Just from this historical record , he could see the effects of climate on storm frequency. A pronounced lull in the frequency of storms in the mid 1700's, was coincident with a climate feature called the Maunder minimum, suggesting a clear link between longer climate cycles and hurricane frequency. Dr. Emanuel made a distinction between storm numbers and intensity because the two are not related and should have very different patterns of interannual variability. Storm number is the feature that is usually studied with the statistical and GCM modeling approaches described by Knaff and Krishnamurti. What determines intensity is less well understood.
Hurricanes are heat pumps which transfer heat from the surface of the ocean to the upper atmosphere.
The intensity of a cyclone is related to the difference between the temperature at the surface of the ocean and the temperature of the upper atmosphere. Dr. Emanuel presented figures showing the relationship between these two temperatures and the central pressure of a storm. Particularly for strong storms, a very small change in ocean temperature will result in a very large change in the potential strength of a storm. However, most storms never reach the theoretical limit of intensity . Although the reason that storms are less strong than predicted is still unclear, it is an area of active research.