The phrase "unprecedented rate" means that something is happening or changing very quickly or at a level that has never been seen before. Full definition
The earth and its ecological processes are changing at unprecedented rates due to human activity; the effects of these changes are uncertain. (neonscience.org)
For example, we have seen the ice and snow extent greatly decrease throughout the world, reflecting unprecedented rates of melting in modern time. (realclimate.org)
Climate change is happening, being driven by the human combustion of fossil fuels at unprecedented rates for more than a century. (climatenewsnetwork.net)