If the Earth was a simple barren orb, like the Moon, with no ocean, no forest, and no clouds, its surface temperature would be a frosty minus 6°C.
(This is derived from very well established physics, and an easily repeated measure of solar output.)
Add an atmosphere of oxygen and nitrogen and guess what — no difference. Add a little water vapour and carbon dioxide, in the proportion we see around us, and the temperature you can expect rises in excess of +58°C. Again, this employs only very well established physics, following a simple analysis of the greenhouse effect.
In fact, the planetary surface on which we stand has a surface temperature of about +14°C — just right for life. It is cooled by, in a word, weather. Rising air and water vapour, after solar evaporation, transport vast amounts of heat high into the sky. The air and water, after condensation, comes back down. Most of the heat does not.
The mechanisms by which heat, water, and carbon, are transported, vertically, around the planet, and in and out of the ocean, are (often strongly) interrelated and form a complex dynamic system. As a result, the consequences of any change, however small, are 1) extremely difficult to predict, and 2) quite possibly profound.
This should be very scary for two reasons. First, the climatic variation with which our civilization can cope is very small — we need to grow crops, find adequate water, and control disease. Second, we have made a significant change, and no one can reliably tell us what the result will be.
We, and our children, are all now guinea pigs in our own lab experiment.