Hurricane Florence at its strongest on September 9th, 2018. (Photo Credit NOAA)

Climate change didn’t cause Hurricane Florence, but it probably made it worse

Hurricane Florence has caused the worst East Coast flooding in history.

Next Generation Science Standards: 

  • MS-ESS2-4. Develop a model to describe the cycling of water through Earth’s systems driven by energy from the sun and the force of gravity.
  • MS-ESS2-5. Collect data to provide evidence for how the motions and complex interactions of air masses result in changes in weather conditions.
  • MS-ESS2-6. Develop and use a model to describe how unequal heating and rotation of the Earth cause patterns of atmospheric and oceanic circulation that determine regional climates.
  • MS-ESS3-2. Analyze and interpret data on natural hazards to forecast future catastrophic events and inform the development of technologies to mitigate their effects.

Key Vocabulary: hurricanes, equator, heat energy, atmosphere, atmospheric pressure, storm surge, evaporation, moisture, condense, frequency, precipitation,

Article Guide: Hurricane Florence_Article Guide

Hurricane Florence slammed into the Eastern United States last weekend, dumping a record-breaking amount of rain and leaving more than a million people without power. While Florence had weakened from a category 4 to a category 1 hurricane by the time it made landfall Friday, the storm had slowed to crawl, essentially stalling over the Carolinas and causing the worst East Coast flooding on record. The storm has already resulted in 32 deaths, but the worst may be still yet to come as 16 rivers in the state of North Carolina are at high flood stage. Roads and highways have been rendered impassible, cutting off residents from food, water, and other critical supplies. Officials are planning to airlift supplies to Wilmington, NC, a city of nearly 120,000. Additionally, there is an impending environmental disaster as open-air manure pits on hog farms have also flooded, dumping animal feces into the local water supply. With flooding not expected to peak until Wednesday or Thursday, the disaster has no end in sight.

As the same time the Carolinas were being evacuated from Florence, the strongest hurricane of the year, Typhoon Manghkut, was wrecking havoc on the Philippines with wind gusts reaching over 200 mph. At one point this year, there were up to four tropical storms or hurricanes in the mid-Atlantic at the same time. These storms come only a year after a record-breaking 2017 hurricane season. Hurricane Harvey’s floodwaters reached over 25 ft and Hurricane Maria was responsible for the deaths of nearly 3000 Americans in Puerto Rico. Have the last two years just been outliers, or is climate change really to blame for the increase in hurricanes?

The answer is complicated; weather and climate are complex systems. Some of the most powerful computers in the world are used to predict atmospheric conditions, and even those models are sometimes wrong. That said, climate change is expected to increase the number of extreme weather events, including hurricanes. Increasing global temperatures might not necessarily cause more storms, but will likely cause storms to be stronger and more destructive.

Observed rainfall in North Carolina from Hurricane Florence (Photo Credit: National Weather Service)

How Hurricanes Form

Hurricanes are rotating systems of thunderstorms around a center of low atmospheric pressure. Scientifically, they are referred to as tropical cyclones, and they are the most powerful storms in the world, characterized by strong surface winds, heavy rain, and storm surges.

Hurricanes form above the oceans around the equator, usually in late summer when sea temperatures are over 80° F (26.6°C). When the ocean gets that warm, there is enough heat energy to bring a significant amount of water into the atmosphere through evaporation. As the ocean heats the air above it, the air rises, bringing with it heat and moisture. This causes a low-pressure system to form, bringing more air towards the center of the storm, and with it even more heat and moisture, and developing it into a hurricane.

The warm water is what powers hurricanes; the higher the sea temperatures, the more heat energy and moisture are able to enter the atmosphere and the stronger the storm. Additionally, warmer air is able to hold more water once it evaporates. According to Penn State climatologist Michael Mann, an increase of about 1°F (0.5° C) correlates with roughly a 3% increase in atmospheric moisture content. This means that hurricanes which form over warmer waters are not only stronger but have a higher capacity for precipitation.

The Effect of Climate Change

So as global temperatures continue to rise, shouldn’t that have an effect on hurricanes?

In 2010, the NOAA did a study on the effect of increasing temperatures on hurricane frequency. The results were mixed: while some models foretasted more hurricanes in the coming years, there was no provable causal relationship between an increase in atmospheric temperatures and the number of hurricanes each year. The observational data supports this: while the last two years have had many named hurricanes, there was no evidence to show a statistically significant change. However, there was evidence to show that there would be a higher frequency of stronger hurricanes. Warmer oceans allow for more heat and water to enter the atmosphere, strengthening storms. And since warmer air can hold more moisture, even weaker hurricanes can still cause a lot of damage through excess rainfall and flooding.

In the case of Hurricane Florence, the storm formed off the western coast of Africa at the end of August when the water temperatures were around 87.5°F (30.6°C), about 2°F higher than the historical average. As Florence moved through the Atlantic, it picked up a lot of strength and moisture before weakening to a catagory 1 storm just before landfall. However, the Saffir-Simpson Scale for classifying hurricanes is based only on wind speed, not precipitation. So while the wind damage from Florence was relatively minor, the flooding was catastrophic. Nearly 36 inches of rain fell in just four days in Elizabethtown, North Carolina, smashing the previous rain record for North Carolina by nearly a foot. A preliminary study on Hurricane Florence modeled the storm in a world that hadn’t been subject to climate change and found that the warmer atmosphere allowed Florence to dump 50 percent more rainfall.

Ocean temperatures have been increasing worldwide over the last 50 years. Source: NOAA

 

It bears repeating that we cannot point to any single weather event as caused by climate change. The amount of damage a particular hurricane causes depends on a host of different factors; how fast a storm moves, the natural geography of the area affected, and the land use by humans. One of the reasons Hurricane Harvey caused so much damage was due to Houston’s urban sprawl. Much of the area’s 600,000 acres of natural coastal prairie has been paved over to create roads, parking lots, and shopping centers. Replacing the grasses and marshes with concrete meant that virtually none of the 27 trillion gallons of rain could be absorbed by the ground. All that water had nowhere to go but up, and the floods rose to historic levels.

That said, climate change is going to mean more volatility in the atmosphere, and likely, stronger and more damaging storms.

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