 Over the last few centuries, the amount of carbon dioxide or CO2 in the atmosphere has risen by about 40%. To explain why this is the case, we need to understand how the carbon cycle operates. The carbon cycle describes the flows of carbon between the atmosphere, the oceans and the biosphere as shown in this slide. Carbon sources release carbon dioxide into the atmosphere, carbon sinks absorb carbon dioxide out of the atmosphere. Some of these flows are the result of natural processes. For example, when plants grow, they absorb CO2. However, human activities also play a part. We release CO2 into the atmosphere when we burn fossil fuels. Land use change, such as deforestation, also releases CO2. Ice cores provide invaluable information on how atmospheric CO2 has changed over time. As the Antarctic ice sheet formed, it trapped small bubbles of air which stay trapped for thousands of years. Cores drilled deep into the ice show us that prior to the Industrial Revolution, atmospheric CO2 had been fairly stable for several thousand years. The carbon cycle was in a state of natural balance. Carbon sources were roughly matched by carbon sinks. Human activity has upset this natural balance. Fossil fuels were formed millions of years ago from the remains of plants. When these plants died and were buried, the carbon was taken permanently out of the active carbon cycle. When we burn fossil fuels, we release this carbon dioxide back into the atmosphere. As a result, CO2 levels have been rising. Scientists became more aware of this after 1958 when accurate measurements of CO2 concentrations were first made at the Mourna Lower Observatory in Hawaii. Together with the ice core record, we see that atmospheric CO2 began to rapidly grow soon after the start of the Industrial Revolution. It would be a bit of a coincidence if this were a natural phenomenon. Furthermore, the increase in atmospheric CO2 has closely tracked the amount of CO2 that we've been releasing. This would have been even greater coincidence if the rise in atmospheric CO2 were natural. In fact, the amount of extra CO2 in the air has consistently been only about half of the CO2 we've released. So the evidence is clear that humans are raising CO2 levels, but not everybody accepts this. One myth is to argue that because the CO2 that humans release is small compared to the CO2 released by nature, our influence must be negligible. Unfortunately, this is incorrect because it only looks at half of the carbon cycle. It fails to consider that carbon sinks also absorb CO2 from the atmosphere. It's the difference between total sinks and total sources that actually governs the rise in atmospheric carbon dioxide. There's a simple analogy that demonstrates that the rise in CO2 is caused by humans. Imagine my wife and I share a bank account which pays no interest and attracts no bank charges. I pay in £1,000 per month but take no money out. If I notice that the balance rises by only £500, then I know that my wife has taken £500 more out of the account than she has put in. Now, I don't have any direct knowledge of my wife's transactions, but whether she put in £1,000,000 a month or only £1 a month, I still know that she is taking out £500 more than she is putting in. The account obeys the principle of conservation of money. Likewise, the carbon cycle obeys the principle of conservation of mass. The carbon we release doesn't just disappear, it must either be removed by natural sinks or it ends up in the atmosphere. In other words, the change in atmospheric CO2 depends on the difference between the total sources and the total sinks. CO2 levels are rising more slowly than we are releasing CO2. This means that natural environment must be a net carbon sink. It soaks up carbon. It is taking more CO2 out of the atmosphere than it puts in. Nature has been a net carbon sink every year for at least the last 50 years. This fact alone establishes that the increase in atmospheric CO2 is not a natural phenomenon. In fact, nature is actively resisting the rise. I don't need to know the details of my wife's transactions to know that she was opposing the rise in our bank balance. Likewise, we don't need to know the exact strength of the individual sources and sinks to know that nature is opposing the rise in atmospheric CO2. We only need reliable measurements of atmospheric CO2 and of human emissions, which we already have. The error here is in taking an overly simplistic view of the carbon cycle. In this case, considering only CO2 sources and ignoring the CO2 sinks altogether. Scientists have developed complex models in order to understand how carbon moves around our carbon cycle. They have done this for a good reason. As Einstein is often paraphrased, everything should be made as simple as possible, but no simpler.