Carbon dioxide levels are claimed to play an important role in climate change, in part because of how climate drivers and the greenhouse gas effect works.
In the guide below, we look at the history of Earth’s carbon dioxide levels (including how fast they might be rising), recent CO2 concentration levels, and what the significance of each might be.
Summary – Earth’s Carbon Dioxide Levels Throughout History, & How Fast They Are Currently Increasing
Carbon Dioxide Levels Have Varied Throughout History
From the available ancient Earth data and related studies & analysis, Earth’s carbon dioxide levels have varied significantly throughout history
How CO2 Concentration Levels Have Been Estimated From The Past
Modern direct ongoing flask and in situ measurements (also called modern sampling) of atmospheric CO2 concentration levels started in 1958, and have progressed to satellite and other instrumental measuring since.
But, before that, we relied on examining proxies and Earth material such as ice cores, rock sediment samples, fossil leaves, and so on.
CO2 Levels Right Now
In June 2022, CO2 concentration in the atmosphere was at 419 ppm (parts per million), up from 412 ppm in September 2019
When Were CO2 Levels Last This High?
The last time CO2 levels are estimated to have been this high (according to various reports) is anywhere from 3 million to 20 million years ago
According to ice cores though, some reports indicate that at the very least CO2 levels haven’t been this high since the last 800,000 years
What Was Earth’s Climate Claimed To Be Like When CO2 Was Last This High?
Some of the mentioned climate and Earth conditions when CO2 levels were last this high in the past were warmer global and Arctic temperatures, as well as higher sea levels
Although, one report indicates that there’s no one single agreed upon answer of when CO2 levels were last this high, or what the climate was like at this point
Recent CO2 Level Trends – How Much Have They Increasing, & How Fast Are They Increasing?
In recent times (over the last 50 to 100 years), CO2 levels have been rising reasonably quickly – roughly at an average rate of 2ppm annually.
CO2 levels have risen recently from close to 300ppm in 1950, to what they are now.
Since 1850, Earth’s global surface temperature has also risen around 0.9 degrees celsius
Trends In How CO2 Levels Have Increased Throughout Earth’s History vs How They Are Increasing Now
Compared to recent CO2 increases which are happening quickly, some reports indicate that CO2 levels increased more slowly in the past – gradually over the course of millenia
Were CO2 Levels Higher At Any Point In The Past Compared To Now?
One report indicates that CO2 levels reached as high as 3000ppm between 150 to 200 million years ago
Several reports indicate that CO2 levels reached as high as 6000ppm between 400 to 600 million years ago.
What Was The Impact Of Higher CO2 Levels On The Climate In The Past, & Why?
One report indicates that in the past when CO2 levels were higher than they are now, there may not have been the same warming effect on the climate because there are other climate drivers (other than man made CO2), such as natural drivers
Some of these potential natural drivers and the potential impact on the climate are outlined in the guide below
Additionally, the speed at which CO2 increased was much slower and much more gradual, compared to how quickly CO2 is increasing today
Estimated Timeline Of Earth’s Historical CO2 Levels
An estimated timeline of Earth’s CO2 levels throughout history is provided in the guide below, ranging back to 500 million years ago
Potential Disputes Over Interpreting Past CO2 Levels & Greenhouse Gas Levels From Available Data
When it comes to interpretation by researchers and scientists of past CO2 levels and greenhouse gas levels from the available data, there might be a wider acceptance of some studies going back say hundreds or thousands of years.
However, there’s also several reports that dispute the interpretations Earth’s past CO2 levels and greenhouse gas levels, especially further back in history to the hundreds of thousands and millions of years ago.
Common disputes are that the interpretations aren’t conclusive or definitive, or that the evidence/data (such as proxies and other ancient material) isn’t clear enough.
Not All CO2 Ends Up In The Atmosphere
It’s relevant to note that not all CO2 emissions end up in the atmosphere – some ends up in the ocean and vegetation too.
It is only the atmospheric CO2 from these emissions that we measure though.
More Information On Climate Change
You can read more about the broader issue of climate change, and how CO2 and other greenhouse gases fit into the climate change and global warming in this separate guide
What Are CO2 Levels Right Now?
[In June 2022, CO2 was at 419 ppm, up from 412 ppm in September 2019] (climate.nasa.gov)
‘Ppm’ means ‘parts per million’, and is a measure of the concentration of CO2 in the atmosphere
Updated global CO2 levels can usually be checked on a monthly or yearly basis.
How Much Have CO2 Levels Increased Since Pre-Industrial Times, Or Since The 1950 Baseline?
Various reports indicate that the amount of CO2 in the atmosphere (measured by CO2 concentration in the atmosphere) has increased over 40% since pre-industrial times (or, since the Industrial Revolution)
One report indicating that much of the CO2 has been emitted since 1970 specifically
According to another report, other greenhouse gases – methane and nitrous oxide – have both increased since pre industrial times too, with methane having the largest % increase since pre-industrial times.
Increase In Just CO2 Since 1950
climate.nasa.gov indicates that CO2 levels have increased from:
300ppm in 1950 to … 415ppm in 2019
wikipedia.org indicates that:
… global annual mean CO2 concentration has increased by more than 45% since the start of the Industrial Revolution, from 280 ppm during the 10,000 years up to the mid-18th century to 415 ppm as of May 2019
[Another way to say this is that] … carbon dioxide has gradually accumulated in the atmosphere, and as of 2019, its concentration is almost 48% above pre-industrial levels
Increase In Just CO2 Since 1970
More than half of the increase in CO2 has occurred since 1970 (royalsociety.org)
Increase In CO2 & Other Greenhouse Gases (Methane & Nitrous Oxide) Since Pre-Industrial Times
From royalsociety.org:
The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased significantly since the Industrial Revolution began.
Since pre-industrial times, the atmospheric concentration of CO2 has increased by 40%, methane has increased by about 150%, and nitrous oxide has increased by roughly 20%.
What Has Been The Recent Yearly Increase In CO2 Levels?
Various reports indicate the increase in CO2 concentration in the atmosphere might average out to roughly 2 to 3 ppm a year
Average Annual Increase
The recent annual average [of annual increase in CO2 ppm] has been hovering around 2.5 ppm (scientificamerican.com)
The global mean CO2 concentration is currently rising at a rate of approximately 2 ppm/year and accelerating (wikipedia.org)
Tables & Graphs Showing CO2 Concentration Over Time
You can view the average rates of CO2 concentration growth rate by decade, in terms of ppm per year, from 1961 until 2020 in the co2.earth resource
Since 2001, the two decades since have both had CO2 concentration growth rates over 2ppm
climate.nasa.gov also has a live graph showing direct measurements of CO2 ppm in the atmosphere from 2005 to the present
How Fast Is Today’s Rate Of Increase Of CO2 In The Atmosphere Compared To The Past?
Referencing NOAA and a Scripps news release, co2.earth indicates that:
Today’s rate of [CO2] increase is more than 100 times faster than the increase that occurred when the last ice age ended.
It’s worth noting though, that this is a comparison to just one point in the past.
How High Could CO2 Levels Potentially Get In The Future?
Apart from plotting out current emissions rates into specific years into the future, it might be impossible to definitively say how high CO2 levels might get in the future, because much of it depends on the policies and emissions of different countries in the future.
Other big variables like various forms of mitigation and CO2 absorption also play a role.
However, two considerations that may provide some guide as to where CO2 levels could go might be:
– Considering the average recent rate of annual increase in CO2 levels
This rate (of roughly 2 to 3 ppm a year) can be multiplied out to a certain year in the future to get an estimation of where CO2 levels could possibly go
– Considering other estimates
One type of estimate is how much CO2 would be emitted into the atmosphere if we burnt all remaining fossil fuel reserves.
One estimate by climate.nasa.gov indicates that:
[Based on some calculations of the fossil fuels we have left] If fossil-fuel burning continues at a business-as-usual rate, such that humanity exhausts the reserves over the next few centuries, CO2 will continue to rise to levels of order of 1500 ppm
When Were CO2 Levels Last This High? What Happened On Earth Last Time CO2 Levels Were This Level?
Estimates for when CO2 levels were previously this high vary anywhere between 800,000 years ago, up to 20 million years ago.
Some of the mentioned climate and Earth conditions when CO2 levels were last this high in the past were warmer global and Arctic temperatures, as well as higher sea levels
One report indicates that there’s no one single agreed upon answer of when CO2 levels were last this high, or what the climate was like at this point
When Were CO2 Levels Last This High?
The last time CO2 was similar to current levels was around 3 million years ago, during the Pliocene.
Back then, CO2 levels remained at around 365 to 410 ppm for thousands of years.
– skepticalscience.com
Another skepticalscience.com report indicates:
… current CO2 levels are the highest they’ve been in the last 15 million years
… scientists analyzed shells in deep sea sediments to estimate past CO2 levels, and found that CO2 levels have not been as high as they are now for at least the past 10 to 15 million years, during the Miocene epoch [and at this point in time … modern humans didn’t exist (climatecentral.org)
… the concentration of carbon dioxide in the atmosphere is this high “for the first time in 55 years of measurement—and probably more than 3 million years of Earth history.”
The current concentration may be the highest in the last 20 million years
– wikipedia.org
Another wikipedia.org reports indicates that:
The present concentration is the highest for 14 million years
Climatecentral.org also mentions:
… when was the last time that CO2 levels were [as high as 400ppm], and what was the climate like back then?
There is no single, agreed-upon answer to those questions as studies show a wide date range from between 800,000 to 15 million years ago.
The most direct evidence comes from tiny bubbles of ancient air trapped in the vast ice sheets of Antarctica.
By drilling for ice cores and analyzing the air bubbles, scientists have found that, at no point during at least the past 800,000 years have atmospheric CO2 levels been as high as they are now.
What Happened When CO2 Levels Were Last This High?
From skepticalscience.com:
The last time CO2 was similar to current levels …
Arctic temperatures were 11 to 16°C warmer …
Global temperatures over this period is estimated to be 3 to 4°C warmer than pre-industrial temperatures …
[The last time there was this much carbon dioxide in the atmosphere …] Megatoothed sharks prowled the oceans, the world’s seas were up to 100 feet higher than they are today, and the global average surface temperature was up to 11°F warmer than it is now (climatecentral.org)
Have CO2 Levels Been Higher In The Past Than They Are Now? If So, How High?
One report indicates that CO2 levels reached as high as 3000ppm between 150 to 200 million years ago
Several reports indicate that CO2 levels reached as high as 6000ppm between 400 to 600 million years ago.
[It’s likely CO2 levels were higher at points in the past, and it’s possible that around 440 million years ago CO2 levels topped 5000ppm …] (skepticalscience.com)
There is evidence for high CO2 concentrations between 150 and 200 million years ago of over 3,000 ppm, and between 400 and 600 million years ago of over 6,000 ppm (wikipedia.org)
What Was The Effect Of Higher CO2 Levels In The Past On The Climate, & Why?
One report indicates that in the past when CO2 levels were higher than they are now, there may not have been the same warming effect on the climate because there are other drivers of climate (other than man made CO2), such as natural drivers
Solar output, volcanic activity, and rock weathering and other variables may have had an impact on CO2 levels and climate
From skepticalscience.com:
[The reason there wasn’t always a warming effect in the past when CO2 levels were higher than they were now is because] CO2 isn’t the only driver of climate.
[For example, it’s possible that around 440 million years ago when CO2 topped 5000ppm, solar output was] 4% less than current levels we see today.
[Throughout different points in Earth’s history] the ice threshold changes [depending on the output of the Sun, and this can change whether the Earth warms or heads into glaciation/widespread ice]
[The examination of strontium isotopes from rocks found that] volcanic activity dropped while rock weathering remained high [during this period around 446 million years ago, and this caused CO2 levels to fall below 3000ppm and caused] the late Ordovician glaciation
If CO2 Levels Were Higher In The Past – Why Are Current CO2 Levels A Concern?
Scientists and researchers think they can identify factors other than CO2 that played a role in influencing climate change events in the past on a longer term scale when CO2 levels were at the same or higher levels.
One example of this is the output of sun (skepticalscience.com), which was ‘about 4% less than current levels’
Right now, some scientists and researches claims those additional factors aren’t at play.
Additionally, in the past, there might have been a more gradual increase in CO2 over hundreds of years and millenia (and the environment could and carbon sinks could adapt) – but, the increase we are seeing now is happening over a very short period of time and is claimed to be happening much faster than past shifts in the climate
A few sources that comment on on this topic further are …
From skepticalscience.com:
[One of the major differences in sudden vs slow change in CO2 and greenhouse levels … is that the environment has a long time to respond to gradual increases in GHG levels]
… there have been several times in Earth’s past when Earth’s temperature jumped rapidly, in much the same way as they are doing today [and] Those times were caused by large and rapid greenhouse gas emissions, just like humans are causing today.
[There were also often] rapid global warming events that happened as a result [of these rapid emissions, and these events could have destructive and dire consequences]
[Read more in this resource by skepticalscience.com]
While there have been past periods in Earth’s history when temperatures were warmer than they are now, the rate of change that is currently taking place is faster than most of the climate shifts that have occurred in the past, and therefore it will likely be more difficult to adapt to … (climatecentral.org)
CO2 Levels In The Past, & Throughout Earth’s History – An Estimated Timeline
When discussing Earth’s CO2 levels throughout history, wikipedia.org notes that ‘Carbon dioxide concentrations have varied widely over the Earth’s 4.54 billion year history’
Below is a chronological timeline of CO2 levels throughout Earth’s history.
We’ve compiled this timeline from different reports …
Estimated Timeline Of CO2 Levels Throughout Earth’s History
Below in a rough timeline of what those CO2 levels might have been at certain points throughout Earth’s history, ranging from the last 160 to 2000 years, up to 500 million years ago:
Last 160, & 2000 Years
There’s several graphs of CO2 levels over the past 160 and 2000 years available at science.org.au
10,000 Years Before The Industrial Revolution
[Widely accepted studies of a] variety of Antarctic cores and indicate that atmospheric CO2 concentrations were about 260–280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years (wikipedia.org)
Last 800,000 Years
From wikipedia.org:
The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years.
During this time, the atmospheric carbon dioxide concentration has varied between 180–210 ppm during ice ages, increasing to 280–300 ppm during warmer interglacials.
A CO2 graph of the last 800,000 years can be found at climate.nasa.gov
There’s also a graph of CO2 and methane levels over the past 800,000 years available at skepticalscience.com
Last Million Years
Carbon dioxide concentrations have shown several cycles of variation from about 180 parts per million during the deep glaciations of the Holocene and Pleistocene to 280 parts per million during the interglacial periods (wikipedia.org)
Last 2 Million Years Ago
Concentrations of CO2 in the atmosphere were as low as 180 ppm during the Quaternary glaciation of the last two million years (wikipedia.org)
Last 200 to 420 Million Years
Reconstructed temperature records for the last 420 million years indicate that atmospheric CO2 concentrations peaked at ~2000 ppm during the Devonian (∼400 Million Years ago) period, and again in the Triassic (220–200 Million Years ago) period (wikipedia.org).
There’s also a graph of the estimated CO2 levels from proxies and fossil soil carbonate over the last 400 million years available at skepticalscience.com
Last 500 Million Years Ago
Concentrations of CO2 in the atmosphere were as high as 4,000 parts per million by mass (ppm) during the Cambrian period about 500 million years ago (wikipedia.org)
Other Information On Earth’s CO2 Levels Throughout History
You can read wikipedia.org’s guide for a full description of what atmospheric CO2 concentration levels might have been at various points throughout Earth’s history
The ‘Geologic Temperature Record’ Wiki resource listed in the sources list below also shows a graph of where these time periods fit into each other.
How CO2 Levels Are Interpreted/Determined From The Past – Different Methods
There’s a few different ways depending on the time period.
Modern sampling is the most recent
Ice cores, rock and sediment samples, and proxy measurements are also mentioned below
– Modern Sampling
As the name suggests, modern sampling is the most recent form of CO2 sampling.
From wikipedia.org:
[Ongoing reproducibly accurate measurements of CO2 from flask samples began in 1958, and] Now, there are … several surface measurement (including flasks and continuous in situ) networks and sites globally that do these CO2 measurements
Ongoing ground-based total column measurements began more recently [and …] satellites have significantly improved the data density and precision of global measurements [in general]
– Ice Cores
Ice cores are for periods before modern sampling started.
From wikipedia.org:
The most direct method for measuring atmospheric carbon dioxide concentrations for periods before instrumental sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice sheets ..
[Most widely accepted ice core studies come from Antarctic cores]
– Rock & Sediments Samples
Rock and sediment samples are for CO2 levels on much longer long time scales, such as hundreds of millions of years ago.
On long timescales, atmospheric CO2 concentration is determined by … organic carbon burial in sediments, silicate rock weathering, and volcanism (wikipedia.org)
[For] 450 Million Years Ago – strontium isotopes from rock sediments found in the ocean, and other proxy records [may suggest at CO2 concentrations] (skepticalscience.com)
– Proxy Measurements (In General)
Proxy measurements is a general phrase used to describe a range potential data sets that might provide an indication of ranges of CO2 concentrations from millions of years ago.
From wikipedia.org:
Various proxy measurements have been used to attempt to determine atmospheric carbon dioxide concentrations millions of years in the past.
These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves
Potential Disputes Over Interpreting Past CO2 Levels & Greenhouse Gas Levels From Available Data
Different reports may dispute the interpretation of CO2 levels or greenhouse gas levels from the past from the available data sets.
This may lead to some reports concluding that certain interpretations or claims are not conclusive, or, that they lack certainty.
They may also claim that evidence or records are sparse, or don’t offer a clear conclusion.
Having said that, some groups do offer clarifications and rebuttals as to why some of the evidence use, such as ice cores, may show variations.
Below are some examples of disputes or disagreements over past CO2 level data:
– Interpretations of fossil leaves
– Interpretations of ice cores
– CO2 data being sparse, or not being conclusive
– Conflict between claims about earlier greenhouse gas levels in Earth’s history, and existing experimental evidence
– Dispute Over Past CO2 Levels From Analysis Of Fossil Leaves
From wikipedia.org:
Based on an analysis of fossil leaves, [some sources] argued that atmospheric CO2 concentrations during the last 7,000–10,000 year period were significantly higher than 300 ppm and contained substantial variations that may be correlated to climate variations.
Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO2
– Dispute Over Past CO2 Levels From Analysis Of Ice Cores
From wikipedia.org:
Relevant to [the fossil leaves] dispute is the observation that Greenland ice cores often report higher and more variable CO2 values than similar measurements in Antarctica.
However, the groups responsible for such measurements … believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice.
When dust concentrations in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between measurements of Antarctic and Greenland CO2 concentrations
– CO2 Data Being Sparse Or Not Conclusive From Hundreds Of Millions Of Years Ago
CO2 data covering the late Ordovician [about 450 million years ago] is sparse with one data point … Given the low temporal resolution of the CO2 record, the data was not conclusive (skepticalscience.com)
– Greenhouse Gas Levels On Earth In Early History Not Clear
If we go back say 2,000 to 4,400 million years ago, or even 4.5 billion years ago when the Earth first formed, some reports indicate that it might be unclear as to what greenhouse gas levels might have been.
For example, it’s thought there was ‘… larger greenhouse gas concentrations in Earth’s early history, though such proposals are poorly constrained by existing experimental evidence’ (wikipedia.org)
Potential Disputes & Uncertainty Over Linking CO2 & Greenhouse Gas Levels To Past Climate Events
There may also be other disputes about different aspects of climate change, with one example being disputes over whether, and how heavily CO2 and greenhouse gas levels can be linked to past climate events.
We explain this dispute in a more comprehensive guide about climate change.
However, a summary of the points of that dispute are:
– Dispute About Around Greenhouse Gases Being The Primary Cause Of Abrupt Climate Events
– Dispute Over CO2 Being The Primary Cause Of Intensification Of An Ice Age
– Greenhouse Gases & The 100,000 Year Problem
Why Is There Concern Over CO2 Levels? (& Why Do We Monitor & Measure Them?)
The concern of CO2 emissions relates to several different aspects of how climate change studies identify CO2 as a notable driver of the climate trend we are seeing, and how emissions today may impact Earth’s climate for years to come.
But, as a brief summary:
Linked To Past Climate Changes
Greenhouse gasses – mainly CO2, but also methane – have been implicated in most of the climate changes in Earth’s past. When they were reduced, the global climate became colder. When they were increased, the global climate became warmer (skepticalscience.com)
Carbon dioxide is believed to have played an important effect in regulating Earth’s temperature throughout its 4.7 billion year history (wikipedia.org)
Linked To Recent Warming Trend
Greenhouse gas emissions are linked as the primary cause of the recent warming trend we are seeing with Earth’s climate
CO2 levels have increased with human emissions and the recent temperature increase since 1850 – around 0.9 degrees celsius.
CO2 Is The Most Relevant Human Emitted Greenhouse Gas
On Earth, carbon dioxide is the most relevant, direct anthropologically influenced greenhouse gas (wikipedia.org)
Human CO2 Emissions May Exceed The Amount That Earth’s Systems Can Take Up Or Balance Naturally
[Natural sources of CO2 emissions are essentially balanced by natural CO2 sinks.] But, Anthropogenic [human] carbon emissions exceed the amount that can be taken up or balanced out by natural sinks (wikipedia.org)
Studies On CO2’s Proven Absorption & Radiation Of Radiation Back To Earth
It’s been proven that CO2 absorbs and emits infrared radiation back to Earth (at specific wavelengths) (wikipedia.org)
Concern Over Long Term Effect Of Current CO2 Emissions’ Potential Impact On Earth’s Temperatures
Excess CO2 emitted since the pre-industrial era is projected to remain in the atmosphere for centuries to millennia, even after emissions stop. Even if human carbon dioxide emissions were to completely cease, atmospheric temperatures are not expected to decrease significantly for thousands of years (wikipedia.org)
Some Potential Effects Of Increased CO2 Levels
A few resources that may outline the effects of increased CO2 levels in the atmosphere might be:
The wikipedia.org resource ‘Carbon Dioxide In Earth’s Atmosphere’, where they discuss the effects of CO2 on plants and crops, and in the ocean
Additionally, you can read this guide for a fuller list of potential effects of climate change in general.
More Information About CO2 Emissions & Climate Change
We put together a more comprehensive guide about emissions and different aspects of climate change
Sources
1. https://climate.nasa.gov/vital-signs/carbon-dioxide/
2. https://en.wikipedia.org/wiki/Geologic_temperature_record
3. https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere#Measuring_ancient-Earth_carbon_dioxide_concentration
4. https://www.skepticalscience.com/print.php?r=77
5. https://en.wikipedia.org/wiki/Abrupt_climate_change
6. https://en.wikipedia.org/wiki/100,000-year_problem
7. https://climate.nasa.gov/climate_resources/24/graphic-the-relentless-rise-of-carbon-dioxide/
8. https://www.scientificamerican.com/article/co2-levels-just-hit-another-record-heres-why-it-matters/
9. https://skepticalscience.com/co2-levels-airborne-fraction-increasing.htm
10. https://www.bettermeetsreality.com/our-most-complete-updated-guide-on-climate-change-global-warming/
11. https://skepticalscience.com/Lee-commentary-on-Burgess-et-al-PNAS-Permian-Dating.html
12. https://skepticalscience.com/climate-change-little-ice-age-medieval-warm-period-intermediate.htm
13. https://www.co2.earth/co2-acceleration
14. https://www.climatecentral.org/news/the-last-time-co2-was-this-high-humans-didnt-exist-15938
15. https://climate.nasa.gov/evidence/
16. https://www.science.org.au/learning/general-audience/science-booklets-0/science-climate-change/2-how-has-climate-changed
17. https://royalsociety.org/topics-policy/projects/climate-change-evidence-causes/basics-of-climate-change/
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You gotta love it.
“Those rapid global warming events were almost always highly destructive for life, causing mass extinctions such as at the end of the Permian, Triassic, or even mid-Cambrian periods.”
Increased temperature did no such thing. Major extinctions have mostly, if not entirely, occurred when the Earth was cold, e.g. from asteroids, volcanoes, and yes possible even precipitous drops in carbon dioxide that essentially wiped out photosynthesis and most life forms.
Just think of the earth as a body. If you get rid of heat the carbon is in the air.
If you need heat most of the carbon is in the cold ocean. If the world is total tropic, the air needs to be high in carbon in the air to get rid of heat into space. The ice cover oceans all over the world need to store carbon to warm up. The cycle of warmer or cooler is just like a body needing or get rid of heat. I call this the earth body effect.