Education - Stable Isotopes NOAA GML

The Data: The Story Told from CO₂ Samples

A Mosaic of Stories

Different sources of carbon dioxide have their own, unique isotopic fingerprints. However, the story of a sample of carbon dioxide becomes more complex when carbon dioxide from many different sources mix together in the atmosphere. While carbon dioxide from the burning of fossil fuels does not have ¹⁴C, and carbon dioxide from terrestrial plants has less ¹³C than from the ocean, an air sample contains carbon dioxide from all of these sources. The story becomes even more complicated because the ratio of isotopes in the mixture also reflects how much atmospheric carbon dioxide is taken up from different sinks. Therefore, an air sample will show the effect of all these different components–each with their unique characteristic isotopic fingerprints.

The carbon dioxide in an air sample is a mixture of carbon dioxide coming from different sources. Some of this carbon dioxide is also taken up in different ways.

The Detective Work

NOAA Carbon Cycle and Greenhouse Gases scientists measure the carbon isotopic composition in each air sample. For ¹⁴C, this ratio is given as a Δ¹⁴C value (pronounced delta 14-cee–the triangle is a capital delta, which is the Greek symbol for a “D”). The smaller the Δ¹⁴C value in a measured sample, the fewer ¹⁴C atoms is in it, and the more of its carbon dioxide is from fossil fuel emissions.

Similarly, the ratio for ¹³C is written as δ¹³C (pronounced delta 13-Cee, and δ is the lowercase “d” in ancient Greek). Smaller δ¹³C values mean that there are relatively fewer ¹³C atoms in the sample. The units are ‰, pronounced “permil” for both Δ¹⁴C and δ¹³C.

CO₂ Pool	Δ¹⁴C Value (‰)	δ¹³C Value (‰)
Fossil Fuels	-1,000	-28
Terrestrial Biosphere	+45	-26
Ocean	+45	-10
Atmosphere	+45	-8

Average delta values–both for stable carbon and radiocarbon–of the different major pools of carbon dioxide.

Learn more about the formula for determining delta values.

The Balancing Act

Just like you (or your parents) have to balance your check book by looking at the amount of money that goes into your account and the amount you spend, atmospheric scientists also have to balance the amount of carbon dioxide in the atmosphere. Using the special isotopic fingerprints for each source, scientists can figure out how much of the carbon dioxide in a sample comes from each individual source.

An Example Calculation…How the Sources Affect the Delta Value

Let's look at an example: Right now (in 2010) the natural atmosphere has about 380 parts per million, or ppm, of CO₂, with Δ¹⁴C of +45‰. If we add 1 ppm of fossil fuel CO₂, with a Δ¹⁴C of -1000‰, then the new Δ¹⁴C value in the atmosphere is a little bit lower: +42‰. On the other hand, if we instead add 1 ppm of CO₂ from a forest fire, the CO₂ from the fire would have the same Δ¹⁴C value as the atmosphere (or very close to it). The Δ¹⁴C of the atmosphere would not change.

Patterns in the Story

So, we know that the ratio of carbon isotopes in atmospheric carbon dioxide samples is from a mixture of sources, and we also know the unique isotopic fingerprint of each of those sources. Using these two pieces of information, scientists can figure out why trends in Δ¹⁴C and δ¹³C occur. Globally, as atmospheric carbon dioxide levels continue to increase, both Δ¹⁴C and δ¹³C are decreasing over time. This is called the Suess Effect – named after Dr. Suess who first discovered this phenomenon.

Dr. Seuss seemed to truly understand the work of science–the need to collect data at all times of days and in every location. Only with years of work in all conditions can you have enough data to show a pattern as significant as the Suess effect. As Dr. Seuss said,

“if you want to catch beasts you don't see everyday, You have to be out-of-the-way, You have to go places no others can get to. You have to get cold, and you have to get wet, too.”

Wait a minute…that quote comes from If I Ran the Zoo, by Dr. Seuss…the Dr. Seuss that wrote The Cat in the Hat and How the Grinch Stole Christmas NOT the Dr. Hans Suess that discovered the Suess Effect. Either way, both seemed to understand a great deal about our environment and mankind's power to alter natural systems.

At a regional level, Δ¹⁴C and δ¹³C values can be quite variable. Changes on a daily or weekly basis in fossil fuel use in the surrounding region will greatly affect both of these measurements. Additionally, the nearby vegetation, and its uptake of carbon dioxide depending on the season, will alter the δ¹³C of the region.

Measurement	Trend	Why?
CO₂ Levels		CO₂ is currently being released from burning fossil fuels faster than it is taken up either on land or in the ocean.
Global Δ¹⁴C		The percent of atmospheric CO₂ that comes from fossil fuel emissions is increasing.
Local Δ¹⁴C	But Highly Variable	Highly polluted areas have lower Δ¹⁴C values from fossil fuel burning. This value varies greatly depending on the quantity of fossil fuels emitted on a short time scale.
δ¹³C		Fossil fuels add CO₂ to the atmosphere that have less ¹³C. This addition of CO₂ is more than the amount of CO₂ removed by the terrestrial biosphere.

Explore up-to-date atmospheric measurements from NOAA's Carbon Cycle and Greenhouse Gases Group. You can see real data from around the world–from any site in the NOAA network!