Sea surface temperatures (SSTs) in most regions of tropical cyclone formation have increased by several tenths of a degree Celsius during the past several decades. The Intergovernmental Panel on Climate Change (IPCC) fourth assessment report concluded that most of the global surface temperature increase over the past half century is very likely due to the observed increase in anthropogenic greenhouse-gas concentrations, and the US Climate Change Science Program 3.3 report extended this by concluding that human-induced greenhouse-gas increases have very likely contributed to the increase in sea surface temperatures (SSTs) in hurricane formation regions. These results have raised the question of how substantial further warming, coupled with other changes in the tropical environment, would affect tropical cyclone activity.