A scientist may develop a theory linking effects to their causes by observation and logical induction. Archimedes while taking a bath and Newton when dozing under an apple tree had such an insight. The scientific work that remains to be done is then to bring experimental validation of the theory, or to demonstrate that it can be falsified; that is a difficult word reserved to philosophers or counterfeiters meaning to show evidence that a theory is false, one relevant negative example being sufficient to disprove all the theory. Galileo falsified the geocentric model of the universe.
Also, a theory may remain valid only within a given application range. Newton’s laws of motion cease to be applicable at speeds near the velocity of light but are quite useful in our day to day slower life and for precise astronomical calculations.
In absence of validation, a scientific hypothesis cannot be promoted to be a theory. It took 48 years and billions of dollars to obtain some evidence that the boson model postulated by Higgs (and others) could be confirmed. And even if it’s not yet absolutely sure, it sufficed for a Nobel Prize.
In pure logic it is impossible to demonstrate the inexistence of anything. Thus when no falsification can ever be expected, like for example denying the existence of god, then the scientific community cannot offer any critical help. Such beliefs or speculations remain in the realm of metaphysics, personal convictions, or dogma.
Also, highly complex systems cannot be studied by a proper combination of all relevant physical laws in such order that a solution would be found. In the 19th century, when science and mathematics made spectacular progresses, determinism was believing that, given the necessary calculation resources, it would be theoretically possible to compute the exact time and place of every event that will ever occur. Nowadays we know that it is impossible either in theory (uncertainty principle) or in practice (the needed calculation power is beyond universe’s resources) to ascertain the whole universe in precise spatial and time dimensions.
But mankind has lived without valid theories and could invent and develop artefacts without actually understanding why and how they work. This could be done by observing that things are somehow linked together.
The sole and only experiment that can prove or disprove hypothesis about climate is the Earth history itself. And being just one unique experiment it may just provide an anecdote. No isolated, small-scale laboratory set-up can replace the complex, non-linear and chaotic system that links together solar and cosmic input with geology, oceans, atmosphere, flora and fauna, and human industry. Of course, scientific methods are used to make and to analyse observations, to interpret historical data, to describe individual phenomena, and even to link together a few of them. A wealth of information and data has been assembled, as we could glimpse in the fact and observation section, and much more is to come. But, at this time and in the foreseeable future (if such exists), no unifying theory can be elaborated that will fully describe the evolution of climate on Earth. The complexity of the whole lets believe that it may never be the case.
This is why we are left with three practical approaches: seek correlation between observed parameters, elaborate simplified models, or go fishing and accept that we aren’t even aware of our ignorance and that the sky may fall on our heads.
With correlations and models, any attempt to make some predictions will have a probability close to 1 to be invalid. Thus we can only speak of prophecies.