Climate change key driver of catastrophic impacts of Hurricane Helene that devastated both coastal and inland communities

Late on September 26th, Hurricane Helene made landfall at category 4 on the Saffir-Simpson scale on the panhandle of Florida, bringing high winds, extreme rainfall and storm surges to coastal areas.

Over the next two days, it struck inland in a North-northeasterly direction, bringing torrential rainfall to a large region including most of Georgia, western North and South Carolina, eastern Tennessee and southern Virginia. This led to extensive and in many cases unprecedented flash flooding across many of these regions. At least 227 people were killed and nearly 2 million were left without power (AP News, 2024; CNN, 2024).

The influence of climate change on tropical cyclones is more complex compared to other types of extreme weather events as impacts are driven by both heavy rainfall and extreme winds. In addition, the surrounding conditions, in particular the sea surface temperatures in the Gulf of Mexico and wider basin, are crucial for the formation of hurricanes. Here, scientists from the United States, the United Kingdom, Sweden and the Netherlands, use several different approaches to investigate the influence of climate change on these different aspects of hurricane Helene, following the same approach as with Typhoon Gaemi that led to severe impacts in the Philippines, Taiwan and Hunan province of China earlier in the year, to analyse whether and to what extent human-induced climate change affected wind speeds and rainfalls associated with Hurricane Helene. For the assessment of the role of climate change in the heavy rainfall, we divide the most impacted region into two subregions, the coastal region encompassing 29.5-32 °N and 81.5-86 °W (figure 1.1) where Helene made landfall and caused 2 days of extreme rainfall, and the inland region, encompassing 34.5-38 °N and 80-85 °W (figure 1.1), in the Southern and Central Appalachians where the rainfall from the hurricane combined with an earlier heavy rainfall system and led to 3 days of unprecedented rainfall. To study the conditions that formed and fuelled Helene, we also analyse the role of climate change in high sea surface temperatures and potential intensity, a metric combining sea surface temperature, air temperature and air humidity data to predict maximum hurricane wind speeds.

A map showing Accumulated precipitation over the southeastern US from 25-27th September 2024. The track of Hurricane Helene is shown as a series of red points, and the red regions are those used for extreme rainfall analysis in section 2 of this report. The borders of the affected states are shown for reference. — Figure 1.1: Accumulated precipitation over the southeastern US from 25-27th September 2024. The track of Hurricane Helene is shown as a series of red points, and the red regions are those used for extreme rainfall analysis in section 2 of this report. The borders of the affected states are shown for reference. Data from IMERG.

Hurricane Helene formed in the Gulf of Mexico above record-hot sea surface temperatures (SSTs). In the days leading up to Helene’s landfall, a line of slow-moving storms formed along a stalled cold front, drawing in tropical moisture from Helene’s outer edges. This system, stretching from Atlanta through the southern Appal