When you say that climate naturally fluctuates, what you really mean is that physical changes occur that affect the system. For example, ice ages don't just happen by 'magic' ... they are a consequence of some drop in temperature, caused, say, by a reduction in the amount of sunlight we receive, caused by solar variations or the earth's orbit around the sun bringing us slightly farther away, and enhanced by ice reflecting sunlight away from the earth, and oceans retaining more CO2, and so on.
The temperature rise we have observed has a physical cause. We have ruled out the sun (temperatures rose even when solar activity dropped). We have ruled out orbital perturbations and volcanoes. What we have observed is an increase in radiative forcing directly as a result of increasing CO2 levels, linked that rise in CO2 to human activities, and shown that this, along with an increase in water vapour due to higher temperatures, is the physical mechanism that best explains the data.