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How Much CO2 Is in the Atmosphere and Can That Level Have Dramatic Effects?

Ask a Scientist - March 2011

J. Cooper of Austin, TX, asks "I've seen efforts to discount the impact of carbon dioxide (CO2) emissions on global warming, which focus on the minute percentage of CO2 in the atmosphere and state that this small percentage cannot significantly factor into warming.  How much CO2 is in the atmosphere and can such a small component have dramatic effects?"  and is answered by Climate Scientist Todd Sanford, Ph.D.

This is an interesting question because it highlights how a bit of truth—in this case, that the percentage of carbon dioxide (CO2) in the atmosphere is small—can be distorted to seem to support something inaccurate. The truth is that the CO2 currently in our atmosphere is already having dramatic effects on Earth’s climate.

The amount of our planet’s atmosphere that is made up of CO2 is small compared to other gases such as nitrogen and oxygen, so scientists like me don’t track it in terms of percentages.  Instead, we use a measurement called “ppm” or “parts per million.” Current CO2 levels are around 391 ppm. This means that for every million molecules in the atmosphere, 391 of them are CO2. (If you wanted to convert that to percentages, it would be 0.0391 percent.)

So yes, that is a small percentage. But that’s looking at CO2 in relation to the atmosphere as a whole, which is massive. That 391 ppm is equal to around 3 trillion tons of CO2 in the air. And that “small” component can still have a big impact. 

One way to measure the factors that influence Earth’s climate, or “climate drivers,” is by measuring each driver’s “radiative forcing” (RF). Basically, RF measures the balance between incoming solar radiation and outgoing heat radiation from Earth. In a stable climate these are balanced. When the drivers change, this balance is disturbed and the climate responds by either warming or cooling. In 2007, the Intergovernmental Panel on Climate Change (IPCC) summarized all climate drivers—such as CO2 and other heat-trapping gases—and their RF values. CO2, with an RF value of +1.7 W/m2 (watts per square meter) was found to be the driver having the greatest impact on the climate.  The positive value indicates that CO2 is trapping heat in the atmosphere and leading to an overall warming.  Other heat-trapping gases are much more potent than CO2, but the amount of CO2 we’ve added far outweighs the others leading to a larger RF value and a bigger effect on climate.

And we’re definitely adding CO2 to the atmosphere at a rapid pace. Antarctic ice core records vividly illustrate that atmospheric CO2 levels today are higher than levels recorded over the past 800,000 years. These levels have risen by nearly 40 percent since the Industrial Revolution, with half of that rise occurring only in the last three decades. 

In addition, CO2 remains in the atmosphere longer than the other major heat-trapping gases emitted as a result of human activities.  Research has shown that even if we cut all of our emissions today, 1000 years from now around 40 percent of the  CO2 we’ve added will still be in the atmosphere—leading to warming from CO2 that will be with us for a very, very long time. This literally means that the heat-trapping emissions we release today from our cars and power plants are setting the climate our children and grandchildren and many generations after them will inherit. CO2’s long life in the atmosphere, and the nearly irreversible effect on climate it brings, provides the clearest possible rationale for reducing our CO2 emissions without delay.

Todd Sanford is a climate scientist in the Climate & Energy Program at the Union of Concerned Scientists. Prior to joining UCS, he worked at NOAA Earth System Research Laboratory. While there he carried out atmospheric measurements on aerosols and worked on climate system modeling with a special interest in the interaction of atmospheric lifetimes of various climate drivers to the timescales of their associated climate effects. He holds a doctorate degree in physical chemistry from University of Colorado at Boulder.

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