Remaining Uncertainties in the Causes of Past and Future Atlantic Hurricane Activity

Abstract:

There is no debate that hurricane activity in the North Atlantic has increased substantially since the relatively quiescent period of the 1970s and 1980s, but there is still uncertainty in the dominant cause of the increase. Increases in anthropogenic greenhouse gases (aGHG) have contributed to the observed increase in tropical sea surface temperatures (SST) over the past century, while shorter-term decadal variability in regions where hurricanes form and track is generally dominated by 1) internal variability, 2) natural factors such as volcanic eruptions and mineral aerosol variability, and 3) changes in anthropogenic aerosols. Direct SST warming from globally well-mixed aGHG is understood to have a much smaller effect on hurricane formation and intensification compared to the effect of regional warming due to changes in the three factors noted above. While most recent papers implicate both internal and external anthropogenic causes for the presently heightened Atlantic hurricane activity, some show that internal variability dominates and others show that anthropogenic factors dominate.

In the Atlantic, model projection-based consensus indicates no change in storm frequency over the next century but the uncertainty is large and spans –50% to +50%. Mean storm intensity and rainfall rates are projected to increase with continued warming, and the models tend to agree better when projecting these measures of activity. Models that are capable of producing very strong hurricanes usually project increases in the frequency of the most intense hurricanes. This measure is highly relevant to physical and societal impacts. In the Atlantic, model-based consensus indicates substantial increases in the strongest hurricanes, but the uncertainty is large and spans –100% to +200% change over the next century.