Special Report: What can three-kilometre long ice cores, fossilised leaves and Whanganui's oceanside cliffs tell us about the climate of ancient Earth - and what warnings do they hold for humanity's current, high-carbon trajectory? Marc Daalder reports
In 1996, scientists found a time machine buried deep in the Antarctic ice.
Or rather, it is more accurate to say that the ice itself was the time machine. More than 3.6 kilometres of the stuff, drilled from the centre of the East Antarctic Ice Sheet and sliced into even pillars, two metres long and 13 centimetres in diameter.
If you look around hard enough, you can find time machines like this one just about everywhere. Take the pores on the surface of ancient leaves or the rings of petrified trees or even the striations in the formerly-marine sediments that line Whanganui's beaches and you can glean the level of carbon dioxide in prehistoric atmospheres. Measure the ratios of faintly different types of oxygen trapped in the fossilised shells of microscopic marine organisms and you'll be able to tell the temperature at the ocean's surface 15 million years ago.
These time machines offer us a glimpse at ancient Earths, at once familiar and alien to our own. Taken together, they also show an undeniable trend - when carbon dioxide levels rise in the atmosphere, so do temperatures.
Climate change deniers like to say the "climate is always changing", as if that is a reassuring thought. The truth is that the entirety of human civilisation has taken place in the blink of a geologic eye. In the length of that blink, the climate has been remarkably stable, rarely varying more than half a degree from the norm, but this is an outlier when the broader history of the planet is taken into view.
Now, in just a century, through what amounts to a planetary-level experiment in terraforming, we have thrust our civilisation into new and uncharted waters. At least, waters uncharted by humans. The deniers are right - the planet itself has undergone these changes and worse many times before. But once we take a look through the time machines all around us at what those changes truly meant, that thought should not be a mollifying one - it should terrify us.
The Holocene
We lucked out. Nearly all of human civilisation, from the invention of agriculture to the turn of the 21st Century, has occurred in a narrow band of global temperatures about one degree wide called the Holocene.
The burning of fossil fuels has put an end to that. Temperatures are now 1.1 degrees Celsius above the preindustrial average and only rising - all at an unprecedented rate. Some scientists have devised a new name for this novel epoch in the geological history of our planet: the Anthropocene.
This is not to say there was no variation in temperature over the course of human civilisation. There were several notable periods in which above- or below-average temperatures were recorded and their impacts keenly felt. But these anomalies began slower than anthropogenic warming did and resulted in less overall deviation from the norm.
Take the Little Ice Age, often dealt part of the blame for a range of famines, natural disasters, wars and social and political breakdowns between 1500 and 1850. This cooling occurred, but it involved less than a degree of change from the Holocene average and then mostly in the Northern Hemisphere. Other regions were affected - temperatures dropped by about 0.56 degrees in New Zealand - but unevenly and at different times.
The warming we have witnessed in just the past four decades puts the centuries-long wobbles of the Little Ice Age into perspective. Over the length of human civilisation, this level of change is unprecedented.
And its impacts are already being felt. In just the last couple of months, we've seen record heat waves in North America, the Middle East and South Asia, flooding across Europe, China and even here in New Zealand, and wildfires in Russia, the Mediterranean and the American West. All of this will have been influenced by climate change.
Yet the changes we have experienced so far represent only a tiny fraction of what we have locked in via carbon emissions to date. That's because the climate changes far more slowly than we have been emitting.
The past century and a half has seen an unprecedentedly fast release of carbon dioxide into the atmosphere. Since 1889, the concentration of CO2 in the atmosphere has jumped by more than 40 percent. In 1905, CO2 concentration reached 299 parts per million (ppm) - at that stage the highest level found in the 800,000-year-long record, as the time machines will show us. Just over a century later, in April of this year, the concentration hit 419 ppm.
This is by far the fastest increase in CO2 concentration in 66 million years of records.
A stark report from the Intergovernmental Panel on Climate Change (IPCC), released in August, had similar conclusions.
"Human influence has warmed the climate at a rate that is unprecedented in at least the last 2000 years," the IPCC found.
"The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to many thousands of years."
According to Olaf Morgenstern, a principal scientist at NIWA and the lead author of the IPCC report's chapter on human influence, "human civilisation has never existed in a climate this hot".
Earth is used to climatic changes on a great scale, but not at such great speed. In a matter of centuries, we have discovered vast stores of carbon that took hundreds of millions of years to create and burned them up in a relative heartbeat. We've been spending on a carbon credit card with no thought as to how we will pay our debts. Over the coming decades and centuries, those debts will come due in the form of catastrophic change if we fail to halt emissions.
This is why climate journalist David Wallace-Wells calls climate change "a revenge of time". The catastrophic weather we have experienced so far is largely a result of our past emissions, not our ongoing ones. The effects of today's emissions have yet to be felt.
The climate deniers' adage that "the climate is always changing" reckons neither with how the changes we have wrought are a radical departure from past changes, nor with the gravity of what those past changes actually meant. Deniers bring up historic climate change to imply either that the changes we are seeing today are natural, or that the continued existence of human civilisation shows those changes can't have been that bad.
They point to the Little Ice Age, as if that is the limit of devastation that climatic changes have wreaked. They are unequivocally wrong. As the late Wally Broecker, one of the world's preeminent climate scientists, was fond of saying, "The paleoclimate record shouts out to us that, far from being self-stabilising, the Earth’s climate system is an ornery beast which overreacts even to small nudges."
That vast array of time machines we mentioned earlier can show us exactly how the climate system has reacted to nudges in the distant past, large and small alike. That, in turn, displays that we are only now beginning to experience, in temperature and extreme weather, the impacts of our extraordinarily large, carbon-fuelled nudge.
Last Glacial Maximum, 20,000 years ago
Our first stop is not so long ago, if you take a geological view of time.
For the past 800,000 years, the climate has been on a rollercoaster, with temperatures dropping to plunge the world into long ice ages, then briefly rising for a few thousand years before descending into a frozen Earth once more. Every 100,000 or so years, this cycle repeats itself.
We are now 20,000 years in the past and in the thick of the most recent ice age. Called the Last Glacial Period, this ice age will 14,000 years, in which temperatures at New Zealand's latitude are about six degrees colder than the present day.
That may look like a small number, but it is enough to grow an ice sheet over most of the South Island. The Southern Alps are covered by a single, gargantuan glacier which has advanced and retreated at least three times during this period as a result of minor temperature fluctuations. Icebergs float by the Chatham Islands, at the same latitude as Christchurch.
Christchurch is no longer a coastal region and instead finds itself nearly 100 kilometres inland. As more of the world's water concentrates into glaciers and ice sheets, sea levels drop as much as 125 metres at the height of the ice age.
In this environment, there is no need for a ferry from Wellington to Picton - you can simply walk.
In this icy world, it is now cold enough that continuous forest doesn't extend south of Tauranga. Even patchy forest is virtually non-existent below Whanganui. Native conifers grow in Auckland and the Coromandel, while most of New Zealand's landmass is covered either in glacier, grassland or shrubs and tussock.
This cycle of defreezing and refreezing is a result of complex changes in the Earth's orbit - but the direct cause is greenhouse gases. Shifts in the Earth's own tilt and its proximity to the sun kick off climate changes which are then amplified by the release or sequestration of carbon dioxide.
We can see a clear correlation between temperature and CO2 levels over the course of the past eight glacial cycles. At the height of the last glacial period, about 20,000 years ago, the concentration of carbon dioxide in the atmosphere was as low as 182 parts per million. That's a significant fall from the Holocene when, prior to industrialisation, CO2 levels fluctuated between 257 ppm and 287 ppm.
For context, atmospheric carbon dioxide concentration is likely to average 416 ppm over the course of this year.
Clearly, if we want to understand the impacts of our current level emissions on the climate, we'll have to journey further back.
Last Interglacial, 128 thousand years ago
It is now 128,000 years before the present day. Sea levels in New Zealand have not only returned to modern measures but exceeded them by a few metres.
The level of carbon dioxide in the atmosphere is now back to the interglacial norm - around 286.8 parts per million at its peak. This correlates to temperatures a couple degrees above preindustrial levels at the poles and one to two degrees above the preindustrial average in New Zealand.
How do we know so much about a world so distant from our own? These are the secrets of those time machines, and particularly of the ice cores.
In 1996, scientists drilled 3.6 kilometres into the East Antarctic Ice Sheet, at a base called Vostok Station.
Although previous ice cores had been drilled at Vostok, the 1996 core set records as both the longest core ever harvested and - at that stage - the longest history of glaciation in any ice core.
Dating an ice core can be as simple as counting the rings of a tree. The difference is the rings in an ice core are stacked on top of one another vertically. In Greenland and Antarctica, summer snow melts and refreezes, losing air bubbles as it does so, while winter snow can be accompanied by sediment and dust from elsewhere in the world. After hundreds or thousands of years of compression, this shows up as alternating bands of clear, bright summer ice and darker, cloudy winter ice. Each pair of rings is a year.
The further down you go into an ice sheet, however, the harder it is to count. At times, layers run together. The lower levels of the ice sheet are often spreading towards the coast, meaning the signal becomes muddled and unusable by the end.
Vostok had two additional challenges with its core. First, it turns out the station is built atop Antarctica's largest subglacial lake. The last few hundred metres of the 1996 core are therefore mixed in with the refrozen waters of that lake. The second issue is that, as with a number of other Antarctic drilling sites, Vostok receives relatively little snow. That makes the layers exceedingly difficult to count - the scientists who drilled the 1996 core could only manually count the first 50,000 or so years, out of a total of 414,000 years' worth of ice.
Other methods have since been pioneered. Summer snow and winter snow have different acidities, due to chemical changes in atmospheric composition between seasons. If you run a current of electricity through an ice core, a stronger signal will pass through the acidic parts, allowing scientists to engage in a higher precision form of ring-counting.
Ice core dates can also be calibrated with reference to climatic or geologic events that occurred worldwide. A large volcanic eruption, for example, may have cast ash over most of the world. If you can find that band of ash partway through an ice core and know the date of the eruption, that helps anchor the date of that part of the core.
What about CO2 levels and temperature?
The former is easy enough. As snow hardens under pressure into ice, the air within it is compressed into bubbles. The concentration of CO2 in the air in those bubbles can be measured in the same way we measure carbon dioxide levels in the atmosphere today.
There is a catch, however. Because it takes several decades for snow to harden into ice, the air captured in a given ring is likely to be newer than the ring itself. In other words, if it takes a hundred years for snow to be compressed, then the air that's finally trapped in those bubbles will be a century younger than the ice itself.
That problem is compounded for sites like Vostok, which receive less snow and therefore take longer to form ice. In some places, the differential between the age of ice and the age of the gas trapped within that ice can be as large as a thousand years, though there are ways to correct for this.
Determining ancient temperatures from ice cores is a bit more complex than determining CO2 levels. This usually revolves around isotopes - atoms of oxygen and hydrogen that have one or two more neutrons than usual. This difference doesn't change how the element functions, but it does make it heavier.
More than 99 percent of oxygen and hydrogen are the lightest variants. But that means there's still a small amount of "heavier" atoms in almost every sample of either of the two elements. When they combine to make water, therefore, there will be a little heavy water mixed in with the usual stuff.
The key to determining temperature differences is the ratio of heavy to light water in a given sample.
Evaporation of heavy water requires more heat and energy and therefore happens more in a warmer climate. That means water which evaporates from the ocean and forms clouds is going to have, on average, a lower heavy-to-light ratio than the ocean itself. Because it is heavier, this heavy water will also be the first to fall as rain before the clouds travel far enough north or south to form snow and, eventually, the ice in our ice cores.
In a cooler world, there's even less energy to evaporate heavy water, meaning that a section of an ice core which has a lower heavy-to-light ratio will come from a colder period.
It turns out that the relationship between this ratio and temperature is nearly linear. If you send a paleoclimatologist a sample of rainwater from your hometown, they'll probably be able to tell you the average annual temperature.
The information that ice cores reveal about temperature and climate at the poles may slightly overstate global conditions due to a phenomenon known as polar amplification. Nonetheless, the direction of travel is clear and - as we shall see - other time machines can tell us about global averages.
So, what do the ice cores tell us about the last interglacial? If you take a sample 1.9 kilometres down the Vostok ice core and analyse CO2 concentration of the air bubbles, you'll find carbon dioxide rose to 286.8 parts per million about 129,000 years ago. Analyse the isotopic ratios from a similar spot and you'll see temperatures rising to as much as 3 degrees above present levels in Antarctica.
If you use a more recent ice core, drilled from EPICA Dome C (560 km northeast of Vostok), you'll get a similar result. Air bubbles from just over 128,000 years ago (or 1690 metres down the Dome C core) show CO2 concentration peaking at 290.49 ppm.
The isotopic ratio captured by Dome C ice indicates temperatures rose by as much as 5.5 degrees above present-day levels in this period, though they mostly kept between 2 and 4 degrees.
This interglacial world, despite having similar CO2 concentrations to preindustrial times, is warm enough that hippopotamuses now wallow in Germany's Rhine River and forests grow at the northernmost point in mainland Europe.
Climatic differences are milder in New Zealand, where analysis of buried pollen from the period found temperatures may have only risen 1 to 2 degrees.
Scientists remain unsure of why temperatures in this period were so much higher if carbon dioxide was not. It is possible this may relate to the changes in the Earth's orbit which drove previous climate changes.
Either way, it's clear the last interglacial is not quite a suitable comparator for today's highly-carbonised atmosphere. Instead, we'll have to go further back.
Middle Pleistocene, 335,000 years ago
Imagine a world where temperatures are 4 degrees higher than preindustrial levels. The seas have risen 74 metres in a few millennia and modern humans have yet to walk the Earth. That is the world of 335,000 years ago, when the level of carbon dioxide in the atmosphere last approached 300 ppm. That is also the sort of change we had already locked in by 1905, when CO2 concentrations again reached that mark.
According to those Vostok air bubbles, now sampled from 3.1 kilometres below the surface of the East Antarctic Ice Sheet, CO2 peaks at 298.7 ppm in this interglacial period. Temperature data from 2.5 kilometres down the Dome C ice core shows the world grew rapidly warmer in just a couple thousand years, rising from about present day levels to 3.75 degrees above modern levels. In just as short a time, temperatures will plunge 3 degrees again.
Sea levels operate on a bit of a lag here, taking about 8000 years to rise by 74 metres as the previous ice age ended. That's still a rate of just under 1 centimetre a year - triple the modern-day rate of sea level rise.
Homo sapiens have yet to walk the Earth. Homo erectus is dominant across Africa and Eurasia, having learned to use tools and mastered fire. Some have already managed to travel by sea to Indonesia and other southeast Asian islands.
In New Zealand, although the coastlines look similar to today's, the topography is radically different. The volcanic fields that today characterise Taupō, Auckland and Taranaki have yet to erupt. The Fouveaux Strait separating the South Island from Stewart Island is much shallower and, in the preceding ice age, would not have been submerged.
This may feel ancient, but it still isn't an analog for today's CO2 levels. If we want to truly understand what climate change deniers unknowingly mean when they say the climate has always changed, we must leap even farther back in time, now by millions of years, to a period where glacial and interglacial cycles are shorter but average temperatures across the board are still much higher.
Middle Pliocene, 3 million years ago
We now find ourselves in the Pliocene epoch, between three and four million years ago. Almost all of the North Island south of Tūrangi is now underwater and ancient penguins and megalodon sharks roam New Zealand's seas - none of which are cooler than the coast of Northland is today. Massive, toothed pelicans with wingspans as wide as six metres brawl with shearwaters and albatross for domination of rookeries on South Island coastal cliffs and gravel plains extend out from the newborn Southern Alps, encompassing what is now Nelson, Canterbury, Otago and Southland.
The Banks Peninsula is now a massive island 30 kilometres off the coast of a narrower, elongated South Island. Stewart Island is fully part of the mainland and the tip of Waipounamu reaches across the Cook Strait to encompass Wellington and the Kāpiti Coast. The remainder of the North Island is bisected at Auckland, with Northland forming its own, third island.
All of human evolution is now in the rearview mirror.
This world is hotter, with temperatures a few degrees above today's levels on average, but more than 10 degrees warmer close to the poles. That's high enough to foster the herds of camels and horses that are now roaming the high Arctic - and to prevent the ice sheets in West Antarctica and Greenland from forming at the size they are now, contributing to sea levels as much as 30 metres above present day.
We've now gone farther back in time than the ice cores, with their hundreds of thousands of years of climate history, can take us. We must turn to new time machines to teach us about the Pliocene, although some of the principles remain the same.
Take the Whanganui basin as an example. During the Pliocene, much of the land between Whanganui and Taihape was underwater, gathering different sediments steadily as sea levels rose and fell. A period of seismic activity then thrust the basin upward, perfectly preserving the sediment deposits for a team of GNS and VUW Antarctic Research Centre scientists to examine.
The group found up to 23 metres of the sea level rise during the epoch's warm period was due to the melting of Antarctic ice. This means if CO2 levels stabilised around the concentration they sat at in the mid-Pliocene, we could be in for the same magnitude of change if given enough time.
"Our results suggest that major loss of Antarctica’s marine-based ice sheets, and an associated [global mean sea level] rise of up to 23m, is likely if CO2 partial pressures remain above 400 ppm," the lead author of the study, GNS scientist Georgia Grant, wrote.
Sediments also hold the secrets of yet more time machines.
For these, oxygen isotopes remain a crucial tool. Instead of finding them in ice, we harvest them from the shells of single-celled marine organisms called foraminifera, or forams. The size of a grain of sand, these creatures use oxygen and other chemicals in sea water to build shells of calcium carbonate. When they eventually die and fall to the sea floor, their shells are fossilised atop the carapaces of those that went before them, each containing hints as to the climate of the world in which they lived.
Just like with ice cores, we can drill a sediment core from the ocean floor. This is an extract of up to 300 metres of fossilised forams, representing millions of years of climate data. After determining the age of the shells by aligning ash deposits with known volcanic eruptions or measuring the magnetic field in the sediment, isotopic analysis is a simple inversion of the process used for ice cores.
For the forams, the higher the heavy-to-light ratio, the more water will have been evaporated from the oceans and preserved in ice sheets. So careful analysis of the fossils of these long-dead marine creatures can tell us the amount of ice on Earth when they lived. The ratio is also affected by the temperature at the ocean floor, which means they can also be used to determine global temperature.
Unlike ice cores, the forams are unaffected by polar amplification and give us a look at the temperatures where they lived and died. Take enough deposits from around the world and you can reconstruct the world's average temperature at a given point in time, as well as a range of local temperatures.
Scientists have also used other trace elements within forams, like boron or the ratio of magnesium to calcium, to calculate the acidity of ancient oceans, sea surface temperatures and CO2 concentration.
This is how we know the mid-Pliocene occurred in a period where atmospheric carbon dioxide levels were significantly elevated above the preindustrial average. CO2 in this period fluctuated in a range of between 330 and 400 ppm.
The Pliocene is the most recent time period that CO2 levels settled above preindustrial levels. But we have, in a matter of decades, surpassed even these carbon excesses of three million years ago.
For comparison, CO2 concentration hit 330 parts per million in New Zealand in June 1975 - just a few years after NIWA began taking measurements at Baring Head. It hit that threshold three years earlier at the American National Oceanic and Atmospheric Administration's (NOAA) Mauna Loa lab in Hawaii, which is usually seen as the world standard for CO2 measurements.
On May 6, 2013, Mauna Loa recorded carbon dioxide concentrations of 400 parts per million, marking the first time the atmosphere has been this carbonised in millions of years. Baring Head registered the same measurement a few years later.
Since then, the pace of global emissions has only increased. Mauna Loa hit 410 ppm in 2017 and the daily average surpassed 420 on April 30 of this year, for the first time in millions of years.
That means even the Pliocene, with its high Arctic camels and dwindling Antarctic ice sheets, is not a suitable comparator for the world we are creating with greenhouse gas emissions. There is one journey left before us, to the depths of the Miocene epoch, some 14 to 16 million years ago.
Middle Miocene, 16 million years ago
Temperatures are now 6 to 7 degrees above modern levels. It's warm enough even in the South Island that crocodiles haunt freshwater lakes in central Otago. The Southern Alps have not yet developed and the volcano that will eventually erode into the Otago Peninsula has just begun to erupt. The Banks Peninsula is still 5 million years away from being jolted up from the ocean floor to form the Pliocene island we saw earlier.
In Africa, apes are just beginning to evolve as a distinct lineage from monkeys. Conifers line the shores of Antarctica and the southern ice cap is much smaller than its present-day, continent-spanning form.
New Zealand is now a single, spindly landmass, stretching from south of Stewart Island to north of where Cape Reinga sits today. Taranaki, the East Coast and most of Canterbury are submerged.
A subtropical climate in Otago has fostered the development of an ecosystem more Australian than anything seen in this country since, all centred around an enormous shallow lake nine times larger than Taupō is today. It is now called Lake Manuherikia after the river that flows through its dry basin, past the town of Saint Bathans.
Eucalyptuses and palm trees dot the shoreline of the lake and it borders carbon-sucking peat wetlands and swamps.
Over the course of some three million years, hundreds of animals will die in the lake and surrounding wetlands and their fossils will be preserved in sediments and bogs for future study.
That's how we know the first moa have already evolved and are stalking the nearby beech forest. They are accompanied by the earliest ancestors to the kiwi, as well as a giant flightless parrot that stands a metre tall, and a bird belonging to an extinct family of flamingo-like creatures.
A handful of fossils from a small terrestrial mammal have been found at the Saint Bathans site, showing ground mammals did exist in New Zealand at one stage prior to the arrival of humans. The remains of a giant burrowing bat three times larger than New Zealand's surviving endemic bat species have also been found.
Given the warmer temperatures, this lake is also able to support a much more diverse range of reptiles and amphibians than are found in New Zealand today. That includes a three metre long crocodilian and large land turtles from a now-extinct lineage.
Although this world seems radically different to the one we inhabit today, it was created by atmospheric CO2 levels similar to those we are now measuring. Deep sea forams tell us that carbon dioxide concentration peaked at no more than 450 parts per million in the middle Miocene - a level we are on track to surpass in just decades.
So, by travelling back 14 to 16 million years, we can catch up to the last time there was around this much carbon dioxide in the atmosphere. What about the amount of CO2 we expect to see by the time emissions are finally reduced to zero?
Early Miocene, 23 million years ago
For that, we must take one last, brief journey even further back in time. We're now a little earlier in the Miocene, 23 million years ago, scouting the rainforests surrounding the Foulden Maar volcano in central Otago.
Study of the leaves from this rainforest, which better suits modern-day Queensland than Otago, can tell us that carbon dioxide concentration sat between 450 and 550 ppm in this period.
Average annual temperatures in this region are about 18 degrees - that's hotter than anywhere in New Zealand today, or even than Nairobi, Pretoria, Damascus or Mexico City.
The leaves have been perfectly preserved by repeated volcanic eruptions, which have left no oxygen behind to support microbes that might otherwise break down the organic material. Scientists used two methods to determine CO2 concentration. One is familiar to us already: Isotopic analysis, this time of carbon isotopes found in the fossils.
The second is novel but simple. Plants still need the same amount of CO2 to live, regardless of the changing atmospheric conditions. Therefore, species evolve more or fewer stomata (pores) on their leaves to regulate their CO2 intake. As CO2 levels rise, the number of stomata tends to fall because they need fewer pores to absorb the same amount of the gas. If CO2 levels fall, then the number increases to suck up more.
By comparing different samples of the same species from different time periods, we can establish the amount of CO2 in the atmosphere based on how many stomata are visible on each fossil.
The leaves can also help us divine ancient temperatures. As it turns out, plants tend to evolve smooth-edged leaves in hotter climates and serrated edges in colder regions. Compare enough leaves from similar species in different times and we can get a good guess at average annual temperatures.
The Anthropocene, 2021
This steamy, early Miocene world, with rainforests south of Queenstown, was produced by CO2 concentrations that we could reach as soon as 2050 if no action is taken to reduce emissions.
Even limiting warming to 2 degrees could see CO2 concentration rise to 500 ppm by the end of the century, while current policies set us on course for as much as 600 ppm by 2100.
There's another problem raised by these carbon-heavy past worlds: They seem too hot. Experts have had trouble using existing climate models to recreate worlds of Miocene temperatures with Miocene carbon concentrations.
This could mean the temperature or CO2 data is flawed. But it also raises the spectre that there's some sort of positive feedback loop we don't understand, in which warming becomes self-reinforcing and magnifies the impact of greenhouse gases. Given the models we use to understand past climates are the same as those we use to predict the future impacts of climate change, the possibility we could be missing a major feedback loop is troubling, to say the least.
"This indicates that climate sensitivity must have been elevated during the MCO [Miocene Climate Optimum], leading to highly elevated temperatures at moderately elevated [atmospheric] pCO2. With 415 ppm measured for the first time in spring 2019, and with no sign yet of decreasing emissions, we are fast approaching MCO-level pCO2," one study of climate sensitivity recently warned.
"The race is now on to improve our knowledge of the Earth system in order to understand whether such moderate levels of pCO2 may also cause a devastating temperature increase of up to 7°C in the (near?) future, and if so, take action to prevent it."
This is not to say that we are, by definition, guaranteed Miocene temperatures just because we have achieved Miocene CO2 concentrations. About a third of the carbon dioxide we release into the atmosphere today is reabsorbed by the natural world - mostly the oceans.
If all emissions were to cease tomorrow, then those same carbon sinks would begin to reduce the CO2 stockpile in the atmosphere. However, as the oceans become more saturated with carbon dioxide, their ability to absorb it diminishes. Eventually, we would reach a new equilibrium, probably stabilising slightly above the level of warming we have currently achieved. In this scenario, the danger of massive warming "locked in" doesn't come to pass.
Of course, all emissions aren't about to cease tomorrow. So far, in fact, emissions increases show no sign of slowing, let alone reversing. And, as we've seen, even the stated policies of world governments aren't currently consistent with the sort of radical action needed to limit warming to 1.5 or 2 degrees.
The longer we wait to reach net zero emissions, the worse the changes we lock in will be. We will probably, eventually, reach an equilibrium, but the later we do so, the worse that stable climate will be for human civilisation.
Icarus
Taking in the broad sweep of climate history, we humans have achieved a remarkable and horrible thing. In a matter of centuries, we have undone millions of years of cooling. We have thrust our civilisation and the health of countless ecosystems to the brink of the unknown.
The power we have wielded in terraforming Earth is truly awesome. And given that we are the root of the problem, surely we can also implement the solution.
Or can we? This hope of changing our warming course is reliant on a continual ability to influence the atmosphere, albeit in an opposite way to how humanity has done so up until now. In a few decades, that choice may be taken out of our hands.
Study of climatic changes in the past and of today's Earth systems shows certain "tipping points" can be reached, at which stage the impacts of climate change become irreversible in a human lifetime.
These tipping points include the self-reinforcing dieback of the Amazon rainforest, which holds 76 billion tonnes of CO2, or the melting of methane-rich Arctic permafrost.
In some cases, they have been reached in the past.
One of the conclusions of a 2016 study of Foulden Maar leaf fossils was that CO2 levels briefly doubled in the early Miocene from 515 ppm to more than 1000 ppm, before falling to 425 ppm. This all occurred in a geological flicker, over just 20,000 years, but it was enough to trigger the melting of more than half of the Antarctic ice sheet over the next 100,000 years, even though CO2 had returned to lower levels.
Similarly, the August IPCC report warned some sea level rise is now unavoidable.
Over the next two millennia, sea levels will rise by at least two metres even if warming is limited to 1.5 degrees. A 2 degree pathway locks in two to six metres of sea level rise. A 5 degree scenario would see between 19 and 22 metres of sea level rise.
Glaciers will continue to melt for decades or centuries, the IPCC found. "Permafrost melt is irreversible at centennial timescales."
Changes in global ocean temperature, the acidification of the deep ocean and the deoxygenation of the ocean can't be undone for centuries to millennia.
This is the stark warning that the paleoclimate record has issued to us. It is Wallace-Wells' "revenge of time", in that the devastation we are experiencing now is the irreversible result of past actions, and that our current and future actions can only avert so much. It is Broecker's "ornery beast", a catastrophe that has an easily flicked "on" switch but an "off" switch that might not even be connected to anything.
In the end, it is hard to escape the thought that climate change is the result of uniquely human hubris.
All of the fruits of human civilisation were only made possible by the remarkable stability of the climate, from the first intentional planting of ancient grains through to the present day.
We are now venturing into new territory. The speed of the change we are causing is not only unprecedented over the length of human history but over 66 million years of records. For better or for worse, this unique and anomalous experiment is the most significant thing humans have done so far.
On a global scale, we have reenacted the ancient Greek tale of Icarus, who flew so close to the sun that the wax binding together his mechanical wings melted and he plunged into the ocean. Global emissions continue to rise - we continue to approach the sun, flirting with fiery ruin.
By the time we realise the wax is melting, it may be too late to safely descend.
Whether we veer away from our current course of self-immolation is still - for now - up to us. Every year, month, or even week we delay sends another ancient forest up in flames or drowns another coastal wetland. In mere minutes of emitting, we reverse the geological and climatic clock by years, if not decades. The world we are forging is irreconcilable with the one in which our civilisation came to be.
No human has ever experienced what we are about to.
Special thanks to Dr. Christina Riesselman at the University of Otago and to Jacob Anderson for their help with this article.
