As we brace for the start of yet another hurricane season on June 1st, I can’t help but compare last year’s hurricane season outlook from the National Weather Service with the one for this year. The first thing that strikes me is that, even though they look very similar, and the number of predicted hurricanes is the same, there is a 10 percent lower probability of this year being an above-normal season than last year.
The 2017 outlook predicted a 35 percent chance of a near-normal season, while this year’s is 40 percent. Lower chance of above-normal and higher chance of near-normal? I think we can all say YEAH to that!
I mentioned last year that since the mid-1970s the number of hurricanes reaching categories 4 and 5 in strength has roughly doubled. It may be unsurprising then that 2017 set some records. With 17 named storms, including 10 hurricanes and six major hurricanes (those reaching categories 3-5), 2017 was the most active season since 2005 and the seventh most active since record-keeping began in 1851. And September 2017 was the most active month on record for Atlantic hurricanes. Hurricanes Harvey, Irma, and Maria are among the five costliest storms of all time. The season was terribly destructive and definitely not “normal.”
We know that it only takes one hurricane making landfall to make a season destructive, and we should always be prepared. But there is nothing wrong about looking at this year’s prediction and hoping for a better season!
Sea surface temperatures are near normal – a good thing
Current North Atlantic sea surface temperature (SST) anomaly pattern looks like opposite of May SST patterns associated with active Atlantic #hurricane seasons historically. pic.twitter.com/1eLt541Q9y
— Philip Klotzbach (@philklotzbach) May 29, 2018
What a difference a year makes! According to NOAA, this year’s slightly less severe outlook is mainly due to temperatures in the tropical Atlantic Ocean and Caribbean Sea. Last year in early May, sea surface temperatures in that area were running consistently above average. But not this year – temperatures are running below average. In addition, there is the possibility for a weak El Niño to develop, and this phenomenon tends to suppress hurricane activity in the Atlantic.
Why does that matter? While many factors affect hurricane formation, the main driver is the sea surface temperature. It is a known fact that hurricanes “feed” on warm waters. Surface ocean temperatures above about 79°F (26°C) are one of the key factors that strengthen hurricane development when overall conditions – high humidity, warm and moist air above the ocean, and relatively constant winds at different altitudes – are conducive for their formation and growth. So yes, lower sea surface temperatures bode well for a potentially weaker season.
The climate change connection
Although we are not off the hook for a bad season, we hope that this year will be nothing like 2017, and certainly wish that Puerto Rico and other locations hit hard by last year’s hurricanes are spared this year.
We know that global warming is not making things any easier when it comes to hurricanes. As mentioned before, warm waters are the fuel for hurricanes, and have the potential to increase hurricane power. But it is not just the wind and strength: the amount of rain that hurricanes bring can also be increased by global warming. The potential moisture content (water vapor) in the atmosphere increases with every 1°C (1.8°F) increase in temperature, and we are currently 2.0°F (1.1°C) warmer than we were in the late 1800’s. That means more water can fall when it rains. It is therefore not surprising that we are seeing not only more powerful hurricanes, but ones bringing more rain too.
Hurricane Harvey is a prime example: it set rainfall records in the Houston region, and that was not by chance. A study published a few months after Harvey hit found that human-caused global warming made the record rainfall roughly three times more likely and 15 percent more intense. Another study found that higher ocean heat content and sea surface temperatures make hurricanes such as Harvey “more intense, bigger, and longer lasting and greatly increase their flooding rains.” As the authors put it, “Harvey could not have produced so much rain without human‐induced climate change.”
Water, water everywhere
I cannot close without mentioning another type of event that often happens during the same season as hurricanes, and that is extreme precipitation – heavy rainfall unrelated to hurricanes or tropical storms – that can bring intense flooding. The increase in moisture content in a warmer atmosphere and the potential for more intense rain events applies whether there is a hurricane or not.
The historic town of Ellicott City, Maryland, was devastated by flooding brought by extreme precipitation just this past weekend. This is the second time in less than two years. We know there are several factors affecting flooding risk in addition to the rain amount itself, such as the configuration of the storm and patterns of land use, but the fact remains that extreme rain events are on the rise. This is something we must come to terms with – and plan for.
Devastating floods in both rural and urban areas are nothing new, but the human and economic toll are increasing as more floodplain and coastal areas are developed. We must act at the federal, state, and local level to rally the resources, policies, and coordination needed to respond adequately to the magnitude and severity of floods not only today, but into the future.
In addition, we must support and act on policies and measures at all levels – federal, state, and local – to reduce coal, oil, and natural gas emissions that cause global warming. Our actions in the next few decades will be crucial in determining how much more weather patterns will change and how destructive they may become.
Support from UCS members make work like this possible. Will you join us? Help UCS advance independent science for a healthy environment and a safer world.