Tropical storms

Hurricanes thrive in warm water, weakening if sea surface temperatures fall below 27° Celsius. Partly for this reason, the possible effects of global warming upon hurricanes—particularly their frequency and intensity—is a subject of serious concern among climate scientists and world governments.

Background

A tropical storm is a cyclonic storm pattern originating in the tropics that has the potential to develop into a hurricane. Sea surface temperatures (SSTs) are believed to play a role in hurricane frequency and intensity, since hurricanes usually require SSTs of at least 27° Celsius to form and sustain themselves. Many other factors contribute to hurricane development, however, making a simple linear association of hurricane intensity with SST, or with global warming generally, difficult to prove.

As is the case with other phenomena, global warming’s effect on hurricanes plays out in a broader climatic context. The relationship (or lack thereof) between hurricane intensity and warming atmospheric and oceanic temperatures is complicated by the fact that water temperatures (like air temperatures) sometimes vary over periods of several decades. The long-term effect of rising greenhouse gas (GHG) levels upon climate is only part of the total picture.

Cyclonic oceanic storms that generate over the oceans are known by different names around the world. “Tropical storms” contain winds of less than hurricane strength (under 119 kilometers per hour). Storms that are called “hurricanes” in the Atlantic Ocean are known as “typhoons” in the Pacific Ocean and “cyclones” in the Indian Ocean. These storms are also rated numerically on the Saffir-Simpson scale, from 1, a minimal hurricane, to 5, a storm with winds of more than 251 kilometers per hour that can produce catastrophic damage.

Most research on tropical storms and climate change has been performed on hurricanes. The debate in the United States also has been parsed mainly regarding hurricanes, since these are the storms that affect that country. Since tropical storms are similar worldwide, however, this research may be generalized to other regions.

Hurricane Intensity and Frequency

SSTs in the Atlantic Ocean, which produces nearly all the hurricanes that have an impact on the United States, have been rising slowly but steadily since the 1970s, paralleling a general global rise in air temperatures. Frequency and intensity of hurricanes (as well as the number hitting US coastlines and inflicting major damage) also have been rising during the same period. While heat is important to hurricanes, they are very sensitive to other influences as well. After the devastating hurricane seasons of 2004 and 2005, the next year was very quiet. A major reason for the drastic decrease in hurricane activity in 2006 appears to have been an increase in the level of dust from the Sahara Desert present in the air over the Atlantic Ocean. During the 2006 hurricane season, SSTs also remained relatively cool, and only five hurricanes formed, one-third the number that formed in 2005.

Still other factors influence hurricane number and intensity, including El Niño conditions in the tropical eastern Pacific Ocean and the strength of the West African monsoon. If other conditions are right, a hurricane season can be intense even if SSTs are cooler than usual, and vice versa.

Any study of tropical storms that begins in the 1970s will indicate a very close relationship between ocean warming, hurricane intensity, and air temperatures. However, during the 1950s and 1960s, air temperatures were generally cooler than during the 1970s, but average hurricane intensity was higher. By 2005, this divergence was fueling a debate between some hurricane experts regarding whether, and to what degree, hurricane intensity and frequency is related to an overall warming trend. During the 2010s and 2020s, climate scientists saw a significant increase in the intensity of hurricanes. They attributed these changes to global climate change, with most scientists warning that coastal communities should prepare for storms to continue to increase in intensity over the coming decades.

The Temperature-Intensity Argument

In 1988, Kerry Emanuel, a hurricane specialist at the Massachusetts Institute of Technology, published an article in the Journal of the Atmospheric Sciences in which he argued that hurricane intensity is governed, in part, by the degree of between the atmosphere and the underlying ocean. Therefore, Emanuel reasoned, warmer ocean waters should breed more intense tropical cyclones. While tracing any specific storm to warming is a tenuous exercise, hurricanes may intensify generally as oceans warm. Hurricanes are essentially heat engines, so storms that approach their upper limits of intensity are expected to be slightly stronger—and produce more rainfall—in warmer climates as a result of higher SSTs.

According to a simulation study by a group of scientists at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL), a 5 to 12 percent increase in wind speeds for the strongest typhoons in the northwest tropical Pacific is projected if tropical SSTs increase by a little over 2° Celsius. Thomas R. Knutson and Robert E. Tuleya’s models indicate that given SST increases of 0.8° to 2.4° Celsius, hurricanes would become 14 percent more intense (based on central pressure), with a 6 percent increase in maximum wind speeds and an 18 percent rise in average precipitation rates within 100 kilometers of storm centers.

• Key Concepts
• El Niño-Southern Oscillation (ENSO): an ocean-circulation pattern that warms the Pacific Ocean west of Peru, affecting world weather
• hurricane: a severe oceanic cyclonic storm in the Atlantic Ocean—called a typhoon in the Pacific Ocean and a cyclone in the Indian Ocean
• La Niña: an ocean-circulation pattern that cools the Pacific Ocean west of Peru, affecting world weather
• thermodynamic disequilibrium: a state of instability in the energy distribution of a system; indicative of tropical storm intensity
• West African monsoon: a seasonal climatic variation that can affect hurricane intensity and frequency in the Atlantic Ocean

Bibliography

"A Force of Nature: Hurricanes in a Changing Climate." NASA, 1 June 2022, science.nasa.gov/earth/climate-change/a-force-of-nature-hurricanes-in-a-changing-climate/. Accessed 10 Dec. 2024.

Donnelly, Jeffrey P., and Jonathan D. Woodruff. “Intense Hurricane Activity over the Past Five Thousand Years Controlled by El Niño and the West African Monsoon.” Nature 447 (May 24, 2007): 465-468.

Emanuel, Kerry A. “The Maximum Intensity of Hurricanes.” Journal of the Atmospheric Sciences 45(1988): 1143-1156.

Gray, William M. “Hurricanes and Hot Air.” Wall Street Journal, July 26, 2006, p. A-12.

Knutson, T. R., R. E. Tuleya, and Y. Kurihara. “Simulated Increase in Hurricane Intensities in a CO₂-Warmed Climate.” Science 279 (February 13, 1998): 1018-1020.

Landsea, C. W., N. Nicholls, W. M. Gray, and L. A. Avila. “Downward Trends of Atlantic Hurricanes During the Past Five Decades.” Geophysical Research Letters 23 (1996): 1697-1700.