The Earth's climate isn't in "constant flux". "Constant flux" implies rapid, constant change, which isn't the case. Earth's climate as a whole gradually changes on a roughly cyclical basis over geological time periods, if that's what you're trying to get at.
Survey's have consistently shown that a high percentage of "scientists", i.e. people with scientific qualifications working in some capacity which could be described as "a scientist", (a ludicrously broad category, I agree) believe in man-made climate change. The most-quoted example found 97% support the theory. While it may be a broad category, "scientists" are by definition highly educated and familiar with the scientific method. If a high proportion of people with the relevant qualifications believe that a particular theory is true, then it is generally accepted that the supporting evidence is good enough to justify belief. That's basically science in a nutshell.
Which leads me neatly onto my next point: jumping up and down yelling "it's just a theory" isn't a criticism, because in a scientific context, "theory" does not equate to "guesstimate". It means "a coherent group of propositions formulated to explain a group of facts or phenomena in the natural world and repeatedly confirmed through experiment or observation". In other words, to all intents and purposes, a fact.
The average global temperature of the earth has been increasing steadily since records began. This is a simple, observable, measurable fact.