evidence: Substantial multidecadal variability in the basin confounds efforts to detect long-term (century-scale) trends of the type that might be anticipated as responses to increasing greenhouse gases. Human activities may have already caused other changes in tropical cyclone activity that are not yet clearly apparent due to the small magnitude of these changes compared to estimated natural variability, or due to observational limitations. [...] Despite the difficulty in identifying a detectable influence of human-caused greenhouse gas increases on global tropical cyclone activity, greenhouse gas-induced climate change may have already caused changes in tropical cyclone activity that are not yet detectable due to their small magnitude compared to estimated natural variability, or due to observational limitations. [...] An implication of the GFDL studies is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse-gas induced warming may lead to an increasing risk globally in the occurrence of highly destructive category-5 storms.