What was done
The authors conducted a new analysis of the characteristics of Atlantic hurricanes (tropical cyclones or TCs, whose peak winds exceeded 33 m/s) for the period 1878-2008, based on the widely-used HURDAT database, developing a new estimate of the number of hurricanes that occurred in the pre-satellite era (1878-1965), based on analyses of TC storm tracks and the geographical distribution of the tracks of the ships that reported TC encounters.

What was learned
Vecchi and Knutson report that "both the adjusted and unadjusted basin-wide hurricane data indicate the existence of strong interannual and decadal swings," and although they say that "existing records of Atlantic hurricanes show a substantial increase since the late 1800s," their analysis suggests that "this increase could have been due to increased observational capability." In fact, they say that "after adjusting for an estimated number of 'missed' hurricanes (including hurricanes that likely would have been mis-classified as tropical storms), the secular change since the late-nineteenth century in Atlantic hurricane frequency is nominally negative -- though not statistically significant."

What it means
The two researchers from NOAA's Geophysical Fluid Dynamics Laboratory state that their results "do not support the hypothesis that the warming of the tropical North Atlantic due to anthropogenic greenhouse gas emissions has caused Atlantic hurricane frequency to increase."