Why is predicting our future oil supply controversial and involves some uncertainty?
i just need some explaining.

Natural resources such as oil that are as yet undiscovered are very difficult to predict or estimate; this creates huge uncertainty and large errors in even the most rigorous scientific efforts to predict future supply. Changes in oil drilling and discovery technology can and will completely change the eventual results in the search for undiscovered resources. The controversy lies in the desire to eliminate oil from the world's energy mix in order to prevent a perceived but uncertain risk of global warming and the desire to encourage the use of alternative energy in order to reduce that risk, even though so far no alternative energy source other than natural gas, coal, and nuclear energy has been able to compete with oil economically.

The Problem with predicting things like our natural supply of oil can be picked out to a LOT of different variables. For starters, we find a lot of oil in a myriad of different places. The frequency in which we find this oil could theoretically be linked to a computer and predicted like Stocks. But then again, since the rate changes almost as much as the finding sites, You can easily be slapped aside by prehistoric biology and geology. The other problem with predicting our supply is often controversial; "How much do we use" well, you have to look at individual countries or the entire planet. And that alone is  a lot of work. Imagine knocking on your neighbor's door and asking exactly how much hot water he uses Every day For a year!  Pretty creepy right? Not to mention intrusive. But the internet has some of these things, So lets say you managed to find the frequency of oil findings globally, and the rate of use for these areas. Well now you have even more problems. Because there are many different people looking for oil. And when they find it. They sell it. Or they sue eachother over who found it first. Because underground supplies are huge at times. Often companies will be drilling the same one. To get an exact count from one company you would need to track all of the oil possessed by them and the buyers, which is paperwork. Which is Highly variable depending on who you are counting from. Without Tainting your next variable. "Current processed supply." ie. The stuff already out of the ground. If you have got all of that counted and punched into the smartest computer you can find. Then you still have some data to collect. Because the numbers are always changing. And everyone uses a different amount every day. This alone can stop any predictions cold for obvious. Reasons. So in conclusion, There literally is a large amount of Static variables and a few constant variables to consider when predicting future supply. This is why simply internet searching these things often gives numbers that are highly different from eachother. Being both controversial and uncertain.

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).

It's potassium i believe