The phrase "natural fluxes" refers to the regular and continuous changes that occur in nature. It basically means the natural flow or movement of things, such as the cycling of water, energy, or nutrients in ecosystems. Full definition
And we are currently exceeding the natural flux of entry of new carbon by about a factor of 20, because you can't say that nature is not putting carbon up into the circulation and that we are doing it [all]. (scientificamerican.com)
According to the IPCC reports, the carbon cycle has actually intensified, with the natural fluxes both into the atmosphere and out of it increasing. (skepticalscience.com)
Some will argue that coral conditions are in a natural flux, or that reefs will have time to adapt to our changing oceans. (natureworldnews.com)