A summary by Till Stoltenow

The article I will summarize here, written by science writer Peter Brannen, takes us on a journey back to several climatic times of the earth. Understanding how our planet’s climate worked in past periods can help us to understand what devastating effects the anthropogenic climate change might have in the future.

For those who are interested, I recommend to read original article, which is written in a fantastic style, and has lots of profound information and additional links. But since reading and fully understanding it, will take you way more than one hour, I hope this summary will help the rest of you to get an overview.

The climatic importance of CO2

To begin we need to understand, that our planet is fickle and so is its climate. In fact, in all of its history, the earth has undergone a lot of changes, induced by its own forces of nature like plate tectonics, and even the influence of other celestial bodies. Today it’s more important that a variation in the composition of the Earth’s atmosphere of as little as 0.1 per cent has meant the difference between Arctic rainforests and a half-mile of ice atop North America. That negligible amount of air is carbon dioxide.

During the entire half-billion-year long history of animal life, CO2 has been the primary driver of the Earth’s climate. There were warmer times with high CO2 levels changing gradually with cooler times and low CO2 levels. Sometimes though, when massive amounts of CO2 at once were released into the atmosphere, it had catastrophic effects.

Today, atmospheric CO2 sits at 410 parts per million, a higher level than at any point in more than 3 million years, and is still rising with an incredible speed. This is endangering the stability the world’s climate had over the entire time of recorded human history, which took place in perhaps the most stable climate window of the past 650,000 years. Levels of CO2 that high could push the climate into a state it hasn’t seen in tens of millions of years, a world for which Homo sapiens did not evolve.

To truly appreciate the changes coming to our planet Branen takes us on a trip back in time, that will begin with the familiar climate of recorded history and end in the feverish, high-CO2 greenhouse of the early age of mammals, 50 million years ago. The first couple of steps back will illuminate what sort of ill-tempered planet we’re dealing with, which has in fact been an ice-age planet for the past 3 million years, one marked by the swelling and disintegration of massive polar ice sheets in response to tiny changes in sunlight and CO2 levels.

Ten thousand years ago the history of civilisation started, as humans, after 200,000 years of wandering, developed agriculture and settled down. Humans, with a surfeit of calories, began to divide their labour, and the Earth’s oldest cities, such as Jericho, were founded.

By 5,000 years ago the last bigger change of world climate ended, as the glaciers from the last ice age had stopped melting and oceans, which had been surging for 15,000 years, finally settled on modern shorelines. Sunlight had waned in the Northern summer, and rains drifted south toward the equator again. The at that time green Sahara began to die, as it had many times before. People who for thousands of years had lived in it abandoned the now-arid wastelands and gathered along the Nile and the age of pharaohs began.

By geologic standards, the climate has been remarkably stable ever since, until the sudden warming of the past few decades. That’s unsettling because history tells us that even local, trivial climate misadventures during this otherwise peaceful span, like droughts of several decades, volcanic eruptions, or the Little Ice Age, lasting roughly from 1500 to 1850, helped to bring societies to ruin. But these are not even remotely on the same scale of disruption as that which might come in our future.

As we jump back 20,000 years the world ceases to be recognizable. We see what a difference 5 to 6 degrees can make. A scale of change similar to the one that humans may engineer in only the next century or so, though in this case, the world is 5 to 6 degrees colder, not warmer.

Ice shields the size of today Antarctica’s now rest atop North America, and Northern Europe, and as a result, the sea level is 120 meters lower. The retreat of the seas made most of Indonesia a peninsula of mainland Asia. Vast savannas and swamps linked Australia and New Guinea, and of course Russia shared a tundra handshake with Alaska. There were reindeer in Spain, and glaciers in Morocco. And everywhere was dust, created by the ice, polverizing rocks and mountains in its way. All of this dust seeded the seas with iron, which then nurished plankton, which bloomed around Antarctica and pulled gigatons of CO2 out of the air, freezing the planet further.

CO2 in the atmosphere registered only 180 ppm, less than half of what it is today. In fact, CO2 was so low, it might have been unable to drop any further. Photosynthesis starts to shut down at such low  levels, a negative-feedback effect that might have left more CO2, unused by plants,in the air above, acting as a brake on the deep freeze. This was the strange world of the Ice Age.

For almost all of the Earth’s history, the planet was a much warmer place than it is today, with much higher CO2 levels. Acknowledging this does nothing to take away from the potential catastrophe of future warming. Because, we humans, along with everything else alive today, evolved to live in our familiar low-CO2 world, a process that took a long time.

Over tens of millions of years the stately march of plate tectonics seems to have driven long-term climate change toward a colder, lower-CO2 world, by balancing volcanic CO2 and rock weathering, a pogress binding a lot of CO2. When Earth’s blanket of CO2 was finally thin enough the ice ages began. But the climate was not stable during this period, as the ice advanced and retreated. But while the depths of planetary winter took tens of thousands of years to arrive, the leap out of the cold tended to be sudden and violent. This is where positive feedback loops come in: When the last ice age ended, it ended fast.

As, around 13,000 years ago, the ice sheets of the Northern Hemisphere lost their grip, darker land around the melting margins became exposed to the sun for the first time in 100,000 years, accelerating the ice’s retreat. Permafrost melted, and methane rose up in the atmosphere. Colder, more CO2-soluble oceans warmed, and gave up the carbon they had kept in the Ice Age, warming the Earth even more. Relieved of their glacial burden, volcanoes awoke, adding further CO2 to the atmosphere. And by this ending the last ice age.

This was the most recent interglacial period, the last of many breaks between the ice ages, and the last time the planet was roughly as warm as it is today. But sea level was 7 to 10 meters higher than it is now. Modelers have tried and mostly failed to explain how a world about as warm as today’s could produce seas so strangely high.

Yery soon though, we may well have warmed the planet enough to trigger similarly dramatic sea-level rise, even if it takes centuries to play out. But our civilization is headed well beyond the warmth of the last interglacial, or any other interglacial period. Therefore we must take our first truly big leap into geologic time, millions of years into the past.

More than 3 million years in the past carbon dioxide in the atmosphere is at 400 parts per million, a level the planet will not see again until September 2016. This world is 3 to 4 degrees Celsius warmer than ours, and the sea level is up to 25 meters higher. There are forests not far from the South Pole. We are now outside the evolutionary envelope of our modern world, but as to atmospheric carbon dioxide, 3 million years is how far back we have to go to arrive at an analogue for 2021. The differences are notable. In the Canadian High Arctic, where today tundra spreads to the horizon, evergreen forests come right to the edge of an ice-free Arctic Ocean. Though the world as a whole is only a few degrees warmer, it’s a full 10 to 15 degrees Celsius warmer in the long twilight of northern Canada.

A common projection for our own warming world is that, while the wet places will get wetter, the dry places will get drier. But the past seems to defy this predictions, for reasons not yet fully understood. This ancient wetness might come down to inadequacies in how we model clouds, which are under no obligation to behave in physical reality as they do in simplified lines of computer code.

So this is the world of the distant present. While today’s projections of future warming tend to end in 2100, this look back illuminates just what sort of long-term changes might inevitably be set in motion by the atmosphere we’ve already engineered. As the great ice sheets melt, the permafrost awakens, and darker forested land encroaches on the world’s tundra, positive feedbacks may eventually launch our planet into a different state altogether, one that might resemble this bygone world. Nevertheless, human civilization is unlikely to keep atmospheric CO2 at this level. So even more ancient and extreme analogues must be used.

We’re now deeper in the past, and the planet appears truly exotic.

The were some of extremely rare and world-changing volcanic eruptions known as large igneous provinces, or LIPs. Some LIPs in Earth’s history span millions of square miles, erupt for millions of years, inject tens of thousands of gigatons of CO2 into the air, and are responsible for most of the worst mass extinctions in the history of the planet. But the eruptions from 16 million years ago were still rather small as far as LIPs go, and so the planet was spared mass death. Nevertheless, the volcanoes raised atmospheric CO2 up to about 500 ppm, a level that today represents something close to the most ambitious and optimistic scenario possible for limiting our future carbon emissions. In this time, this volcanic CO2 warmed up the world to at least 4 degrees Celsius and perhaps as much as  8  degrees above modern temperatures. As a result, there were turtles and parrots in Siberia Instead of grasslands, there were forests everywhere, which made this planet darker than our own world and allowed it to absorb more heat. This change in the planet’s color is just one of the many long-term feedback loops awaiting us after todays ice melts. Long after our initial pulse of CO2, they will make our future world warmer and more alien still. And with CO2 at 500 ppm, the sea level was about 45 meters higher than today.

Today there is no volcano with the powers to launch the planet into a new climate for hundreds of thousands of years, or kill most life on the surface. But the industrial civilization might be able to do this. With CO2 likely to soar past 500 ppm from future emissions, even this might not tell us everything we need to know about our future climate. So we need to go back to a global greenhouse climate that ranks among the warmest climate regimes complex life has ever endured. In the final leap backwards, CO2 at last reaches levels that humans might reproduce in the next 100 years or so.

We have arrived, finally at the end of our journey, in the greenhouse world of the early age of mammals. There are rainforests in Canada, that nevertheless endure months of Arctic twilight and polar night.

For each degree Celsius the planet warms, the atmosphere holds about 6 percent more water vapor, and given that global temperatures at the beginning of the age of mammals were roughly 13 degrees warmer than today, it’s difficult to imagine how uncomfortable this planet would be for Ice Age creatures like us. In fact, much of the planet would be rendered off-limits to us, far too hot and humid for human physiology.

This periodwas also punctuated by some of the most profound and sudden CO2-driven global-warming events in geologic history. Deep under the North Atlantic, 56 million years ago, massive sheets of magma that spread sideways through the crust, ignited vast, diffuse deposits of fossil fuels at the bottom of the ocean. This ignition of the underworld injected something like the carbon equivalent of all currently known fossil-fuel reserves into the seas and atmosphere in less than 20,000 years, warming the planet by another 5 to 9 degrees Celsius. Geological evidence tells about violent storms and megafloods during this ancient example of climate change. Seas near the equator may have been too hot for most complex life. As for the rest of the planet, all of this excess CO2 acidified the oceans, and the world’s coral reefs collapsed. Ocean chemistry took 200,000 years to recover.

To conclude our journey…

This ancient planet is far more extreme than anything being predicted for the end of the century by the United Nations or anyone else. After all, the world that hosted the rainforests of Canada was 13 degrees Celsius warmer than our own, while the current global ambition, enshrined in the Paris Agreement, is to limit warming to less than 2 or even 1.5 degrees. Part of what explains this glaring disparity is that most climate projections end at the end of the century. Feedbacks that might get you to these ancient level warmth play out over much longer timescales than a century. But the other, much scarier insight that Earth’s history is telling us is that we have been missing something crucial in the models we use to predict the future.

This sauna of our early mammalian ancestors represents something close to the worst possible scenario for future warming (although some studies claim that humans, under truly nihilistic emissions scenarios, could make the planet even warmer). The good news is the inertia of the Earth’s climate system is such that we still have time to rapidly reverse course, heading off an repetition of this world in the coming decades. All it will require is instantaneously halting the super-eruption of CO2 emitted into the atmosphere that began with the Industrial Revolution.

We know how to do this, and we cannot underplay the urgency. The fact is that none of these ancient periods is actually a suitable analogue for the future if things go wrong. It took millions of years to produce those ancient climates and the rate of change right now is almost unprecedented in the history of animal life. Our environment changes faster than evolution can accommodate, leading to mass extinction. Life has speed limits.

The moral of the story is, that next few fleeting moments are ours, but they will echo for hundreds of thousands, even millions, of years. This is one of the most important times to be alive in the history of life.


https://www.theatlantic.com/magazine/archive/2021/03/extreme-climate-change-history/61779 3/

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