A paper published in the current issue of Nature Climate Change finds climate models predict North Atlantic tropical storms could increase over the first half of the 21st century, due to undetermined forcing from "other than increasing CO2 (probably aerosols)" but that the "trends over the entire twenty-first century are of ambiguous sign." Translation: they could increase or decrease, we have no clue.
Assessing potential changes in North Atlantic (NA) tropical storm (TS) activity this century is of paramount societal and economic significance, and the topic of intense scientific research1. We explore projections of NA TS changes over the twenty-first century by applying a statistical downscaling methodology2, 3 to a suite of experiments with the latest state-of-the-art global coupled climate models4. We also apply a methodology5 to partition the dominant sources of uncertainty in the TS projections. We find that over the first half of the twenty-first century radiative forcing changes act to increase NA TS frequency; this increase arises from radiative forcings other than increasing CO2(probably aerosols). However, NA TS trends over the entire twenty-first century are of ambiguous sign. We find that for NA TS frequency, in contrast to sea surface temperature (SST), the largest uncertainties are driven by the chaotic nature of the climate system and by the climate response to radiative forcing. These results highlight the need to better understand the processes controlling patterns of SST change in response to radiative forcing and internal climate variability to constrain estimates of future NA TS activity. Coordinated experiments isolating forcing agents in projections should improve our understanding, and would enable better assessment of future TS activity.