While it’s rapidly becoming established wisdom that both sea level rises and increased storm severity are separately seen as major threats to the US’s east coast, scientists are now looking at the interrelations between the two factors to see what impact their combined efforts are having on the future of flooding in the region. The predictions are that such interactions are likely to cause major spikes in both the height and duration of major flooding events.

As global temperatures rise, naturally so does the temperature of Earth’s seas. Warmer water is quicker to evaporate and has been linked to stronger storm activity. With more heat energy in the water, tropical storms have become less frequent but more intense. Worldwide, the number of severe hurricanes has doubled even though the total number of all hurricanes over the last 35 years has dropped.

Sea-level rises are easier to observe and have been rising by 0.1 to 0.25 centimetres per year, according to NOAA. Already, the east coast of the US is experiencing sea-level rises above the global average of eight inches. Sinking land in the region combined with expanding, warmer oceans mean that sea-levels have risen by up to a foot in much of the northeast and the mid-Atlantic. By 2100, the region is predicted to experience a further two to four foot rise, with worst-case scenarios predicting six feet.

Five locations where the climate model was analysed, with redder areas signifying warmest water predictions (Image: Little et al, Nature Climate Change)

A new study has combined the upward projections of these two factors in order to predict how they might impact the region’s coasts. Using 15 different climate models, the study analysed the probability that the high sea-level and intense storm activity could react together over time to produce more than the sum of their parts. ‘When you look at hazards separately, it’s bad enough,’ says Radley Horton, climate scientist at Columbia University and co-author of the study. ‘But when you consider the joint effects of two hazards together, you can get some surprises. Sometimes, one plus one can equal three.’

The 15 climate models were studied at five key locations across the east coast: Atlantic City (New Jersey), Pensacola (Florida), Charleston (South Carolina), Key West (Florida) and the somewhat more southern Galveston on the east coast of Texas. Five of the 15 models simulated both high sea-level rises and increases in the strongest storms. 

With this data, the study has created two scenarios for the 21st century: the first incorporates the current trend in greenhouse gas emissions while the second uses greatly reduced emissions. With reduced emissions, the locations on the east coast were predicted to see a four to 75-fold increase in their flood index (the combined heights and durations of expected floods). 

However, without any change in emissions, the study states that the east coast flood index could increase by 35 to 350 times. However, because the study does not include any scenarios for further sea-level rise from melting glaciers – just data from expanding waters – the authors warn that the reality could be even worse.