Data on atmospheric oxygen concentration from the Mauna Kea observatory (link). This result shows that oxygen goes down as carbon dioxide goes up - it is what you would expect as the result of the combustion of fossil fuels. And, yes, it is us who burn them.
While the preferred weapon against climate science seems to be character assassination, it is also true that climate is a complex subject and there are perfectly correct reasons for perplexity. One is whether the CO2 increase in the atmosphere is really the cause of global warming change - and not an effect of it.
The reasoning goes as follows: we see an increase in the concentration of CO2 in the atmosphere and, yes, we also see an increase of planetary temperatures. But how can we be sure that CO2 causes warming and not the reverse? After all, if something else (say, the Sun) were to heat the planet, then we would expect the oceans to release CO2! And in those plots that Al Gore was so fond to show in his movie, warming comes first, then CO2 increases. So, it could really be the Sun (or cosmic rays, or whatever else coming from outer space) and not us.
Alas, it doesn't work that way. The additional CO2 in the atmosphere does not come from the oceans. On the contrary, oceans are a sink that absorbs part of the CO2 we humans generate. How do we know it? Well, there are several ways; one is that ocean water is becoming more acidic. Now, CO2 is an acidic molecule. If oceans were emitting CO2, they would become less acid, not more acid. That shows that oceans are absorbing CO2, not emitting it. Another way, even simpler, is to look at the oxygen record - shown in the figure above.
Suppose that is the sun that warms the oceans and, consequently, causes them to release CO2 in the atmosphere. Then, oxygen is not consumed and you wouldn't expect its atmospheric concentration to change. Actually, if oceans are warming, the may release some oxygen together with the CO2, so the concentration of oxygen should increase. But this is not what we see. We see that the oxygen concentration in the atmosphere is going down, not up.
Suppose instead that CO2 comes from the combustion of fossil fuels. "Combustion" means combining the carbon and the hydrogen contained in these fuels with oxygen. So, the more we burn fuels, the less oxygen remains. The oxygen concentration, then, should go down with time. And this is exactly what we see (see figure above).
With plenty of fantasy, you might find a way to concoct a different theory explaining these data. But remember that there is something called the "Occam's razor." What we see is simple: we burn fossil fuels, we increase the CO2 concentration in the atmosphere, that causes warming. It is straightforward: it is not the Sun, it is us.
- About the loss of oxygen in the atmosphere and its long term effects, see this post on Cassandra.
- For a more detailed treatment of the same question discussed here, see this post on Real Climate
- About Al Gore's movie and the lag between temperature and CO2 concentration also see Real Climate