Key Vocabulary: hurricanes, equator, heat energy, atmosphere, fossil fuels, atmospheric pressure, storm surge, evaporation, moisture, condense, frequency, precipitation, sprawl
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.
Classroom Activity: Hurricanes_Article Guide
Hurricane Harvey made landfall in Houston, Texas last week, breaking rainfall and flooding records, causing up to $180 billion dollars in damage, and claiming 60 lives as of this writing. Some meteorologists have called it a once in the 1000 year storm. Yet even as the city of Houston begins the recovery process, another storm, Hurricane Irma, has become the strongest Atlantic storm ever recorded and is already wreaking havoc in the Caribbean.
Many in the media have taken to blame climate change for these hurricanes, but are rising global temperatures to blame for the historic storms? Hurricanes have been occurring on Earth for eons, but in the last 100 years, human activity, namely the burning of fossil fuels, has caused average global air temperatures worldwide to increase by about 1.33°F (0.74°C).
Meteorology is complicated, our atmosphere is a complex system with many different variables contributing to weather events. However, there is research out there that will allow us to draw some conclusions on how Hurricane Harvey formed, why it caused so much destruction, and what is in store for the future.
How hurricanes form
Before we can get into whether climate change can or cannot be blamed for Harvey and Irma, let’s go over some basics of hurricane science. 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). At that temperature, there is enough heat energy to bring a significant amount of water in 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 storm, picking up more heat and moisture, and developing it into a hurricane.
The warm water is what gives hurricanes their strength. 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
Since warmer oceans form stronger hurricanes, shouldn’t climate change have an effect on hurricanes? And if so, how?
In 2010, the NOAA did a study on the effect of increasing temperatures on hurricane frequency. The results were mixed: while some models forecasted 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 there have been some recent years with more hurricanes, there was no evidence to show a statistically significant change in the number of hurricanes.
However, there was evidence to show that there would be a higher frequency of category 4 and 5 hurricanes. Warmer air means warmer oceans, and warmer oceans allow for more heat and water to enter the atmosphere, strengthening storms. Tom Knutson of the NOAA built a computer model for hurricanes at the end of the 21st century compared to today, which showed an increase in Category 4 and 5 storms worldwide.
In the case of Hurricane Harvey, the storm formed off the western coast of Africa in Mid-August when the water temperatures were around 85°F (29.4°C), about 2°F higher than the historical average. However, as Harvey moved through the Gulf of Mexico, it picked up a lot of strength. Water temperatures in the Gulf of Mexico when Harvey passed through were around 86.7°F (30.4°C), though only slightly above normal, gave the storm a lot of power.
The Gulf also had an abnormally warm winter, with sea temperatures not falling below 73° F (22.7°C) while they normally get down to. 57° F(13.9°C). Meteorologists were concerned back in March that this could lead to stronger storms in the fall. That extra heat may have contributed to the record 50 inches of rainfall on the Houston area.
So while climate change probably didn’t cause Hurricane Harvey, the increases in air and water temperature probably did make it stronger and the damage more costly. But even as you read this, Hurricane Irma, a Category 5 storm, is moving through the Caribbean, with sustained winds of 185 mph (297 km/h). Depending on its direction, Irma could cost even more in damage and in lives.
The other problem with Hurricane Harvey
There’s another issue with the devastation caused by Hurricane Harvey that is not directly related to climate change. While most major U.S. urban centers have experienced rapid suburbanization since the end of World War II, Houston has become one of the most sprawled cities in the country, with only about 3,500 people per square mile. While it is the 4th biggest city in the United States, it’s 51st in population density.
With cheap land, and city planners willing to expand the highway system, the Houston metro area built away from the city center. From 1960 to 2010, the city of Houston grew from less than a million people to over 2.1 million people. However, the metro area around Houston grew from 1.1 million to nearly 6 million residents within the same time frame. In the first decade of the 21st century alone, Houston area suburbs grew at an astonishing 39.3%.
That sprawl destroyed much of the natural environment of coastal prairie that could have absorbed the extra precipitation. A ProPublica report from last year found that over 600,000 acres of habitat have been paved over. 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. While the suburban sprawl certainly did cause the hurricane, at least not directly, it likely made the effects worse.
- As the winds of a hurricane move towards the center of the lower pressure, they cause the storm to spin due to the Coriolis Effect. Learn more about the phenomenon here.
- Vox has a very nice video summary of climate change and hurricanes.
- Debunk your misconceptions on climate change with our three part series: Part 1, Part 2, and Part 3.