Carbon dioxide levels play an important role in climate change, because of how climate drivers and the greenhouse gas effect works.
In this guide, we look at the history of Earth’s carbon dioxide levels, 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
From the available ancient Earth data and related studies & analysis, Earth’s carbon dioxide levels have varied significantly throughout history
Modern direct ongoing flask and in situ measurements of atmospheric CO2 concentration levels started in 1958, and have progressed to satellite and other instrumental measuring since
But, before that, we rely on examining proxies and Earth material such as ice cores, rock sediments, fossil leaves, and so on.
Although there can be wide acceptance of studies going back for example 10,000 years, there can also be some level of uncertainty in the studies of proxies and materials when determining CO2 levels before that, especially when going more than tens of thousands and millions of years back
The same can be said for levels of greenhouse gases – some sources say going back thousands of millions and billions of years ago – it’s unclear what the levels of GHGs would have been
At present, CO2 levels have reached over 400 ppm (parts per million), standing at 408ppm in November of 2019
The last time CO2 levels are estimated to have been this high (according to various reports) is anywhere from 3 to 20 million years ago
According to ice cores, it is thought at the very least CO2 levels haven’t been this high since the last 800,000 years
In recent times (over the last 50 to 100 years), CO2 levels have been rising quickly – roughly at a 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 celcius
It’s thought that CO2 levels increased more slowly comparatively in the past – gradually over the course of millenia
Some estimations say that CO2 levels were higher than they are now both between 150 and 200 million years ago, and also about 400 to 600 million years ago
With all the available data, evidence and studies, and taking into account what is thought to have caused a change in the climate in the past, there is a current scientific consensus that human CO2 and greenhouse gas emissions from the burning fossil fuels is the primary cause/driver behind the recent Earth warming trend (i.e. the last century or so)
What Are CO2 Levels Right Now?
[In September 2019, they were 412ppm]
How Much Have CO2 Levels Increased Since Industrial Times, Or Since The 1950 Baseline?
In 1950, … 300ppm
In 2019, … 415ppm
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
… carbon dioxide has gradually accumulated in the atmosphere, and as of 2019, its concentration is almost 48% above pre-industrial levels
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%.
More than half of the increase in CO2 has occurred since 1970.
What Has Been The Recent Yearly Increase In CO2 Levels?
The recent annual average [of annual increase in CO2 ppm] has been hovering around 2.5 ppm
The global mean CO2 concentration is currently rising at a rate of approximately 2 ppm/year and accelerating
Today’s rate of increase is more than 100 times faster than the increase that occurred when the last ice age ended.
The trend is that the ppm concentration rate is increasing with time.
How High Could CO2 Levels Potentially Get In The Future?
It’s impossible to say, because much of it depends on the policies and emissions of different countries in the future.
But, as a thought experiment:
We can look at the recent rate of annual increase in CO2 levels, and
[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 (climate.nasa.gov)
This gives as an idea of how we are currently tracking, and where it’s possible we might go if nothing is done by major emitting countries and cities to reduce emissions, and also absorb current emissions from the atmosphere.
[Something else that should be noted is that of the total CO2 that is emitted by humans, some of it ends up in the ocean and vegetation, and the rest mainly ends up in the atmosphere. It is only the atmospheric CO2 that we measure.
So, total CO2 emissions do not equal CO2 in the atmosphere (or an increase in ppm)]
When Were CO2 Levels Last This High? What Happened On Earth Last Time CO2 Levels Were This Level?
Estimates vary anywhere between 3, up to 20 million years ago.
Some sources even say there is no one agreed upon answer.
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.
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 …
… 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.
The last time there was this much carbon dioxide (CO2) in the Earth’s atmosphere, modern humans didn’t exist.
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.
Other estimates say current CO2 levels are the highest they’ve been in the last 15 million years
… 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
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.
Have CO2 Levels Been Higher In The Past Than They Are Now? How High?
Some sources indicate CO2 levels reached as high as 6000ppm between 400 to 600 million years ago.
[Yes, and 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 CO2 levels topped 5000ppm, but also during this time, it is thought that 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
[So, solar output and natural factors like volcanic activity at different points in the past could impact warming and glaciation of Earth’s surface]
There is evidence for high CO2 concentrations between 200 and 150 million years ago of over 3,000 ppm, and between 600 and 400 million years ago of over 6,000 ppm
CO2 Levels In The Past, & Throughout Earth’s History
Carbon dioxide concentrations have varied widely over the Earth’s 4.54 billion year history (Wikipedia.org)
We’ve outlined above what CO2 levels were in the recent history up to the Pre Industrial Revolutions times.
CO2 levels from further back in the past may have been:
Last 160, & 2000 Years
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
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. (wikipedia.org).
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).
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)
The ‘Geologic Temperature Record’ Wiki resource listed in the sources list below shows a graph of where these time periods fit into each other.
How CO2 Levels Are Interpreted/Determined From The Past
There’s a few different ways depending on the time period.
[Ongoing reproducibly accurate measurements of CO2 from flask samples began in 1958].
Now, there are … several surface measurement (including flasks and continuous in situ) networks and sites globally that do these CO2 measurements (wikipedia.org)
Ongoing ground-based total column measurements began more recently (wikipedia.org)
More recent satellites have significantly improved the data density and precision of global measurements (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 (wikipedia.org)
[Most widely accepted ice core studies come from Antarctic cores] (wikipedia.org)
Rock & Sediments Samples
Long Timescales – On long timescales, atmospheric CO2 concentration is determined by … organic carbon burial in sediments, silicate rock weathering, and volcanism (wikipedia.org)
From 450 Million Years Ago – strontium isotopes from rock sediments found in the ocean, and other proxy records (skepticalscience.com)
Proxy Measurements In General
In General – 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 (wikipedia.org)
Potential Uncertainty Or Problems In Interpreting CO2 Levels (& Other Greenhouse Gas Levels) From The Past
There can be various levels of dispute with interpreting CO2 levels from the past.
Additionally, some studies may produce conclusions that lack absolute certainty:
– Dispute In Past CO2 Levels Based On Analysis Of Fossil Leaves
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 (wikipedia.org)
– Some Dispute In Past CO2 Levels Based On Ice Cores
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 (wikipedia.org)
– 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 Times 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, it’s unclear 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)
Uncertainty In Linking CO2 & Greenhouse Gas Levels To Climate Events From The Past
There can be disputes or uncertainty in linking CO2 and greenhouse gas levels to climate events from the past:
– Lack Of Certainty Around Greenhouse Gases Being The Primary Cause Of Abrupt Climate Events
[There is a lack of certainty around whether the release of methane from frozen ice was the primary cause of abrupt climate change events roughly 55 to 65 million years during the PETM period, where there was thought to be abrupt thermal spikes] (wikipedia.org)
– Lack Of Certainty Around CO2 Being The Primary Cause Of Intensification Of An Ice Age
The gradual intensification of [the] ice age over the last 3 million years has been associated with declining concentrations of the greenhouse gas carbon dioxide, though it remains unclear if this change is sufficiently large to have caused the changes in temperatures.
[This is because] Decreased temperatures can cause a decrease in carbon dioxide as, by Henry’s Law, carbon dioxide is more soluble in colder waters, which may account for 30ppmv of the 100ppmv decrease in carbon dioxide concentration during the last glacial maximum (wikipedia.org)
– Greenhouse Gases & The 100,000 Year Problem
There is some uncertainty around the role of impact of greenhouse gases in the 100,000 years problem (wikipedia.org)
Potential Effects Of Increased CO2 Levels
Read more about the effects of increased CO2 on plants and crops, and the ocean in this resource (wikipedia.org)
Additionally, you can read this guide for a fuller list of potential effects of climate change in general.
Why Are We So Concerned With CO2 Levels, & Why Do We Bother To Monitor & Measure Them?
But, as a brief summary:
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)
Greenhouse gas emissions are linked so closely 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.
Carbon dioxide is believed to have played an important effect in regulating Earth’s temperature throughout its 4.7 billion year history (wikipedia.org)
On Earth, carbon dioxide is the most relevant, direct anthropologically influenced greenhouse gas (wikipedia.org)
It’s been proven that CO2 absorbs and emits infrared radiation back to Earth (at specific wavelengths) (wikipedia.org)
[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)
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)
If CO2 Levels Were Higher In The Past – Why Are Current CO2 Levels A Concern?
In the past, scientists and researchers think they could identify other factors that played a role in influencing climate change events 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, those same or additional factors aren’t thought to be at play.
Additionally, in the past, there was 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.
A few sources that comment on it further are:
[One of the major differences in sudden vs slow change in CO2 and greenhouse levels … 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.
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.
Comparing Conditions & Variables From The Past, To Earth’s Current Condition & Set Of Variables
What some people argue or point out with climate change and global warming, is that if you are trying to make a theory or conclusion on what the impact of human greenhouse emissions on the climate will be in the current time (or since warming trend started), you have to be able to prove that all of the same variables and the same conditions existed at one point in Earth’s past, and the outcomes on the climate (in the past or now) are the same, or very similar.
If this is not the case (i.e. there isn’t very solid proof of past similar circumstances, similar factors/variables, and similar outcomes to what we are seeing now), these people argue that current climate science and climate change information holds less validity.
This is obviously just one argument or opinion though.