What was done
The authors reviewed the state of our knowledge relative to what is known about tropical cyclone activity over the past century of global warming and the ability of state-of-the-art climate models to predict what we might expect with further global warming.

What was learned
In the words of the authors, "there are no discernible global trends in tropical cyclone number, intensity, or location from historical data analyses."  As for the future, they say that "global and mesoscale model-based predictions for tropical cyclones in greenhouse conditions have not yet demonstrated prediction skill."  In addition, they note that "the popular belief that the region of cyclogenesis will expand with the 26°C SST isotherm is a fallacy."

What it means
All of the media hype of more and stronger hurricanes in a warmer world is just that, hype.  It has no basis in real-world observations, or even in climate model predictions themselves.