Hurricane season in the US is kicking into high gear

Let's take a look at how hurricane frequency and intensity has changed and if that matches IPCC predictions.

Hurricane season in the US…

In the United States, hurricane season primarily refers to the Atlantic and Eastern North Pacific regions, as these are the areas most likely to be impacted by tropical cyclones.

Atlantic Hurricane Season (which includes the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea):

• Starts: June 1

• Ends: November 30

• Peak: Typically mid-August to late October.

Eastern North Pacific Hurricane Season (which includes waters off the west coast of Mexico):

• Starts: May 15

• Ends: November 30

The distribution of hurricanes across the season can vary from year to year, but generally, the season starts slowly in June and early July, with a gradual increase in activity. The peak of the season is in mid-September, which historically has the highest likelihood for major hurricane formation. After mid-September, activity tends to decline but can remain significant through October. By the time November rolls around, hurricane activity typically drops off considerably, though late-season storms are not unheard of.

Factors like sea surface temperatures, atmospheric conditions, and the presence (or absence) of phenomena like El Niño or La Niña can impact the formation and intensity of storms in any given year.

Hurricane landfalls in the US…

The frequency of hurricane landfalls in the USA is a complex topic, depending on numerous factors like climatic conditions, sea surface temperatures, and atmospheric patterns. Although this natural variability exists, it is clear that there is no trend in the frequency of landfall hurricanes in the USA and increasing atmospheric GHG concentration.

In fact, this is what is expected during a time of warming. A recent study published in the journal Nature titled, “Declining tropical cyclone frequency under global warming”, states:

Assessing the role of anthropogenic warming from temporally inhomogeneous historical data in the presence of large natural variability is difficult and has caused conflicting conclusions on detection and attribution of tropical cyclone (TC) trends. Here, using a reconstructed long-term proxy of annual TC numbers together with high-resolution climate model experiments, we show robust declining trends in the annual number of TCs at global and regional scales during the twentieth century.

What does the IPCC say?

Here's a broad summary of what the IPCC has indicated regarding the frequency and intensity of hurricanes and tropical cyclones based on previous assessment reports:

Observed Changes:

• There is evidence to suggest that the intensity of hurricanes, particularly the strongest hurricanes, has increased since the early 1980s in the North Atlantic region. (Interesting that this increase has not been observed in landfall hurricanes on US soil.)

• While there are regional variations, globally, the proportion of intense hurricanes (Category 4 and 5) has likely increased over recent decades, and the frequency of weaker tropical cyclones has decreased.

Future Projections:

• It's more likely than not that we'll see an increase in the frequency of the most intense tropical cyclones by the end of the 21st century.

• There's a projection of more rainfall associated with tropical cyclones, particularly in the North Atlantic and Western North Pacific.

• Despite an expected increase in intensity, there's uncertainty regarding the frequency of tropical cyclones. Some models predict a decrease in the overall number of tropical cyclones, but an increase in the proportion of high-intensity storms.

Influence of Climate Change:

• Warmer sea surface temperatures can provide more energy for hurricanes, potentially contributing to their intensification.

Let’s take a closer look at the claim that there will be “more rainfall associated with tropical cyclones”. A recent study published in the journal Geophysical Research Letters titled, “Has There Been a Recent Shallowing of Tropical Cyclones?”, states

The global inner-core high thick cloud fraction measured by satellite has decreased from 2002 to 2021 by about 10% per decade. The TC inner-core surface rain rate is also found to have decreased during the same period by a similar percentage.

To summarize, observed landfall hurricane data for the US shows no trend in the frequency of hurricanes striking the coast since 1850, contradictory to claims by the IPCC that such an increase was already occurring. Furthermore, a recent analysis of precipitation associated with global hurricanes over the last few decades shows significant decreases in rain rate, also contradictory to expectations of the IPCC. There has been no increase in the frequency or precipitation intensity of hurricanes related to climate change.