One of the common themes in the previous pages is the carbon dioxide, CO₂, in our atmosphere and oceans. CO₂ is it is one of the major drivers of long-term atmosphere and ocean changes on our Earth through time and is often mentioned when discussing climate change. Many people, for good reason, misunderstand why increasing amounts of CO₂ in our atmosphere and oceans are harmful. Commonly, scientists hear “CO₂ was much higher at other times in Earth’s history, so the current increase in CO₂ is the Earth’s natural variation”. The first part of this statement is correct: CO₂, at times, was much higher than today in the geologic past. The other part of the sentence “the current increase in CO₂ is the Earth’s natural variation” is not correct. Human activity on Earth has increased the amount of CO₂ entering the atmosphere and oceans, altering the natural cycling of CO₂ in the Earth’s systems (oceans, atmosphere, rock, and biosphere).

The rock record provides us with a chronological record of changes in climate through time. We can use these data and compare them to the current warming event. This investigation allows us to better understand the impacts of human activity on the environment (Kidder and Worsley, 2012). On this page, we’ll go through these data and the levels and trends of CO₂ in the Earth’s past, what those levels were in Earth’s more recent past, and what CO₂ levels should (approximately) be today. We’ll also talk briefly about the CO₂ levels our Earth may experience in the near future.

Main Points

• Throughout its history, the Earth has experienced very high levels of CO₂; these are called greenhouse times. Times of lower CO₂ and significant ice sheets are called icehouse times.
• Over the past 800,000 years, CO₂ has remained largely between 280 and 180 ppmv CO₂, a change of only 100 ppmv CO₂.  This is due to natural variations of the Earth’s system. Colder times within an icehouse time is called an glacial period; warmer times are called an interglacial period.
• The rate of atmospheric CO₂ increase in the atmosphere from an interglacial to a glacial period was very slow.
• The Earth should be going back into a glacial period, but instead, CO₂ is rising at an unprecedented rate which can be traced back to the Industrial Revolution.
• Within the past 57 years alone, the amount of CO₂ in the atmosphere has risen by 100 ppm, the same amount that differentiates a glacial from an interglacial period in the previous 800,000 years of Earth’s history.
• The rate of CO₂ increase in the atmosphere today is on the order of over 30,000% greater than the rate of atmospheric CO₂ increase from an interglacial to a glacial period.
• Humans are already feeling the effects of increased carbon dioxide by acidifying oceans, decreased temperature gradients across the globe, sluggish ocean circulation, increased ocean stratification, melting glaciers, increased storm activity, increased wildfires, drought, and heavy precipitation.
• We are changing our Earth through our existence, and it’s time to make a decision about how we can curb our emissions and change for the better, and the most correct way to do that.

CO₂ in the Geologic Past

Scientists have several methods of how we determine the amount of CO₂ that was in Earth’s oceans and atmosphere at different times in the geologic past. We won’t review all of them here, but will instead talk about them through the ‘Meet the Scientist‘ blog and through our own research in the ‘Science Bytes’ blog. These data, when compiled and plotted together, show us some very interesting patterns in CO₂ through the Phanerozoic (the time of ‘visible life’), or the time from the Cambrian (~542 million years ago) to today.

Reading the Data

Before we really dig into the data, we’ll take a paragraph or two to talk briefly about the figure shown below, as most of this discussion will center around the data presented within it.

This figure shows the amount of CO₂ from the Cambrian to today. The bottom of the chart is geologic time, from about 542 million years ago (Cambrian) to 0 million years (today). The abbreviations stand for the different periods: Cm, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Pennsylvanian; Tr, Triassic; J, Jurassic; K, Cretaceous; Pg, Paleogene; and N, Neogene.

On the left axis of the chart is the amount of CO₂ in the atmosphere, measured in parts per million by volume (ppmv). Let’s say you were to take a certain volume of air, for example, 1 cubic centimeter. If you measure the amount of CO₂ in that cubic centimeter and it comes out to 1500 ppmv, this means that out of every million molecules in that cubic centimeter, 1500 of them are CO₂. Notice that the left side of the chart ranges from 0 to 8000 ppmv.

On the left side of the chart is ‘Times Quaternary Average’. Recall from the ‘Geologic Time’ that the Quaternary is the period of time from 2.6 million years ago to today. So this axis is telling us how much more (or less) CO₂ was in the atmosphere at any time on the chart. For example, at 300 million years ago, CO₂ was about 1 times that of the average Quaternary value.

Now for the meat and potatoes of the chart: the different colored lines that are plotted against CO₂ ppmv, geologic time, and times the Quaternary average represent different climate models that take real data, connect the data points through time, and then put an error range on the data. Error within each model is represented by the lighter-colored ‘envelope’ around the darker colored lines for each respective model. Some of the error is quite large (for example, look at the large envelope around the GEOCARB III model in the Cambrian). This is due to scientists only having a few data points from this period of time. The more data points we have, the less our error is; for example, notice how small error becomes closer to time 0. If the data have large error, this does not mean they aren’t useful. We can still discern long-term trends and patterns from the data. In addition to the three different models, the blue points with vertical lines plotted on the chart are real (from proxies) data points, which are labeled as the ‘Royer Compilation’ in the chart. This is after the study by Royer et al. (2004), who compiled several different atmospheric CO₂ measurements from different proxies and plotted them against geologic time to uncover the general trend of CO₂ in the past.

CO2 Trends Through Time

Deep Time

Now that you are an expert at reading climate data and charts, let’s look at some general trends in the data.

Notice from about 500 to 400 million years ago in the Ordovician and Silurian, atmospheric CO₂ was really high, somewhere between 2,500 and 6,000 ppmv, which is 10 to 23 times the amount of atmospheric CO₂ averaged over the Quaternary (2.6 million years ago to today, or technically 2004, since that is when this data was published). During this time, one of the largest evolutionary events took place: the Great Ordovician Biodiversification Event. This event, which lasted several million years, involved the evolution of new species in the marine realm. So much so that the number of new species during this time nearly tripled (Harper et al., 2015)! Many of these new species had body parts and shells made of calcite.

But wait, if atmospheric CO₂ was high, doesn’t this mean the ocean was acidic from absorbing all that CO₂from the atmosphere, meaning the calcite shells would dissolve? In this case, absolutely not! Remember from the ‘Ocean Chemistry & Acidification’ page that the ocean has a huge buffering capacity, meaning that it has the ability to absorb large amounts of CO₂ from the atmosphere without becoming too acidic and thus harmful to marine life. But, recall from the ‘Ocean Layers & Mixing’ page that the surface ocean, which exchanges gases with the atmosphere rather quickly (10-100 years), completely mixes with the deep ocean on a 1,000 year timescale. The deep ocean, if it becomes too acidic, can also buffer with the carbonaceous ocean sediments.

The main point here is: if atmospheric CO₂rises slowly enough, the entire ocean has time to mix and buffer the CO₂ that it absorbs, without becoming too acidic. That is why during other intervals in Earth’s history when CO₂ was really high, like during the Ordovician, or during the Mesozoic (200-100 million years ago), life flourished and the oceans were habitable to marine organisms. It is important to note here that the Earth was very different during these times of high CO₂, as there were no to very small continental glaciers and the global average temperature was much higher.

These very large-scale trends in CO₂ are due in part to the natural variation of the Earth. Scientists call intervals of very high CO₂, and thus higher average global temperatures, greenhouse times, or a greenhouse world. On the contrary, times of lower CO₂ and cooler average global temperatures are called icehouse times, or an icehouse world.

Earth’s More Recent Past

Let’s zoom into Earth’s more recent past, the Pleistocene and Holocene, and look at CO₂ data from about 800,000 years ago to a few hundred years ago. Like in the above section, we’ll first talk about the graph presented below at left, and what these data mean.

Like the above chart, the one on the left has time plotted against CO₂ in ppmv. Here, the age range is much shorter: from 800,000 years ago to a few hundred years ago (measured in 1000 years before present). This chart is a bit different as it has to different y-axis plotted along the left and right sides. The left y-axis indicates the amount of atmospheric CO₂ in ppmv, which was defined above. The right y-axis indicates global mean temperature in degrees Celsius.

The top line running horizontally across the chart is the amount of atmospheric CO₂ through time, and the bottom line is temperature. Unlike the above chart that used proxies and climate models, this data is very different. The CO₂ measurements presented in this chart come directly from bubbles in ice cores that preserved the air, and thus the amount of gases in the atmosphere, when the bubble was formed. This particular data comes from Antarctica, specifically a location called Dome C. You can see a map of core locations on Antarctica and one of the drilled ice cores on our ‘Proxy Data’ page.

Notice that this data is very cyclical, meaning that there are peaks at regular intervals of time. The peaks represent times of warmer climate within an icehouse world (when there are substantial continental ice sheets) called an interglacial, and the troughs are times of colder climate, called glacials. During glacials periods, continental ice sheets grew, and during interglacial periods, they partially melted back. These cool to warm to cool cycles are completely normal, and it is important to note how regularly they occurred and the amount of CO₂ in the atmosphere during each cycle.

During glacial periods, CO₂  averaged around 180 ppmv. When CO₂  began to rise during an interglacial, it rose to about 280 ppmv. The bottom line: atmospheric CO₂  has largely stayed between 180 and 280 ppmv, a 100 ppmv difference, for the last 800,000 years.

You might notice that on the above chart, it seems that the transition from a glacial period (the troughs in the data, times of higher CO₂) to a glacial (the peaks in the data, times of low CO₂) occur very quickly. Wouldn’t this cause ocean acidification and stratification because the deep ocean wouldn’t be able to mix with the surface ocean quickly enough to buffer the additional CO₂ in the atmosphere? To answer that, we’ll zoom into this data a little closer, specifically a transition from a glacial to an interglacial period.

When we look a bit closer at the transition from a glacial period to an interglacial period, we can see that the amount of atmospheric CO₂  doesn’t rise quite as quickly as we would think. Instead, it takes about 17,672 years to transition from a glacial period to an interglacial period. Let’s do the math to see, on average, how much atmospheric CO₂  increased per year in a transition from a glacial to an interglacial:

From the above equation, we’re basically calculating the slope of the line in the second chart above. By doing this, we can see that on average, atmospheric CO₂ increased by 0.0052 ppmv per year. On a human timescale, this is REALLY slow!! Therefore, the increases in atmospheric CO₂ over the last 800,000 years were slow enough for our oceans to keep pace and buffer the carbon dioxide, which kept seawater pH stable. 

CO₂ Today

One last VERY IMPORTANT point to make before we move on: Notice in the above atmospheric CO₂ charts, at the time closest to today, CO₂ is reaching a peak around 280 ppmv. This means that we should be seeing a leveling-off of atmospheric CO₂, or even a slight decrease today in the most recent decades.

But, that is not the pattern scientists have observed over the last 1,000 years. Instead, there was a ‘plateau’ of the amount of atmospheric CO₂ from 1,000 to about 1700. Between 1750 and 1800, there was a sharp increase in atmospheric CO₂. Coincidentally, this time marks the beginning of the Industrial Revolution, a time when manufacturing and fossil fuel use exploded. Humans began burning coal, natural gas, and oil. Clearing of forests for cities and crops also contributed (and still do contribute) to the rise in atmospheric CO₂. This is because plants take in CO₂ and exhale O₂, or oxygen. Raising livestock and cattle also contributed to the huge increase in atmospheric CO₂. The methane, written as CH₄, that cows fart quickly oxidizes (joins with oxygen) to form more CO₂. All of these factors contributed to the huge rise in atmospheric CO₂ that began during the Industrial Revolution, and are still doing so today.

The Keeling Curve

In the 1950’s, scientists began continuously measuring the amount of atmospheric CO₂ from the Mauna Loa Observatory in Hawaii. The effort was headed by Charles David Keeling, and thus the resulting data has been coined ‘the Keeling curve’. The measurements, shown below, are telling of the rapid rise in atmospheric CO₂ . You can visit the Scripps Institution of Oceanography Keeling Curve webpage to check the current amount of atmospheric CO₂, and look at the trends through time.

Currently, the amount of CO₂ in the atmosphere is flirting with 410 ppm. But is this really a lot of CO₂? Let’s put this into perspective with another figure that combines the data from the previous charts we’ve looked at.

Well, that sure does look like a HUGE increase of CO₂! Let’s do the math again to see on average how much CO₂ has been added to the atmosphere since 1960 and compare that rate to the rate we calculated for a transition from a glacial to an interglacial period.

So, this means that on average, since 1960, 1.614 ppm CO₂ has been added to the atmosphere every year. Remember, CO₂  only increased by 0.0052 ppmv per year in the transition from a glacial to an interglacial period above. This is a 30,938% increase in the rate of CO₂ per year added to the atmosphere!

The bottom line here is: the Earth has not experienced over 400 ppm CO₂ since the Pliocene approximately 3.6 million years ago. As discussed in the earlier ‘Climate Change‘ pages, this huge increase in CO₂ cannot be absorbed by the oceans and buffered on timescales relevant to humans. Remember, it takes the surface ocean about 1,000 or more years to mix with the deep ocean.  For reference, 1,000 years ago is when Old World history was known as the ‘Middle Ages’.

This means that if we keep adding huge amounts of CO2 into the atmosphere, our surface oceans will become more acidic, leading to increased extinctions of animals. In addition, we are causing a decreased temperature gradient from the equator to the poles which is already affecting our weather patterns. Ocean circulation is becoming more sluggish from melting ice, and the oceans are in danger of becoming increasingly stratified, which could lead to decreased oxygen levels in our oceans.

(Notice that here, we have only used one transition from a glacial to an interglacial period to calculate the rate of atmospheric CO₂ increase per year. We have provided the links to access the raw data if you wish to plot it yourself and see what the rates are for other glacial to interglacial cycles, or from an interglacial to a glacial cycle.)

CO₂ in the Future

To get an idea of  the amount of CO₂ in the atmosphere in the near future, scientists have used  climate models to simulate different scenarios. In the chart at right, four different scenarios and their results are presented as indicated by the different colored lines. On the y-axis is the amount of atmospheric CO₂; the x-axis is years into the future. This data has only been modeled to the year 2100, so these scenarios apply to the next few generations. The scenarios, which are called ‘Representative Concentration Pathways, or RPCs, RPCs are climate model standards that are agreed upon and published by scientists all over the world. Commonly, these RPCs are published in the Intergovernmental Panel on Climate Change (IPCC) Reports, which are available and free to read online.

Now, let’s talk a bit about what this chart means. First, let’s focus in on the green line, or RPC3-PD. This line represents what would happen if we stopped emitting CO₂ cold turkey. Initially, the amount of CO₂ in the atmosphere would continue to rise for a number of reasons, but would then level off and begin to decrease.

The middle two scenarios, RPC 4.5 and RCP 6.0, the purple and gold lines respectively, indicate how much CO₂ would rise if we begin to curb our emissions, but not completely. There is a range of uncertainty in these scenarios, indicated by the gray envelope around the lines, as there are several possible options. We could follow the Paris Agreement, or decide to slowly decrease our dependence on fossil fuels by a certain percentage every year, etc. These RCPs, we solemnly hope, are the more likely scenarios.

Let’s talk about the last scenario indicated by the red line, RPC 8.5. This is the scariest of all the scenarios, as it models what will happen if we do nothing to curb our emissions, and continue to heavily rely on fossil fuels, continue chopping down forests, and raise more livestock for consumption. Under RCPC 8.5, CO₂ will skyrocket, which will have devastating effects for humanity. Hopefully, this scenario is the least likely, and after reading through our pages, we hope you will agree that this is a scenario we do not want to see come to fruition.

Where do we go from here?

Humans all over the world are feeling the effects of the huge increase in CO₂, which is causing stronger storms, droughts, sea level rises, and the spread of tropical animals like mosquitoes that carry diseases (e.g., the Zika virus). But where do we go from here? How can we prevent disastrous increases in CO₂? 

The answers to these questions, dear readers, are up to you! That’s right, you  DO have the power to help curb the amount of CO₂ in our atmosphere in coming decades so that future generations can enjoy our beautiful Earth for years to come. First and foremost, you must get out and vote for representatives that are committed to fighting climate change and curbing emissions. Pressure your elected officials at all levels of government by sending emails, calling, and writing letters. Second, start small to begin shrinking your own carbon footprint, or how much CO₂ you emit: recycle, ride a bike when you can, take public transportation, plant trees and flowers, and reduce the amount of meat in your diet.

Here is a list of great websites with information on how to curb your own emissions and waste:

• COTAP.org 25+ ways to reduce your carbon footprint
• Carbonfund.org
• Global Stewards 20 ways to reduce your carbon footprint
• The Guardian How to reduce your carbon footprint
• Climate Care 50 ways to shrink your footprint
• How to Reduce your Household’s Carbon Footprint

There are several of great websites that explain the information on this page or have additional information on CO₂. Here, we have listed just a few:

• University of California San Diego Carbon Dioxide through Geologic Time
• NASA: The relentless rise of CO2 (graphic)
• NASA: The consequences of climate change
• CO2 Earth
• NOAA Trends in Atmospheric CO2
• Scripps Institution of Oceanography The Keeling Curve
• American Chemical Society Keeling Curve
• Union of Concerned Scientists Global Warming Science
• Environmental Protection Agency: Global Greenhouse Gas Emissions Data

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