Has the intensification of hurricanes in recent years been caused by the rise in greenhouse gases in the atmosphere?
A computer model supports this hypothesis.
In 2024, the hurricane season in the United States was particularly deadly: hundreds of people died in hurricanes Helene and Milton that struck Florida.
Two reports published days later claim that their exceptional intensity is related to the increased warming of the sea caused by rising greenhouse gas emissions.
According to the researchers who wrote them, it is unlikely that the storms would have been so deadly without the increasing emissions of greenhouse gases.
How did the researchers reach these conclusions?
The researchers applied climate attribution, a scientific procedure that studies the development of the climate system over time.
They sought to investigate quickly while the extreme weather event was still in public memory whether the event was linked to climate change, so the public could understand firsthand the negative impacts of the crisis.
Soaring Increase
Why is hurricane intensity expected to rise with global warming?
An explanation presented in the 1980s compared the functioning of a hurricane to a heat engine. Ideally, a heat engine uses the instability caused by a temperature difference between a hot environment and a cold environment and converts the instability into work, for example movement. By this analogy, the temperature difference between warm waters in the tropical Atlantic Ocean and the cold upper atmosphere leads to the development of a hurricane, a weather phenomenon that includes strong winds and high rainfall.
Due to the increase in greenhouse gas levels in the atmosphere, the tropical part of the ocean is expected to warm further, and therefore hurricane intensity, meaning their maximum wind speed, is projected to rise in the future.
Essential but Not Enough
To test how much the increase in greenhouse gas levels in the atmosphere affects hurricanes, temperature and hurricane intensity need to be measured, and the simplest tool is to compare the two data sets over different time periods.
For more than a century, the Earth has been warming at an increasing rate, and today the average global temperature is the highest in 100,000 years.
Average temperatures rise year by year, allowing researchers to examine whether average hurricane intensity also increases.
Indeed, between the 1980s and the early 2000s, average hurricane intensity rose alongside ocean warming.
Necessary but Not Sufficient
Is the correlation between global warming and hurricane intensity enough to determine that increased greenhouse gas emissions are causing stronger hurricanes? No.
Even in school science lessons, it is taught that correlation does not necessarily imply causation, and every experiment requires a control group.
Whether studying a drug, a chemical process, or plant development, the effect being tested must be compared to a control group the group where the action is not applied. For example, to test whether a drug is effective for treating a disease, the recovery speed and quality of the test group are compared to a group of patients who did not receive the drug.
Only if the test group recovers faster can the drug’s effectiveness be determined.
To determine that the increase in greenhouse gas emissions influenced hurricanes and possibly other parts of the global climate, it would be useful to have another Earth where greenhouse gas emissions were not rising.
In that case, we could compare the average hurricane intensity on our Earth, where emissions are rising, to the average intensity on the second Earth, where emissions are constant.
This would show whether excess greenhouse gas emissions influence hurricane intensity on Earth. Unfortunately, we have only one Earth, where we emit increasing amounts of greenhouse gases. How can we still know if greenhouse gases are causing stronger hurricanes?
Hurricane on Screen
The main tool used by the scientific community in climate attribution is climate models, computer models for climate prediction.
The models link the effects of the atmosphere, ocean, and sea ice using physical equations that describe their state.
Climate models are like a virtual Earth, and the scientific community uses them to ask questions, for example how increasing greenhouse gases in the atmosphere affect average hurricane intensity.
To answer the question, researchers run a computer model that reproduces the hurricane intensity measured in observations, with increasing greenhouse gas levels, and a model where their amount is constant the control group.
They analyze the output of each model, examine average hurricane intensity in the test and control groups, and compare them.
The answer? As mentioned at the beginning: researchers found that without the rise in greenhouse gas emissions, strong hurricanes like those measured in 2024 would have been unlikely.
Using the same method, researchers showed that the likelihood of strong hurricanes like Helene and Milton increases with rising greenhouse gas emissions.
Is there a silver lining? When greenhouse gas emissions decrease, the rise in hurricane intensity will also stop. Hopefully, that happens sooner rather than later.