“Both IPCC and NASA agree that anthropogenic CO2 is between 3 and 4% of total CO2.”

The person who wrote the statement above likely believed that this information counters the idea that anthropogenic CO_{2} emissions have been responsible for the the rise in atmospheric CO_{2} levels from 280 ppm to 420 ppm.

Yet, it doesn’t counter that idea at all.

It’s important to realize that the following are different questions, with different answers:

**What fraction of the increase in atmospheric CO**_{2}levels since the beginning of the Industrial age is due to anthropogenic CO_{2}emissions?**What fraction of the CO**_{2}that is currently in the atmosphere is of anthropogenic origin?

The quote offers data on the answer to question #2, when question #1 is the important question.

The answer to question #2 turns might well be 3 to 4%. (Based on IPCC carbon flux data the human-caused carbon flux into the atmosphere is about 11 PgC/yr, while natural fluxes into the atmosphere are about 216 PgC/yr, which means that 5% of the carbon injected into the atmosphere each year is anthropogenic.) Yet, that **isn’t in any way inconsistent with the answer to question #1 being “100%.” **

To see how that can be true, let’s consider a situation which offers a simplified analogy. Suppose there is a tank of water that initially contains 1000 gallons of water. Suppose that there is an inlet pipe that adds 100 gallons per hour, and an outlet pipe that drains away 100 gallons per hour. Under these conditions, there will continue to be 1000 gallons of water in the tank.

Then, suppose someone connects a hose to the tank, and that hose adds 4 gallons per hour of water with some dye mixed into it. The inlet and outlet pipes continue to add and take away 100 gallons per hour. However, the net rate of water flowing “in” will now be 104 gallons per hour, while the rate “out” is 100 gallons per hour, so that the amount of water in the tank will now be increasing at a rate of 4 gallons per hour.

After 250 hours (10 days and 10 hours), there will be 2000 gallons in the tank.

The amount of water in the tank is twice what it was initially. How much of that extra 1000 gallons is due to the addition of the hose supplying 4 gallons per minute? 100% of it. If the hose hadn’t been added, there would be no additional water in the tank. So, the hose and the water flowing through it are 100% responsible for the increase in the water level.

What fraction of the water in the tank originated from the hose? One could measure that by measuring the amount of dye in the water in the tank. However, one can also calculate the answer, and it’s going to be just under 4%. (I could write out the exact math if it matters.) There were always 100 gallons coming in from the inlet pipe for every 4 gallons coming from the hose. So, the answer can’t possibly be larger than 4%.

What if, when we change the rate at which water is coming in, the rate at which water leaves also changes? In the figure below, the rate at which water leaves has increased to 102 gallons per hour.

In this example, water is only accumulating at a rate of 2 gallons per hour, so let us wait 500 hours (20 days and 20 hours). The amount of water in the tank will have increased to 2000 gallons.

Again, we can ask, How much of that extra 1000 gallons is due to the addition of the hose supplying 4 gallons per minute? 100% of it. If the hose hadn’t been added, there would be no additional water in the tank. So, the hose and the water flowing through it are 100% responsible for the increase in the water level.

So, having a “feedback” that will increase outflow doesn’t change the main conclusion.

So, I hope you can see in this example that “How much of the increase was due to a particular source?” and “How much of what is present originated with that source” are very different questions with very different answers.

The situation with CO_{2} in the atmosphere has a few extra complications, relative to this example, but the basic principle illustrated by the example remains valid.

If you like, you can look at an IPCC chart of the carbon cycle. This chart indicates that humans are emitting about 11.0 PgC/yr (Petagrams of carbon per year) into the atmosphere. Natural absorption of CO_{2} has increased in response to the increased concentration of CO_{2} in the atmosphere. But, the increase in absorption has only been about 5.9 PgC/yr. Thus, nature is only absorbing around half of what humans are emitting into the atmosphere.

So, nature is not fully compensating for our emissions of CO_{2}.

So, yes, the amount of CO_{2} in the atmosphere which is of anthropogenic origin is only a small fraction. And, that’s fully consistent with the *increase* in atmospheric CO_{2} levels since the beginning of the industrial age being *entirely* due to anthropogenic causes.

## Added Information

Upon further reflection, I’ve realized that question #2 has two variants:

**2A:**What fraction of the CO_{2}that is currently in the atmosphere was**added by humans and has never left the atmosphere?****2B:**What fraction of the CO_{2}that is currently in the atmosphere is**originated from the burning of fossil fuels**, though it may have temporarily left the atmosphere before returning to it?

In the discussion above, I was talking about question 2A. The answer is to 2A is “5% or less.” However, 2A is the least interesting of the three questions that have been posed.

Question 2B has a different answer than question 2A. The reason for that is that carbon enters the atmosphere, then flows into the ocean and land, and then back into the atmosphere. There is a constant interchange.

However, scientists can tell the difference between carbon that was in the atmosphere prior to human intervention, and carbon that entered the system as a result of human burning of fossil fuels. Carbon in the air normal contains small amounts of the radioactive isotope Carbon-14. However, fossil fuels contain no Carbon-14. So, by tracking changes in the amount of Carbon-14 in the atmosphere, one can discern how much of that carbon originated from human burning of fossil fuels. Below is a chart of the measurements.

These measurements seem to suggest that atmospheric levels of Carbon-14 are about 25% lower than they were during the pre-industrial era. (This decline is called the “Seuss effect.”) But, If I’m interpreting the data correctly, it seems to indicate that something like **25% of the carbon in the atmosphere as of the 1950s originated from the burning of fossil fuels.** (The precise number is a bit uncertain because I don’t currently have access to the study from which the data originates. So, there is some uncertainty in how to properly interpret the data.)

Please remember that questions #1, #2A and #2B each have different answers. The answer to question #1 continues to be: it appears that **100% of the increase in atmospheric CO _{2} levels is due to human activities.**