophe in the Earth system. “It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction.”
Peter Raymond, Professor of Ecosystem Ecology at Yale University and Editor and Chief of the American Geophysical Union’s journal Global Biogeochemical Cycles wrote in an email to The Daily Galaxy: “By absorbing all this fossil fuel CO2 the oceans are becoming more acidic. The timescale is a bit harder to comment on (whether it is 2100 or not) but the scale of ocean acidification and ocean warming will lead to a massive disruption of ocean ecosystems.”
In the past 540 million years, the Earth has endured five mass extinction events, each involving processes that upended the normal cycling of carbon through the atmosphere and oceans. These globally fatal perturbations in carbon each unfolded over thousands to millions of years, and are coincident with the widespread extermination of marine species around the world.
The question for many scientists is whether the carbon cycle is now experiencing a significant jolt that could tip the planet toward a sixth mass extinction. In the modern era, carbon dioxide emissions have risen steadily since the 19th century, but deciphering whether this recent spike in carbon could lead to mass extinction has been challenging. That’s mainly because it’s difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today’s disruptions, which have taken place over just a little more than a century.).
Rothman has analyzed significant changes in the carbon cycle over the last 540 million years, including the five mass extinction events.
He has identified “thresholds of catastrophe” in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.
In a paper published in Science Advances, Rothman proposes that mass extinction occurs if one of two thresholds are crossed: For changes in the carbon cycle that occur over long timescales, extinctions will follow if those changes occur at rates faster than global ecosystems can adapt. For carbon perturbations that take place over shorter timescales, the pace of carbon-cycle changes will not matter; instead, the size or magnitude of the change will determine the likelihood of an extinction event.
Taking this reasoning forward in time, Rothman predicts that, given the recent rise in carbon dioxide emissions over a relatively short timescale, a sixth extinction will depend on whether a critical amount of carbon is added to the oceans. That amount, he calculates, is about 310 gigatons, which he estimates to be roughly equivalent to the amount of carbon that human activities will have added to the world’s oceans by the year 2100.
Does this mean that mass extinction will soon follow at the turn of the century? Rothman says it would take some time — about 10,000 years — for such ecological disasters to play out. However, he says that by 2100 the world may have tipped into “unknown territory.”
Rothman had previously done work on the end-Permian extinction, the most severe extinction in Earth’s history, in which a massive pulse of carbon through the Earth’s system was involved in wiping out more than 95 percent of marine species worldwide. Since then, conversations with colleagues spurred him to consider the likelihood of a sixth extinction.
He eventually derived a simple mathematical formula based on basic physical principles that relates the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change. He hypothesized that this formula should predict whether mass extinction, or some other sort of global catastrophe, should occur.
“It became evident that there was a characteristic rate of change that the system basically didn’t like to go past,” Rothman says.
In other words, he observed a common threshold that most of the 31 events appeared to stay under. While these events involved significant changes in carbon, they were relatively benign — not enough to destabilize the system toward catastrophe. In contrast, four of the five mass extinction events lay over the threshold, with the most severe end-Permian extinction being the farthest over the line.
“Then it became a question of figuring out what it meant,” Rothman says. With further analysis, Rothman found that the critical rate for catastrophe is related to a hidden process within the Earth’s natural carbon cycle. The cycle is essentially a loop between photosynthesis and respiration. Normally, there is a “leak” in the cycle, in which a small amount of organic carbon sinks to the ocean bottom and, over time, is buried as sediment and sequestered from the rest of the carbon cycle.
Rothman found that the critical rate was equivalent to the rate of excess production of carbon dioxide that would result from plugging the leak. Any additional carbon dioxide injected into the cycle could not be described by the loop itself. One or more other processes would instead have taken the carbon cycle into unstable territory.
He then determined that the critical rate applies only beyond the timescale at which the marine carbon cycle can re-establish its equilibrium after it is disturbed. Today, this timescale is about 10,000 years. For much shorter events, the critical threshold is no longer tied to the rate at which carbon is added to the oceans but instead to the carbon’s total mass. Both scenarios would leave an excess of carbon circulating through the oceans and atmosphere, likely resulting in global warming and ocean acidification.
From the critical rate and the equilibrium timescale, Rothman calculated the critical mass of carbon for the modern day to be about 310 gigatons..
The best-case scenario projects that humans will add 300 gigatons of carbon to the oceans by 2100, while more than 500 gigatons will be added under the worst-case scenario, far exceeding the critical threshold. In all scenarios, Rothman shows that by 2100, the carbon cycle will either be close to or well beyond the threshold for catastrophe.
“There should be ways of pulling back [emissions of carbon dioxide],” Rothman says. “But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present.”
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Humans Reversing Long-term Cooling Trend Tracing Back 50 Million Years
Our future on Earth may also be our past. In a study published in 2018 in the Proceedings of the National Academy of Sciences, researchers show that humans are reversing a long-term cooling trend tracing back at least 50 million years, all within the past two centuries.
“If we think about the future in terms of the past, where we are going is uncharted territory for human society,” says the study’s lead author, Kevin Burke, who conducted the work while a graduate student in the lab of Jack Williams. “We are moving toward very dramatic changes over an extremely rapid time frame, reversing a planetary cooling trend in a matter of centuries.”
All of the species on Earth today had an ancestor that survived the Eocene and the Pliocene, but whether humans and the flora and fauna we are familiar with can adapt to these rapid changes remains to be seen. The accelerated rate of change appears to be faster than anything life on the planet has experienced before.
The new study builds upon work Williams and colleagues first published in 2007, which compared future climate projections to historical climate data from the early 20th century. The new study relies on extensive data about climate conditions to probe much deeper in Earth’s geologic past and expand those comparisons.
“We can use the past as a yardstick to understand the future, which is so different from anything we have experienced in our lifetimes,” says Williams. “People have a hard time projecting what the world will be like five or 10 years from now. This is a tool for predicting that—how we head down those paths, and using deep geologic analogs from Earth’s history to think about changes in time.”
During the Eocene, Earth’s continents were packed more closely together and global temperatures averaged 23.4 degrees Fahrenheit (13 degrees Celsius) warmer than they are today. Dinosaurs had recently gone extinct and the first mammals, like ancestral whales and horses, were spreading across the globe. The Arctic was occupied by swampy forests like those found today in the southern U.S.
In the Pliocene, North and South America joined tectonically, the climate was arid, land bridges allowed animals to spread across continents and the Himalayas formed. Temperatures were between 3.2 and 6.5 degrees Fahrenheit (1.8 to 3.6 degrees Celsius) warmer than they are today.
For the study, Burke and Williams—along with colleagues at the University of Bristol, Columbia University, University of Leeds, NASA Goddard Institute for Space Studies and the National Center for Atmospheric Research—examined the similarities between future climate projections as set forth by the Intergovernmental Panel on Climate Change Fifth Assessment Report and several periods of geologic history.
These included the Early Eocene, the mid-Pliocene, the Last Interglacial (129 to 116 thousand years ago), the mid-Holocene (6,000 years ago), the pre-industrial era (before A.D. 1850) and the early 20th century.
Deep-Geological Climates Emerges First from the Center of Continents
The models showed these deep-geological climates emerging first from the center of continents and then expanding outward over time. Temperatures rise, precipitation increases, ice caps melt and climates become temperate near the Earth’s poles.
“Madison (Wisconsin) warms up more than Seattle (Washington) does, even though they’re at the same latitude,” Williams explains. “When you read that the world is expected to warm by 3 degrees Celsius this century, in Madison we should expect to roughly double the global average.”.
About a decade ago, Swedish scientist Johan Rockström and colleagues introduced the idea of “safe operating space,” referring to the climate conditions under which modern agricultural societies developed. By comparing to the deep past, Williams and Burke say, we are able to better understand the planetary boundaries and thresholds that delineate this space.
“The further we move from the Holocene, the greater the potential that we move out of safe operating space,” says Williams, a faculty affiliate with the UW-Madison Nelson Institute Center for Climatic Research. “In the roughly 20 to 25 years I have been working in the field, we have gone from expecting climate change to happen, to detecting the effects, and now, we are seeing that it’s causing harm. People are dying, property is being damaged, we’re seeing intensified fires and intensified storms that can be attributed to climate change. There is more energy in the climate system, leading to more intense events.”
In their paper, the researchers try to strike a balance between alarm and optimism. On the one hand, Earth is headed into the unknown in our children’s and grandchildren’s lifetimes. On the other, life has long proven to be resilient. And, Williams says, in many places we are moving away from fossil fuels toward more sustainable and carbon-free energy sources. But more needs to be done.
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The Last Word from Jack Williams –“Biome-Scale Transformations”
“We know from the geological record that species and ecosystems are highly sensitive to climate change: when global temperatures rise, species can shift their ranges across hundreds to thousands of miles. Wildfire regimes intensify and whole ecosystems can experience biome-scale transformations. These changes are already beginning and we as a society must prepare for further ecosystem transformations,” Williams wrote in his email to The Daily Galaxy.
“The good news,” continued Williams in his email, “is that most past large climate changes seem not to have triggered widespread species extinctions. The closest past counterpart to today’s rapid warming is the warming with the Paleoeocene-Eocene Thermal Maximum, which was triggered by a carbon release to the atmosphere equivalent to burning all coal reserves today. This caused widespread species range shifts and ecosystem transformations, but few extinctions.
“The really big mass extinctions in the past seem to have been caused by a toxic cocktail of multiple causes – e.g. carbon cycle disruption combined with acidic sulfur released from volcanoes, or a meteorite impact combined with worldwide dimming of sunlight blocked by all the dust,” observed Williams. “The worrying concern for today is that today’s toxic cocktail will be climate change combined with overharvesting and overhunting, land use change, and other human pressure. If unchecked, these have a high risk of accelerating the already high rates of extinction, leading to a possible Sixth Mass Extinction.”
Echoing Williams and Rothman, mass-extinction authority Peter Brannen referring to our current Anthropocene, wrote in The Ends of the World: “Human history, though environmentally cataclysmic and sedimentologically interesting, is not usefully described in the terms of a geological epoch on par with a yawning span of time like the Early Cretaceous, an epoch that lasted 600,000 times longer than this newly minted one.
“We are pushing our planet into a place it hasn’t seen in tens of millions of years,” a world, writes Brannen, “for which Homo sapiens did not evolve. To comprehend the analogues for the kind of warming we’ll likely see in the coming decades and centuries, we will need to move beyond the past 3 million years of ice ages entirely, and make drastic jumps back into the alien Earths of tens of millions of years ago. Our future,” Brannen warns, “may come to resemble these strange lost worlds.”
The Daily Galaxy via John W. Williams, University of Wisconsin-Madison, MIT News and Science Advances
Image credit Top of Page: Antarctica –Ground Zero for Climate Change, Maria Stenzel, National Geographic
Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research. A Google Scholar, Avi was formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL). His motto, not surprisingly, is a quote from Carl Sagan: “Somewhere, something incredible is waiting to be known.”