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What Scientists Know About Climate Change and Hurricanes: QuickTake Q&A

These days, as soon as winds hit 74 miles per hour or barometric pressure drops below 990 millibars, people want to know: Is climate change behind this hurricane? It’s an even more pressing question when a giant storm like Harvey is followed by an even more gigantic one like Irma, which itself is being followed by Jose and Katia. Climate scientists continue to wrestle with the connection between global warming and individual storms, but they’re more confident than ever that there’s some linkage.

The Coming Storm of Climate Change

These days, as soon as winds hit 74 miles per hour or barometric pressure drops below 990 millibars, people want to know: Is climate change behind this hurricane? It’s an even more pressing question when a giant storm like Harvey is followed by an even more gigantic one like Irma, which itself is being followed by Jose and Katia. Climate scientists continue to wrestle with the connection between global warming and individual storms, but they’re more confident than ever that there’s some linkage.

1. Is climate change to blame for Harvey and Irma?

Climate scientists are increasingly comfortable connecting global warming to the unprecedentedly high ocean temperatures that fuel some storms. Scientists in Germany and the U.K. drew a direct link between global warming and the intensity of Irma and the destructiveness of Harvey. Climate change can’t be blamed for the existence of these two juggernauts -- there have always been hurricanes, after all -- but it does shape the remarkable conditions they’re occurring in. The fuel for tropical storms is ocean heat, and each storm’s top winds have a theoretical speed limit, determined by how much of that fuel is in their tank.

2. How hot is the ocean?

Hotter than at any previous moment in recorded history, thanks to human-driven climate change. The global average sea-surface temperature for July was 1.24 degrees Fahrenheit above the 20th century average, making it the third-hottest July for oceans, behind 2016 and 2015. The waters where Irma was born were about 2 degrees Fahrenheit above normal or 1 degree Celsius.

3. What does the extra heat do?

In addition to raising water temperatures, it heats up the air, which makes storms wetter. Every uptick in temperature increases the air’s water-holding capacity exponentially. As a result, there’s now at least 4 percent more water vapor in the air than a century ago.

4. What does all that mean?

So far, the scientific consensus is that global warming will make more-intense storms more frequent, even if the total number of storms stays the same or drops.

5. How did scientists reach that consensus?

Climate scientists rely on powerful computer models that simulate the behavior of the atmosphere, oceans and land. Through testing, they make sure these model-Earths can reproduce weather events that have already occurred. Then the models can be used to project into the future. There is not as much historical data as scientists would like to support a more confident statement about hurricane frequency going forward.

6. What explains the recent run of quiet hurricane seasons?

The U.S. did indeed go without a strike by a major hurricane (Category 3 or above) between 2005 and 2017, the longest gap on record going back to 1851. But the Atlantic Ocean spun up some monster storms during that time. Last year, Hurricane Matthew killed more than 500 people across the Caribbean. In 2015, Hurricane Joaquin grew with explosive power, trapping the freighter El Faro and sinking it with the loss of all hands. In addition, 2010, 2011 and 2012 all posted 19 named storms, making them tied for most active year, along with 1995 and 1887. And Superstorm Sandy, which hit the U.S. in 2012, started as a hurricane near Jamaica. The U.S. also suffered billions in damages from smaller storms such as Ike in 2008 and Irene in 2011, which caused severe flooding as far north as Vermont.

7. What else are scientists trying to figure out?

How the extra heat is changing overall atmospheric dynamics, and what the impact of those changes might be. Hurricane Harvey, for example, is one of several recent weather disasters marked by a shocking staying power, punishing whole regions for days or weeks on end -- or longer. Researchers are trying to understand climate change’s connection to episodes in which the jet-stream -- the high-flying river of air that meanders around the higher-to-mid latitudes -- gets locked into one place for an extended period. They suspect that may have played a role in a massive heatwave in Russia and flooding in Pakistan in 2010, the Texas drought of 2011 and the multiyear California drought that began around the same time.

8. What do scientists disagree about?

For one thing, how powerful an influence the Arctic, which is warming twice as quickly as the rest of the planet, has on weather extremes in the sub-tropics. There’s also a debate over whether the Atlantic has a long-term cycle of warming and cooling that’s separate from warming induced by industrialization.

9. Are Irma and Harvey likely to change the climate debate?

The U.S. quickly went from world leader in global climate diplomacy to an outlier when President Donald Trump walked away from the 2015 Paris accords, which continue to be supported by virtually all other nations. There is interesting research into behavior and decision-making on climate and other issues. Extreme weather and palpable changes in long-term trends can be influential in helping people understand what’s going on — even if political affiliations restrict what they are comfortable saying out loud. On the other hand, the record hurricane year of 2005, when there were 28 named storms, including Katrina, was followed by deepening political polarization on the issue, in the U.S. if not elsewhere.

The Reference Shelf

  • A QuickTake explainer on hurricanes and typhoons.
  • A study of Superstorm Sandy predicts more extreme weather ahead.
  • The U.S. National Oceanic and Atmospheric Administration’s overview of research on global warming and hurricanes.
  • A Nature article on "gray swan” hurricanes.
  • World Weather Attribution is a research partnership between universities and research groups that evaluates extreme weather events for human influence.

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