 Fossil fuel emissions have caused atmospheric CO2 levels to rise by about 40% over the last few centuries. But it's interesting to ask how long it would take for a return back to pre-industrial levels if we would stop emissions tomorrow. To find out, let's have a closer look at the carbon cycle. Fast amounts of carbon dioxide or CO2 flow between the atmosphere, oceans and stores of carbon on the land. Each year, the ocean absorbs 80 billion tonnes of carbon from the atmosphere, but only releases 78 billion tonnes back into the atmosphere. Likewise, plants extract around 123 billion tonnes of carbon from the atmosphere each year for photosynthesis, but release 119 billion tonnes when they die and decay. So nature emits large amounts of CO2, but it also absorbs large amounts of CO2. They all must match, but not quite. The result is that the natural environment is a net carbon sink. It removes about 6 billion tonnes of carbon dioxide each year from the atmosphere. Nevertheless, an enormous amount of CO2 is constantly moving back and forth between the various parts of the carbon cycle. This vigorous churning does not affect the total amount of CO2 in the atmosphere, but it does have an interesting consequence which is important in understanding the rise and fall of atmospheric CO2. If we follow the path of an individual CO2 molecule, we find it only remains in the atmosphere for a short time before being exchanged with a molecule CO2 from the oceans or from land-based vegetation. The important thing to notice though is that this is a straight swap. It doesn't affect atmospheric CO2 levels. This is most easily understood by a simple analogy. Imagine my wife and I share a sweet jar where we keep our jelly beans. At the start of our marriage, my wife had no jelly beans, but I'd already stashed away 589 green jelly beans. Each month my wife puts in 9 red jelly beans and I put in 198 green jelly beans. However, I also take out 203 jelly beans selected at random and I eat them. Now our sweet jar will rise by 4 jelly beans a month. This rises in cause entirely by her red jelly beans because I'm taking more beans out of the jar than I'm putting in. So how long should we expect a red jelly bean to remain in the jar before I take it out and eat it? It turns out that the average time that a single bean stays in the jar is the number of jelly beans in the jar divided by the flow of jelly beans out of the jar. This is known as the residence time and in this case turns out to be only about 4 months. We can also calculate the proportion of red and green jelly beans in the jar as the months pass. It may seem counter-intuitive but the number of green jelly beans quickly rises while the number of red jelly beans stays relatively low even though my wife's red jelly beans are the solely responsible for the rise. This is because my large exchange of jelly beans are constantly replacing red beans with green ones even though overall I'm taking out more jelly beans from the jar than I'm putting in. This analogy provides a simple model of the carbon cycle. Each red jelly bean represents the release of 1 billion tonnes of carbon from human sources into the atmosphere. Each green jelly bean represents a transfer of 1 billion tonnes between the atmosphere and the natural environment and each month represents a year. Even though the rise in atmospheric CO2 is caused by humans a single molecule of CO2 has a residence time of only about 4 years in the atmosphere. However, we're not really interested in the fate of a single CO2 molecule. We're interested in a much more important question. If we add lots of CO2 into the atmosphere how long does it take to return back to normal? This is known as the adjustment time and is around 50 to 200 years. It is governed by the difference between total uptake and total emission rather than their own individual magnitudes. Since the start of the Industrial Revolution we've raised CO2 by 40%. If we would stop all fossil fuel used tomorrow most of this excess CO2 would be taken up over the next 50 to 200 years however a full return to pre-industrial levels would take many thousands of years. One myth about the carbon cycle is that CO2 has a short residence time and so CO2 levels would fall rapidly if fossil fuel use were cut. However, this is a bit of a red herring. If we're really interested in how long it would take it's the adjustment time that matters and this is not the same thing as the residence time, it's much longer. Even though a single molecule of carbon dioxide only stays in the atmosphere for around 4 years it will actually take the atmosphere thousands of years to return back to the pre-industrial levels after we stop releasing CO2.