Is climate change making hurricanes worse?

Recent events surrounding Hurricane Harvey in Texas and Hurricane Irma in Florida have led to claims that these storms were caused by or made worse by global climate change. Are such disasters part of a trend? If so, will greatly reducing the use of fossil fuels reverse this trend?

Randy Cerveny, a climatologist and an Arizona State University professor in the School of Geographical Sciences and Urban Planning, sheds some light on what science knows and doesn’t know about the effects of human activity on the earth. Cerveny was appointed by the United Nations-affiliated World Meteorological Organization (WMO) to report on climate extremes. In this capacity, he records, verifies and sometimes repudiates weather extremes reported around the world.

Was Irma a record setting hurricane?

While Irma was without question nasty, it wasn’t globally record-setting. The world’s records for most intense hurricane (by wind speed) remain tied: Sustained winds of 215 mph for Typhoon Nancy (1961) in the Northwest Pacific Ocean and Hurricane Patricia (2015) in eastern Pacific Ocean. Irma’s 185 mph winds were some of the strongest recorded in the Atlantic. Irma also sustained those winds for a very long time and we will be evaluating the data to see if it set a record for the Atlantic basin in terms of longest sustained winds. However, there have been two previous Category 5 hurricanes which were Category 5 at the time they made landfall in Florida: The 1935 Florida Keys “Labor Day” hurricane, and Hurricane Andrew, which hit Dade County, Fla., Aug. 24, 1992. Irma was a Category 4 storm when it made landfall.

The strongest hurricane to make landfall in the U. S. (and the only other Category 5 hurricane to make landfall as a Category 5) was Hurricane Camille, which hit Mississippi in 1969.

 With Harvey, Irma and Jose, are we experiencing more hurricanes now than in previous years?

No. This is normal time (early-mid September) that we have the peak of the Atlantic hurricane season. Over the last couple of years, the Atlantic has been relatively quiet due in part to the extraordinary El Niño episode in the Pacific. However, this does demonstrate the accuracy of our long-term seasonal hurricane predictions as this year was predicted to be an above-average hurricane year.

The hurricanes seem bigger and stronger than pervious years. Is that true?

There is some theoretical research that suggests with global warming hurricanes may increase in size and intensity but there is still much debate about that in the scientific community. In general, hurricanes have not changed much in the last few decades in terms of size or intensity. The last hurricane of Category 5 strength to hit the United States was Andrew (1992). The difference is coverage of tropical cyclones (via 24/7 news and social media) brings tropical storm activity much more into public attention.

 Is climate change making these storms more severe?

A lot of things impact hurricanes but, yes, in general, a warmer climate produces hotter ocean surfaces and more evaporation. Because it is water that powers hurricanes, hotter oceans and more evaporation should produce in part more intense hurricanes. The problem is that there are many other factors that also impact hurricanes. For example, “wind shear,” the variations in wind speed and direction with height, can also cause storms under certain circumstances to weaken or to intensify. The impact of climate change on those many other factors is still being studied.

 What is fact/fiction in terms of climate change making storms worse?

In general, hurricanes are producing more rain due to climate change. Warmer warms and more evaporation put more moisture into the atmosphere so that increased precipitation can occur. With that said, Hurricane Harvey’s incredible rainfalls were primarily the result of the hurricane abruptly stalling along the Texas coast (due in part to a giant ridge of high pressure in the Southwest). That lack of movement allowed for continued rainfall over many days – and that lead to massive flooding.

It is always difficult to attribute one storm’s impact to climate change. Just as it is absurd to say that a single major snowstorm in the winter contradicts anthropogenic global warming theory, it is equally absurd to say that one hurricane in summer confirms global warming theory. Climate is “long-term” while meteorology is “short-term.” As climatologists, we look at long-term (years to decades) averages of hurricanes (size, intensity, frequency), for example, to establish the impact of climate change, not single storms.

 What types of weather are we experiencing a spike in?

Most computer climate models indicate an overall increase in precipitation for a warming planet. Again, that generally is the result of warmer ocean temperatures leading to greater evaporation and, consequently, more fuel for heavier rains. Conversely, most computer climate models also indicate shifts in circulation patterns that might produce greater droughts for regions of Earth. So in general the models are indicating an overall greater variability in some aspects of climate (greater floods but also more intense droughts) with a warming planet.

Is there anything else we should know about the current state of weather and how human activity might be affecting it?

One thing that is important to realize is that we are influencing climate and environment at ALL scales, not just the global scale. Some of the really big problems associated with Hurricane Irma, in terms of damage and flooding, can be attributed as the direct result of human activity over the last 50 years—but not so much at the global scale but at local and regional scales. By changing the environment from the Florida marshlands (e.g., Everglades) to massive urban and suburban landscapes, humans have dramatically increased the damages associated with hurricanes. This is because the marshlands have, in the past, acted as critical barriers and reservoirs for storm surge flooding and rainfall associated with hurricanes like Irma. In addition, the leveling and paving of roads and housing developments doesn’t allow the rains to run off as they would in natural marshlands. Sustainability with regards to climate change isn’t just at the global scale, it really begins at the local and regional scales.