The warming of the tropical Atlantic relative to the rest of the global ocean may have been a key factor driving the abnormally fierce Atlantic hurricane season during 2017, a new study suggests. Moreover, the researchers predict that a similar pattern of North Atlantic sea surface warming - occurring in parallel with increases in greenhouse gas concentrations and decreased aerosol pollution - will likely lead to higher numbers of major hurricanes in subsequent seasons. Their data help inform the ongoing mystery of the effect of human activities on hurricane incidence. The 2017 North Atlantic hurricane season was highly active, with a total of six major storms - nearly two standard deviations above the normal number in the average year. Three of these hurricanes fell over the Gulf Coast and the Caribbean, causing substantial loss and damage. Monitoring unique trends in sea surface temperature, or anomalies, and seawater circulation is key to estimating the likelihood of major hurricane seasons in the future. Using high-resolution computer models, Hiroyuki Murakami and colleagues analyzed the season from 1 July to 30 November, finding that moderate 2017 La Niña conditions - water cooling in the equatorial Pacific that's sometimes linked to hurricane activity - were not major influencers of the 2017 hurricane season. Rather, local Atlantic sea surface temperature anomalies were highly correlated with enhanced hurricane activity. Additional simulation experiments run under the predicted trend of a warmer climate (linked to human-made forces), showed a continued risk of high hurricane density over the Caribbean, the U.S. Coast, and northern North Atlantic open ocean. The authors say that further refinement in the simulation models is needed to parse out the impact of specific human-made activities on hurricane frequencies.