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A New 66 Million-Year History of Carbon Dioxide Offers Little Comfort for Today

Posted on 11 December 2023 by John Mason


An important new paper published on December 7th 2023 presents the results of an unprecedented in-depth review of all existing datasets regarding past CO2 levels. The paper was accompanied by a press-release, written by Columbia Climate School's Kevin Krajick and reproduced in full below. In particular, the work provides a more detailed picture of probable conditions here on Earth, centuries to millennia into the future, as the planet responds to CO2 levels not seen for 14 million years. Now, over to Kevin.

A massive new review of ancient atmospheric carbon-dioxide levels and corresponding temperatures lays out a daunting picture of where the Earth’s climate may be headed. The study covers geologic records spanning the past 66 million years, putting present-day concentrations into context with deep time. Among other things, it indicates that the last time atmospheric carbon dioxide consistently reached today’s human-driven levels was 14 million years ago—much longer ago than some existing assessments indicate. It asserts that long-term climate is highly sensitive to greenhouse gas, with cascading effects that may evolve over many millennia.

The study was assembled over seven years by a consortium of more than 80 researchers from 16 nations. It appears today in the journal Science.

“We have long known that adding CO2 to our atmosphere raises the temperature,” said Bärbel Hönisch, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, who coordinated the consortium. “This study gives us a much more robust idea of how sensitive the climate is over long time scales.”

Greeland ice-margin

The edge of the Greenland Ice Sheet, where recent melting has left bare ground. (Kevin Krajick/Earth Institute)

Mainstream estimates indicate that on scales of decades to centuries, every doubling of atmospheric CO2 will drive average global temperatures 1.5  to 4.5 degrees Celsius (2.7 to 8.1 Fahrenheit) higher. However, at least one recent widely read study argues that the current consensus underestimates planetary sensitivity, putting it at 3.6 to 6 C degrees of warming per doubling. In any case, given current trends, all estimates put the planet perilously close to or beyond the 2 degrees warming that could be reached this century, and which many scientists agree we must avoid if at all possible.

In the late 1700s, the air contained about 280 parts per million (ppm) of CO2. We are now up to 420 ppm, an increase of about 50%; by the end of the century, we could reach 600 ppm or more. As a result, we are already somewhere along the uncertain warming curve, with a rise of about 1.2 degrees C (2.2 degrees F) since the late 19th century.

Whatever temperatures eventually become manifest, most estimates of future warming draw information from studies of how temperatures tracked with CO2 levels in the past. For this, scientists analyze materials including air bubbles trapped in ice cores, the chemistry of ancient soils and ocean sediments, and the anatomy of fossil plant leaves.

The consortium’s members did not collect new data; rather, they came together to sort through published studies to assess their reliability, based on evolving knowledge. They excluded some that that they found outdated or incomplete in the light of new findings, and recalibrated others to account for the latest analytical techniques. Then they calculated a new 66-million-year curve of CO2 versus temperatures based on all the evidence so far, coming to a consensus on what they call “earth system sensitivity.” By this measure, they say, a doubling of CO2 is predicted to warm the planet a whopping 5 to 8 degrees C.

The new palaeo-CO2 curve

Temperatures and atmospheric concentrations of carbon dioxide over the past 66 million years. Bottom numbers indicate millions of years in the past; right-hand numbers, carbon dioxide in parts per million. Hotter colors indicate distinct periods of higher temperatures; deeper blues, lower ones. The solid zigzagging line charts contemporaneous carbon dioxide levels; shaded area around it reflects uncertainty in the curve. (Adapted from CenCO2PIP, Science 2023)

The giant caveat: Earth system sensitivity describes climate changes over hundreds of thousands of years, not the decades and centuries that are immediately relevant to humans. The authors say that over long periods, increases in temperature may emerge from intertwined Earth processes that go beyond the immediate greenhouse effect created by CO2 in the air. These include melting of polar ice sheets, which would reduce the Earth’s ability to reflect solar energy; changes in terrestrial plant cover; and changes in clouds and atmospheric aerosols that could either heighten or lower temperatures.

“If you want us to tell you what the temperature will be in the year 2100, this does not tell you that. But it does have a bearing on present climate policy,” said coauthor Dana Royer, a paleoclimatologist at Wesleyan University. “It strengthens what we already thought we knew. It also tells us that there are sluggish, cascading effects that will last for thousands of years.”

Hönisch said the study will be useful for climate modelers trying to predict what will happen in coming decades, because they will be able to feed the newly robust observations into their studies, and disentangle processes that operate on short versus long time scales. She noted that all the project’s data are available in an open database, and will be updated on a rolling basis.

The new study, covering the so-called Cenozoic era, does not radically revise the generally accepted relationship between CO2 and temperature, but it does strengthen the understanding of certain time periods, and refines measurements of others.

The most distant period, from about 66 million to 56 million years ago, has been something of an enigma, because the Earth was largely ice free, yet some studies had suggested CO2 concentrations were relatively low. This cast some doubt on the relationship between CO2 and temperature. However once the consortium excluded estimates they deemed the least dependable, they determined that CO2 was actually quite high—around 600 to 700 parts per million, comparable to what could be reached by the end of this century.

The researchers confirmed the long-held belief that the hottest period was about 50 million years ago, when CO2 spiked to as much as 1,600 ppm, and temperatures were as much as 12 degrees C higher than today. But by around 34 million years ago, CO2 had dropped enough that the present-day Antarctic ice sheet began developing. With some ups and downs, this was followed by a further long-term CO2 decline, during which the ancestors of many modern-day plants and animals evolved. This suggests, the paper’s authors say, that variations in CO2 affect not only climate, but ecosystems.

The new assessment says that about 16 million years ago was the last time CO2 was consistently higher than now, at about 480 ppm; and by 14 million years ago it had sunk to today’s human-induced level of 420 ppm. The decline continued, and by about 2.5 million years ago, CO2 reached about 270 or 280 ppm, kicking off a series of ice ages. It was at or below that when modern humans came into being about 400,000 years ago, and persisted there until we started messing with the atmosphere on a grand scale about 250 years ago.

“Regardless of exactly how many degrees the temperature changes, it’s clear we have already brought the planet into a range of conditions never seen by our species,” said study coauthor Gabriel Bowen, a professor at the University of Utah. “It should make us stop and question what is the right path forward.”

The consortium has now evolved into a larger project that aims to chart how CO2 and climate have evolved over the entire Phanerozoic eon, from 540 million years ago to present.


The Cenozoic CO2 Proxy Integration Project (CenCO2PIP) Consortium (2023): Toward a Cenozoic history of atmospheric CO2. Science 382, eadi5177.


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Comments 1 to 6:

  1. This study is an important improvement of understanding. But previous leadership actions fighting against limiting the harm being done to the future of humanity is not excused by this improved understanding only now becoming available.

    As an engineer with an MBA I am focused on developing sustainable improvements by learning to limit harm done and protect others from harm. I appreciate that my developed perspective is currently not the developed norm. However, for humanity to have a lasting improving future on this amazing planet, the pursuit of learning to be less harmful and more helpful to others needs to become the dominant governing understanding.

    Being a civil/structural engineer, the changes of climate conditions affecting the performance of structures and surface water run-off systems has been my immediate concern. And having an MBA has helped me understand that the pursuit of profit can develop many damaging results, including powerful resistance to correcting developed behaviour. Focusing on obtaining monetary reward, or other perceptions of status, can tempt people to compromise the pursuit of limiting harm done. And it can cause people to make up excuses for benefiting from harm done.

    The following quote from the press release highlights a major issue that should concern everyone: “The giant caveat: Earth system sensitivity describes climate changes over hundreds of thousands of years, not the decades and centuries that are immediately relevant to humans.”

    Saying that the distant future of humanity on this ‘potentially only amazing planet humans can survive and thrive on’ is not ‘immediately relevant to humans’ is a very dangerous thing to do even if that belief is ‘more the norm these days’. It excuses the developed callous lack of concern for ‘Others and the future’ that currently significantly compromises ‘human considerations’. It excuses the ‘discounting of future climate change impacts’ to justify being more harmful today than is ‘necessary for people to live basic decent lives’.

    It is important to understand that the engineering application of science requires the pursuit of significant confidence that harmful results will not occur (a very low probability of harmful results). That is what keeps bridges and buildings standing and planes in the air. That is very different from a scientific reluctance to state the potential that harm is being done until there is significant confidence regarding the specifics of the harm.

    A responsible engineer discovering that something is potentially harmful should immediately act to protect against the potential harm until there is confidence allowing them to ensure the safety of the item of concern. The continuation of the ability to live as a sustainable part of the ecosystem of this amazing planet needs to be the governing human consideration. How rich or successful some people are perceived to be is rather irrelevant.

    I understandably extend my ‘engineering concerns’ to the improvement of the distant future of humanity (developing sustainable improvements), even if that is to the detriment of created ‘perceptions of advancement at this immediate moment in time’. Doing something that is considered to be ‘beneficial today’ but may be significantly harmful in the distant future is unacceptable, especially if the benefits of the harmful activity cannot continue to be enjoyed in the future.

    Fossil fuel use is a doubly unacceptable activity. It produces accumulating harmful results, now and into the future. And it cannot continue to be benefited from in the future that it harms. Any ethical argument for ‘the benefits of fossil fuel use’ is understandably limited to very narrow applications like ‘temporary actions that exclusively improve the life circumstances of people who suffer less than basic decent lives’.

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  2. Some say that current CO2 emissions have no effect on future warming (global warming, climate change deniers).

    Some say that past CO2 emissions have no effect on future warming (no-warming in the pipeline climate modelers).

    This study suggests that maybe we are not in as much control of the environment as these other two groups of people would like to think.

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  3. Evan @2,

    Agreed that the popularity of those ‘beliefs’ are a serious problem.

    Those types of beliefs get some support from statements like the one included in this press release: “The giant caveat: Earth system sensitivity describes climate changes over hundreds of thousands of years, not the decades and centuries that are immediately relevant to humans.”

    That correct statement regarding the developed highly influential, but undeniably harmful, attitude and restricted consideration of many people should always be paired with a clear statement of the unacceptability of any person in leadership, or of high status, thinking that way.

    As an example, the likely global average surface temperature in 2100 may seem like a decent measurement point given the increased uncertainty of modelling into the future. But it is understandably not an appropriate measure of success. Here are a set of scenarios with equal 2100 temperature values of 1.7 C (not the desired 1.5 C, but maybe considered to be well below 2.0 C):

    1. Human rates of impacts that would increase temperature are ended without temperature impacts exceeding 1.7 C. And sustainable measures are being increasingly implemented that will reverse previous impacts. The future temperature is not expected to increase above 1.7 C and will actually be reduced beyond 2100, including confidence that there will not be increasing surface temperature due to long term potential warming feed-backs.
    2. Human impacts peaked above 2.0 C before 2100 and have been reversed and removed back down with efforts to further reduce impacts being curtailed. Less certainty that there will be no feedback warming after 2100. Part of that uncertainty regarding future warming is uncertainty about the long term impacts of the peak temperature value and duration of the higher temperatures.
    3. Human impacts have been been successfully limited so that the total warming by 2100 is 1.7 C. But impacts are continuing to increase after 2100.

    Those 3 scenarios are not ‘equally successful’ at sustainably improving the future for humanity. Only the first one could be considered to be ‘real success’. The others are just perceptions of ‘equal success’.

    Note that adaptations to the impacts of climate change are 'not improvements of the future' They are attempts to maintain developed ways of living. And repair of climate change damage is also 'attempts to maintain developed stuff'. That leads to the understanding that developed perceptions of poverty reduction and perceptions of living better than a basic decent life are unsustainable if they rely on harmful unsustainable actions like the use of fossil fuels (even abated use of fossil fuels is unsustainable).

    The challenge is being clear about what developed perceptions do not deserve to be maintained as the developed harming of the future of humanity is corrected.

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  4. The information on earth system sensitivity of 5 - 8 degrees C is very sobering. There are many accounts of what a 6 degree world is like easily googled and its very inhospitable for humans and other species. Because ESS develops on long time frames we might adapt to some extent, but that doesn't really make it any less inhospitable.

    This is one authors depiction of a 6 degree world based on available research. The description is based on such a world developing over the next couple of centuries and a failure to curb emissions, but even if it takes thousands of years as a result of ESS,  many of the outcomes would be similar.

    "Special coverage is given to the positive feedback mechanisms that could dramatically accelerate climate change. The book explains how the release of methane hydrate and the release of methane from melting permafrost could unleash a major extinction event. Carbon cycle feedbacks, the demise of coral, the destruction of the Amazon rainforest, and extreme desertification are also described, with five or six degrees of warming potentially leading to the complete uninhabitability of the tropics and subtropics, as well as extreme water and food shortages, possibly leading to mass migration of billions of people."



    The IPCC seems to have focused most attention on warming and sea level rise rates by 2100. We have projections of around 3 degrees C of warming and  worst case about 5 degrees, and SLR around 1 metre with a worst case 2 metres. The details on longer term trends several centuries into the future,  or millenia into the future like earth system sensitivity, are buried away in their reports or not given much attention.

    The IPCC have a chart buried in their reports showing a worst case of about 10 degrees C by about 2300 if equilibrium climate sensitivity turns out to be high and we just go on burning fossil fuels. Likewise by 2300 SLR could  be well over 2 metres. This may be somewhat attenuated by the impacts of renewable energy already reducing projected coal use, but it would still be a big number and theres a lot of SLR already baked in even if we stop warming right now.

    I wonder if this focus on year 2100 is a deliberate psychological strategy to focus on our immediate future. If they focused on the longer term trends there might be a risk that people would say why worry that won't effect me or my children.

    However warming of for example 3 degrees by 2100 and one metre or so of SLR  doesnt sound very scary to some people, while numbers like 5- 8 degrees longer term and SLR of 10 - 20 metres are obviously intuitively far more scary and certainly get my attention. Clearly we do need a focus on year 2100, for obvious reasons, because its in our lifetimes and adaptation would be very costly,  but I wonder if a bit more attention on longer term time frames would have really shown people the huge scale of change we are facing.

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    Moderator Response:

    [RH] Shortened link

  5. One Planet Only Forever @3,

    Where do you source your "set of scenarios with equal 2100 temperature values of 1.7 C" ?

    I do note the use of IAMs to create scenarios & MAGICC to calculate the climatic outcomes does provide a route to developing such scenarios with a lot less effort than the SSP -> GCM approach. And I also note the rather worrying way such scenarios are presented simply as % cuts by 2050 along with a temperature rise. This is worrying as the cuts required over shorter and longer timescales are airbrushed away, this often along with the substitution of CO2 emissions timings for CO2(equivalent) emissions which can be very significant (as per AR6 WGIII Fig3.6 below).IPCC AR6 WGIII FIG3.6

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  6. MA Roger,

    Thank you for pointing to alternative approaches to global warming scenario development. And thank you for pointing out those justified concerns about how the limited scope and method of presentation of an evaluation can be misleading.

    In all honesty, I conceptually created the 3 situations to try to make the point that how we get to a global warming impact temperature value at 2100 and what is done after 2100 makes a difference. I hoped to present the understanding that there are a variety of ways of achieving a value of 2100 warming impact, with an ‘important differentiating consideration’ being what happens beyond 2100. And I simply chose to call the three created example situations ‘scenarios’.

    I chose 1.7 C in an attempt to be brief but clear that I am not referring to any of the formally presented scenarios. I am pessimistic that the 2100 warming impact level of SSP1-1.9 is likely to be achieved. But I am hopeful that something better than SSP1-2.9 can be achieved. The important point is that there are many ways to conceptualize getting to the same 2100 impact result. And the variety of ways of getting that 2100 result, including what happens after 2100, are not ‘equivalent levels of harm reduction’.

    I will add that my MBA included Organizational Behaviour and Design where I learned that it is incorrect to believe that a specific set of policy or operational rule changes can be certain to produce a desired ‘objective change’ (I believe this is part of the reason for the range of results for each IPCC scenario). New policies and rules may result in changes, or they may not, or they produce unanticipated changes.

    When there is a need to change the collective behaviour of the members of any organization, including the organization of all of global humanity, a diversity of hoped to be helpful policy and rule changes can be conceptualized. And some of those changes would be based on improved understanding developed in the very hard to investigate fields of social, political, and economic behaviours where irrational behaviours can, and do, significantly occur. Irrational behaviour, including resistance to learning to be less harmful and more helpful to others, can especially occur due to the potential to benefit unjustifiably, especially from from secrecy (people less aware than they could be) or the popularity of misunderstanding (abusing the powerful science of marketing).

    Therefore, when pursuing an objective correction of unsustainable or harmful developed behaviours by changes of policy or rules it is important to diligently monitor the changes that actually occur and revise the policy and rule changes as required to increase the chances of achieving the desired correction of collective behaviours. (hopefully accomplished by each COP session).

    I have to add that when I got my MBA education in the 1980s it was rather rare to have the MBA program include ‘social and psychological’ considerations. Most MBA programs of the time, particularly the most prestigious programs, were focused exclusively on ‘maximizing making money matters most’, like Economics, Accounting, Finance, Efficiency of Operations and Material supply, Marketing, and Legalities related to financial activities and contracts.

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