FOCUS – Co2 concentration in the atmosphere: are we resembling the Pliocene Era

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Written by Dario Ruggiero (July 2013)
Climate change is becoming the most important challenge for humanity in the near future. Understanding the driving factors of global warming is a starting (and key) point in this challenge. One of these factors is the measure of CO2 concentration in the atmosphere. According to the record of CO2 and temperature preserved in ice sheets in Antarctica and Greenland, there is a clear correlation between CO2 concentration in the atmosphere and air temperature (the higher the concentration, the higher air temperature will be). 

On May 9th 2013, CO2 concentration at the Mauna Loa Observatory (the site chosen by Charles D. Keeling, the scientist who started the studies on the proportion of carbon dioxide in the atmosphere) reached 400 parts per million (ppm). During the last glaciations CO2 concentration in the atmosphere was 180 ppm; in 1800 (pre-industrial era) CO2 concentration was 280 ppm; now it has reached 400 ppm. A concentration of 500 ppm is considered by some scientists as an irreversible point, by which Earth will reach a new hotter equilibrium. The last time such values prevailed on Earth was in the Pliocene epoch, 4m years ago, when jungles covered northern Canada. Are we going towards a new hotter period? In this article we described the historic and recent trend in CO2 concentration in the atmosphere, where are such measurements done, and the method behind them.

Source: our elaboration
Thanks are due to ProfPieter Tans, Head of the Carbon Cycle Greenhouse Gases Group, NOAA's Earth System Research Laboratory, for his availability in being interviewed on this topic. (see the interview with Pieter Tans)
“…climate changes forced by CO2 depend primarily on cumulative emissions, 
making it progressively more and more difficult to avoid 
further substantial climate change...”
(NOAA’s ESRL Global Monitoring Division
“…Many scientists are almost certain that an increase of
CO2 concentration in the atmosphere to 500 ppm, now almost unavoidable, 
will cause a deep climate change ...”
(James Lovelock
The Revenge of Gaia, 2006, pag. 71 (Italian edition))
Who measures CO2 concentration in the atmosphere?
The Global Greenhouse Gas Reference Network measures the atmospheric distribution and trends of the three main long-term drivers of climate change, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), as well as carbon monoxide (CO) which is an important indicator of air pollution. The Reference Network is a part of NOAA's Earth System Research Laboratory in Boulder, Colorado. The measurement program includes around the clock measurements at 4 baseline observatories and 8 tall towers, air samples collected by volunteers at more than 50 sites, and air samples collected regularly from small aircraft mostly in North America.
CO2 concentration in the atmosphere: Historic trend

Since 1975, CO2 concentration in the atmosphere has been rising from about 330 ppm to 400 ppm in all four remote locations. CO2 remains in the atmosphere for a very long time, and emissions from any location mix throughout the atmosphere in about one year. The annual oscillations at the two northern hemisphere sites (Barrow, Alaska and Mauna Loa, Hawaii) are due to the seasonal imbalance between the photosynthesis and respiration of plants on land. During the summer photosynthesis exceeds respiration and CO2 is removed from the atmosphere, whereas outside the growing season respiration exceeds photosynthesis and CO2 is returned to the atmosphere. The seasonal cycle is strongest in the northern hemisphere because of the presence of the continents. The difference between Mauna Loa and the South Pole has increased over time as the global rate of fossil fuel burning, most of which takes place in the northern hemisphere, has accelerated.
Graph - Monthly average carbon dioxide data for the four baseline observatories
Source: Global Greenhouse Gas Reference Network
This following graph shows the atmospheric increase of CO2 over 280 ppm in weekly averages of CO2 observed at Mauna Loa. The value of 280 ppm is chosen as representative of pre-industrial air because it is close to the average of CO2 measured and dated with high time resolution between the years 1000 and 1800 in an ice core from Law Dome, Antarctica. [Etheridge et al., 1996].
Graph - Increase of Co2 at Mauna Loa since pre-industrial times
(Difference in ppm on 280 ppm – preindustrial average)
Source: Global Greenhouse Gas Reference Network
"At 400 ppm CO2 is substantially higher now than it has been in the last several million years. Furthermore, CO2 cannot be removed from the atmosphere + ocean + terrestrial biosphere system by natural means on a time scale of many thousands of years. If we stopped emissions now, the excess CO2 would continue to be re-distributed between the atmosphere and oceans, but it will not disappear. The effect of enhanced greenhouse gases on the heat balance of the Earth (and other planets) is also known from well understood physics and chemistry. The implication is that climate change is already programmed in for a very long time. "
Pieter Tans
Interview on LTEconomy (, June 2013
The annual variations of the CO2 growth rate are not due to variations in fossil fuel emissions. The ups and downs in the atmospheric increase are due to variations in the exchange of CO2 between the atmosphere, oceans, and land ecosystems. They are primarily due to small annual fluctuations of temperature and precipitation affecting photosynthesis and respiration on land. It is very important to know that the added CO2 does not disappear, but a portion of it transfers each year from the atmosphere to the oceans and to plants on land. Since CO2 is an acid, the transfer to the oceans causes the surface oceans to acidify.

The variations in the CH4 growth rate are also related to climate anomalies. Analysis of the NOAA data suggests that the recent increase is related to greater-than-average precipitation in tropical regions resulting in above average emissions from tropical wetlands.

Long-term projections of CO2, CH4, and N2O depend on future emissions trajectories, which include land use, and on climate feedbacks as they are incorporated into climate-ecosystem models. An example of the latter would be Arctic warming producing CH4 and CO2 emissions from melting permafrost, out of our control.

The atmospheric (and other) data leave no doubt that the increases of CO2, CH4, and many other greenhouse gases are entirely due to human activities, and that our influence on the heat balance of the Earth continues to grow.
On May 9th 2013, Co 2 concentration in the atmosphere surpassed 400 parts per million 
On May 9th 2013, the daily mean concentration of carbon dioxide in the atmosphere of Mauna Loa, Hawaii, surpassed 400 parts per million (ppm) for the first time since measurements began in 1958. Independent measurements made by both NOAA and the Scripps Institution of Oceanography have been approaching this level during the past week. It marks an important milestone because Mauna Loa, as the oldest continuous carbon dioxide (CO2.) measurement station in the world, is the primary global benchmark site for monitoring the increase of this potent heat-trapping gas.

Carbon dioxide pumped into the atmosphere by fossil fuel burning and other human activities is the most significant greenhouse gas (GHG) contributing to climate change. Its concentration has increased every year since scientists started making measurements on the slopes of the Mauna Loa volcano more than five decades ago. The rate of increase has accelerated since the measurements started, from about 0.7 ppm per year in the late 1950s to 2.1 ppm per year during the last 10 years.

That increase is not a surprise to scientists. According to Pieter Tans, NOAA senior scientist, “The evidence is conclusive that the strong growth of global CO2 emissions from the burning of coal, oil, and natural gas is driving the acceleration.”

Before the Industrial Revolution in the 19th century, global average CO2 was about 280 ppm. During the last 800,000 years, CO2 fluctuated between about 180 ppm during ice ages and 280 ppm during interglacial warm periods. Today’s rate of increase is more than 100 times faster than the increase that occurred when the last ice age ended.

It was the researcher Charles David Keeling of the Scripps Institution of Oceanography, UC San Diego, who began measuring carbon dioxide at Mauna Loa in 1958, initiating what is now known as the “Keeling Curve.” His son, Ralph Keeling, also a geochemist at Scripps, has continued the Scripps measurement record since his father’s death in 2005.

“There’s no stopping CO2 from reaching 400 ppm,” said Ralph Keeling. “That’s now a done deal. But what happens from here on still matters to climate, and it’s still under our control. It mainly comes down to how much we continue to rely on fossil fuels for energy.”

NOAA scientists with the Global Monitoring Division have made around-the-clock measurements there since 1974. Having two programs independently measure the greenhouse gas provides confidence that the measurements are correct. Moreover, similar increases of CO2 are seen all over the world by many international scientists. NOAA, for example, which runs a global, cooperative air sampling network, reported last year that all Arctic sites in its network reached 400 ppm for the first time. These high values were a prelude to what is now being observed at Mauna Loa, a site in the subtropics, this year. Sites in the Southern Hemisphere will follow during the next few years. The increase in the Northern Hemisphere is always a little ahead of the Southern Hemisphere because most of the emissions driving the CO2 increase take place in the north. Once emitted, CO2 added to the atmosphere and oceans remains for thousands of years. Thus, climate changes forced by CO2 depend primarily on cumulative emissions, making it progressively more and more difficult to avoid further substantial climate change.
Recent trend at the Mauna Loa Center
The following graph shows the weekly trend of CO2 concentration at Mauna Loa Center. In the first week of January 2013, it was 395.5 CO2 parts per million and increased gradually during the following weeks, reaching its climax in the 4th week of May at 400.04 CO2 ppm, then sliding to 398.5 ppm in the 2nd week of June. Compared with the average registered in 1800, in June 2013 CO2 concentration in the atmosphere was 116.15 ppm higher.
Graph – CO2 concentration in the atmosphere at Mauna Loa Center
(Weekly average, Jen-June 2013)
Fonte: Nostra elaborazione su Global Greenhouse Gas Reference Network
Recent global trend[1]
As regards with the global monthly average of Co2 concentration in the atmosphere, in April 2013 it was 366.7, up from 393.8 in April 2012. Monthly trend since 2010 is clearly on the rise as in April 2010 CO2 concentration in the atmosphere was 389.7 ppm, 7 ppm less than the current one. As for CO2 concentration annual average in the atmosphere, the greatest increase happened between 1980, when it was 338.8 ppm, and 2010, when it reached 388.6, almost 50 ppm more. Between 2010 and 2012, the annual average of CO2 concentration in the atmosphere increased by almost 4 ppm and, finally, the average of the first 4 month in 2013 reached 395.9 ppm.
Graph – CO2 concentration in the atmosphere
(Monthly global average, Jen2010 -Apr2013)
Source: Our elaboration on Global Greenhouse Gas Reference Network
Graph – CO2 concentration in the atmosphere
(Annually global average, 1980-2013)
Source: Our elaboration on Global Greenhouse Gas Reference Network
Annex 1: The NOAA ESRL Carbon Cycle Greenhouse Gases group

The NOAA ESRL Carbon Cycle Greenhouse Gases (CCGG) group makes ongoing discrete measurements from land and sea surface sites and aircraft, and continuous measurements from baseline observatories and tall towers. These measurements document the spatial and temporal distributions of carbon-cycle gases and provide essential constraints to our understanding of the global carbon cycle.

The NOAA/ESRL/GMD CCGG cooperative air sampling network effort began in 1967 at Niwot Ridge, Colorado.

To obtain detailed understanding of the short term as well as long term variations of the greenhouse gases, CCGG makes on-site measurements at the four NOAA/ESRL/GMD baseline observatories, which are far from any pollution sources affecting the gases of interest.

•Barrow, Alaska

•Mauna Loa, Hawaii

•American Samoa

•South Pole, Antarctica

The NOAA ESRL/GMD tall tower network provides regionally representative measurements of carbon dioxide (CO2) and related gases in the continental boundary layer. ESRL's Global Monitoring Division (GMD) began making measurements from tall towers in the 1990s in order to extend long-term carbon-cycle gas monitoring to continental areas.

Since its inception in 1992, the NOAA/ESRL Carbon Cycle Greenhouse Gases (CCGG) group’s aircraft program has been dedicated to collecting air samples in vertical profiles over North America. The program's mission is to capture seasonal and inter-annual changes in trace gas mixing ratios throughout the boundary layer and free troposphere (up to 8000m). At present, most aircraft program flights collect 12 flask samples at different altitudes.

Annex 2: How carbon dioxide concentration in the atmosphere is measured

Perhaps, understanding how Co2 concentration in the atmosphere is measured can aid to know better the concept and the importance of such a measure.

The technique: First of all, let’s us describe what is the technique behind Co2 measurement. It is called Infrared absorption: Air is slowly pumped through a small cylindrical cell with flat windows on both ends. Infrared light is transmitted through one window, through the cell, through the second window, and is measured by a detector that is sensitive to infrared radiation. In the atmosphere carbon dioxide absorbs infrared radiation, contributing to warming of the earth surface. Also in the cell CO absorbs infrared light. More CO in the cell causes more absorption, leaving less light to hit the detector. Then the detector signal, which is registered in volts, is turned into a measure of the amount of CO in the cell through extensive and automated (always ongoing) calibration procedures.

What is it measured: The quantity researches actually determine is accurately described by the chemical term “mole fraction”, defined as the number of carbon dioxide molecules in a given number of molecules of air, after removal of water vapor. For example, 372 parts per million of CO (abbreviated as ppm) means that in every million molecules of (dry) air there are on average 372 CO molecules. The table below gives an example for 372 ppm CO in dry air. All species have been expressed as ppm, turning 78.09% nitrogen into 780,900 ppm. The rightmost column shows the composition of the same air after 3% water vapor has been added: Only the dry mole fraction reflects the addition and removal of a gas species because its mole fraction in dry air does not change when the air expands upon heating or upon ascending to higher altitude where the pressure is lower. Nor does it change when water evaporates, or condenses into droplets.
Table – Example of the composition of Air in terms of ppm
  dry air 3% wet air
Nitrogen 780,900 757,473
Oxygen 209,400 203,118
Water vapor 0 30,000
Argon 9,300 9,021
Carbon Dioxide 372 360.8
Neon 18 17.5
Helium 5 4.9
Methane 2 2
Krypton 1 1
trace species (each less than 1) 1 1
Total 1,000,000 1,000,000
Source: Global Greenhouse Gas Reference Network
According to several studies and analysis the positive correlation between CO2 concentration in the atmosphere and air temperature is clear. Moreover, observations and important reports (see IPCC, 2007) definitely prove that CO2 increase in the atmosphere is completely man-made. The response of the climate system to anthropogenic forcing is likely to be irreversible over human time scales, and much of the damage is likely to be irreversible even over longer time scales (IPCC, 2007).
According to James Lovelock, “… many scientists are almost certain that an increase of CO2 concentration in the atmosphere to 500 ppm, now almost unavoidable, will cause a deep climate change…” “… due to the collapse of the ocean life, primarily algae – one of the main absorbers of CO2 -, and the irreversible melting of Greenland’s ice …”; “… at the current growth rate of CO2 concentration, this point will be reached by 50 years …”(James Lovelock, 2006).

That said, it is clear that something must be done immediately in order to avoid such a fate. As previously said, it is true, what has already been done is irreversible in the humane time scale, but one thing is to live with 400 ppm of CO2 and another is reaching 500 ppm. According to Pieter Tans, “we have to bring emissions of CO2 down to zero as soon as we can”:

"It is abundantly clear that “we have to bring emissions of CO2 down to zero as soon as we can.” We actually have most of the means to do so. That would also bring to a halt the ongoing acidification of the oceans (CO2 is an acid), threatening the ocean food chain if it continues. The first on my list of priorities is greatly improved energy efficiency and conservation. It creates meaningful jobs, and increases the effectiveness of all the other things we might want to do. Sweden was able to cut its per capita CO2 emissions in half in about 20 years after the oil price shocks of the 1970s. Second on my list is stopping all direct subsidies to fossil fuel consumption and production. The International Monetary Fund recently estimated that globally these subsidies amount to US$480 billion per year, conservatively counted. Instead, that money could be invested in our future by bringing wind and solar energy to the scale that is required. Third, bring population growth under control primarily by bringing equal rights and education to all women, which has proven to be very effective, and fundamentally very desirable. Fourth, hold on to nuclear power for electricity production. In fact, resurrect the development of more inherently safe nuclear reactor designs and proliferation-proof nuclear fuel cycles, such as thorium."
Pieter Tans
Interview on LTEconomy (, June 2013
Finally, two are the key elements that determine our contribution to CO2 accumulation in the atmosphere: 1) The size of the population; 2) Our lifestyle, the energy we need to satisfy it and the instruments we use to satisfy our energy needs. If we don’t act in both directions in order to stop CO2 emissions as soon as we can, probably the earth will do the work for us, and Earth’s responses could be much more hurting that human’s ones.


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This article by Dario Ruggiero
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