No one claims otherwise.From the Executive Summary of Chapter 3 of IPCC’s 2012 Special Report on Extreme Events (SREX): “Many weather and climate extremes are the result of natural [i.e., not man-made] climate variability (including phenomena such as El Niño), and natural decadal or multi-decadal variations in the climate provide the backdrop for anthropogenic [human-caused] climate changes. Even if there were no anthropogenic change in climate, a wide variety of natural weather and climate extremes would still occur.”
What you've been told, and re-insurers are seeing in their own data, is that these occurrences are increasing in frequency, and severity.