The earth system is a very complex self organizing system. Human activity on earth has been a factor that has created a need for the system to respond and we are seeing that response. A lot of time and effort on the part of very good scientists has produced an array of predictions from models. All of these are necessarily incomplete and have to be modified as new aspects of the earth system's response kick in.
The human factor is a multifaceted one. Carbon emissions have been a focus of attention but there is a lot more. Cutting trees, agriculture by factory methods, fracking, and so many more.
These activities can not be studied in isolation since they all interact. That's why a look at what cooking is all about might provide an analogy for the way we might better understand the problem. Read on below and I will explain.
Two broad categories of change are the dominant features of how a system responds when heat energy is added to it. The warming effect is a physical change and the amount of temperature rise per unit of heat added depends on the substance(s) in the system. The heat capacity of a substance is what we use to calculate this. The heat capacity reflects the way heat energy increases the speed of various kinds of motion of the atoms and molecules in the system. It is not a simple idea, but is simpler than what happens if chemical changes also occur.
Chemical changes are the second way heat is involved and in the simplest case a reaction is either endo or exothermic, meaning that as it progresses it either absorbs of gives off heat.
If you have ever made candy you know that the temperature rise is not linear but that it occurs in stages. Every winter I make toffee for the Winter solstice. It is fun to watch the way the the temperature of a mixture of butter, sugar and water progresses as you go through the various stages to the final, hard crack stage.
If I had been heating pure water, the temperature rise would have been almost linear until the boiling point was reached. Then a new phenomenon, evaporation, would change the way the heat was being used. The latent heat of vaporization rather than the heat capacity of water would now play a role in the way the heat being added at a constant rate was affecting the temperature. With candy, the cooking is a chemical rather than just a physical change and things are much more complicated. That is why there are different stages.
I have oversimplified even this common way we experience change in a system to to added heat. Even so, the example has lots to say about why global Warming is so tough to get a complete model for.
The cooking example also explains "tipping points" or irreversible changes that can also involve positive feedback loops. Certainly heating water is reversible. You can heat the water and cool it back down and be back where you started from as far as the water is concerned. The environment has been changed irreversibly in the process as the second law of thermodynamics dictates.
In the case of the cooking of the toffee not so. Once the chemical changes occur, the sugar, water, and butter are no longer the same substance. At each stage the system goes through profound chemical changes that can not be reversed. You actually creat a new substance.
Let us now use these ideas to look at what happens as we look at what humans have done to the earth system. I mentioned cutting trees, for example. This is n irreversible change with myriad ramifications. Certainly the mediation of heat added to the system will be affected. One might naively think that this all can be reversed by planting new trees. Not so fast! The soil erosion and ecological shifts are just the start of what the consequences are. It gets very involved very fast.
The ecosystem globally is a hierarchy of ecosystems from the very local level of soil on up. Evolution itself is less a phenomenon of species as it is one of hierarchical ecosystems. Every change we cause reverberates throughout this complex system causing other changes that cause other changes, etc. All of these changes circle back on the aspect of the system we are impinging on.
Hence, we are hard pressed to understand the details of what we are doing to relatively simple system like toffee and yet we tend to speak about global warming as if it follows much simpler rules.
Human knowledge is limited in so many ways. Many of them we are not aware of. This circularity is inbuilt and is the human condition. We are so proud of our mastery of nature and our science and technology. This, more than anything else, will be our downfall. If we learn how to do something we do it with no real knowledge of the possible consequences. If we do eventually wake up to the damage we do it is already far too late to correct what we have done. Our usual impulse is to employ more science and technology that is also beyond our ability to control or predict the consequences of.
Fortunately, the harm I do when I make toffee is less potent than in these matters. And it sure does taste good.