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Determining the Source of the Atmosphere's Extra CO2 Scientific measurements taken around the world show that the amount of atmospheric CO2 has increased a lot since the mid-1900s, but how do we know where that extra CO2 is coming from? Is the increase in the atmosphere's CO2 caused by human activity, such as burning fossil fuels, or is it just part of a natural cycle? It's important to understand the cause for the increase—maybe then, there is something we will be able to do about it.

Searching for EvidenceMany scientists have been gathering data from around the world and now have strong evidence to support the conclusion that the recent increase in atmospheric CO2 is caused almost entirely by humans buring fossil fuels. The evidence has come from investigations centered around three important areas of research. Those areas of research and the evidence that scientists gathered are summarized below.

Research Question #1: Is the amount of carbon being released from the burning of fossil fuels large enough to cause the increase in CO2 in the atmosphere?

Scientists have measured the amount of extra carbon dioxide being added to the atmosphere each year. They have also calculated the total amount of fossil fuels people around the world use

each year, and how much CO2 is released into the atmosphere from this burning.

When scientists made the calculations, they were surprised to discover not only that the burning of fossil fuels was enough to account for the increase in CO2 in the atmosphere, it was twice the amount they expected. This was evidence to show that the burning of fossil fuels releases enough CO2 to account for the recent changes to Earth’s atmosphere.

Discovering that the amount of fossil fuels burned produces two times as much CO2 as what was found in the atmosphere raised a new question: if half the CO2 from burning fossil

fuels is going into the atmosphere, where is the other half going? Scientists did further investigations and found there are three places the extra CO2 from burning fossil fuels goes—½ goes to the atmosphere, and the other ½ is absorbed by the ocean and forests.

Research Question #2: If you go back over the last 150 years, how has the amount of radioactive carbon in the atmosphere changed?

One way that scientists can tell where the carbon in the atmosphere comes from is by measuring how radioactive the carbon is. Radioactivity is complicated to explain, but it has

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Burning fossil fuels to power cars and other vehicles, to make electricity, and for many other uses releases CO2.

Student Reading—Ocean Sciences Sequence 3.3

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to do with characteristics of the atoms in a molecule. A form of radioactive carbon, called carbon-14 is found in the CO2 that forms naturally in the upper atmosphere. CO2 that is non-radioactive comes from volcanic explosions, the deep ocean, or from the burning of fossil fuels. If the high level of radioactivity of carbon in the atmosphere has stayed the same for the last 150 years, the extra CO2 in the atmosphere must be coming from natural occurrences in the upper atmosphere. If the radioactivity of the atmosphere has decreased

in the past 150 years, the extra CO2 must be coming from volcanic explosions, the deep ocean, or the burning of fossil fuels.

To measure the radioactivity of carbon in the atmosphere at different times in the past several hundred years, scientists study the rings in the wood of trees. Each ring on a tree is from a different year in the tree’s life. Since trees take carbon (CO2) out of the atmosphere and turn it into wood, scientists can get information about what the carbon in the atmosphere was like during different years by examining tree rings from different years.

When scientists examined the tree rings, they found the atmosphere has gotten less and less radioactive in the last 150 years.

This is evidence that the carbon entering the atmosphere must be the kind that comes from volcanoes, the deep ocean, or fossil fuels, and not the radioactive kind that forms naturally.

Research Question #3: Does the extra carbon entering the atmosphere (which is non-radioactive) come from volcanoes, the deep ocean, or from burning fossil fuels?

Two other forms of carbon, carbon-12 and carbon-13, help to pinpoint the source of the extra carbon in the atmosphere. Neither carbon-12 nor carbon-13 are radioactive. Things that were once alive, like fossil fuels*, mostly have carbon-12. Carbon that comes from things that were never alive, such as the carbon from volcanoes and the deep ocean, has more carbon-13. Once again, tree ring studies provide the evidence. If the amount of carbon-13 compared to carbon-12 in tree rings increased over the past several decades, the extra carbon came from sources that were never alive. If the amount of carbon-12 has increased comparatively, then things that were once alive are the source. Tree ring evidence showed that the amount of carbon-12 in the wood of trees is now much greater. This means that the atmosphere's extra carbon comes from fossil fuels, not from volcanoes or the deep sea.

Conclusion:

The evidence from these three areas of investigation strongly supports the explanation that increases in atmospheric CO2 are caused almost entirely by humans burning fossil fuels.

* Fossil fuels are formed from ancient plant and animal remains. When dead plants and animals in swampy areas, lake floors, or the seafloor are buried under layers and layers of sand, mud, and sediments and don't decompose, over million of years, the intense heat and pressure turns them into fossil fuels.

The carbon in tree rings gives evidence about the Earth’s atmosphere in past years.

Student Reading—Ocean Sciences Sequence 3.3

Determining the Source of the Atmosphere's Extra CO2 (continued)