Climate change could see big storms become even bigger

While climate change did not cause Hurricane Harvey, it could explain the intensity of the cyclone as well as other catastrophic storms that have hit the United States in recent years, experts say.

Harvey is the latest in little more than a decade of ‘‘500-year’’ and ‘‘100-year’’ floods that once were considered rare.

Climate change won’t mean more storms overall, but it probably will mean that the biggest storms become even bigger, scientists say.

For example, rising ocean temperatures could be making storms like Harvey bigger than they would be otherwise, said Michael Mann, a climate scientist at Pennsylvania State University. That’s because as the ocean’s surface temperature rises, the atmosphere’s ability to hold moisture goes up.

According to the ClausiusClapeyron equation, every 0.5 degrees C of warming means an extra 3 per cent or so of moisture in the air. Harvey grew over an ocean that was 1 to 1.5C warmer than just a few decades ago, which means there is around 5 per cent more moisture in the atmosphere.

More moisture means a higher chance of heavier rainfall and greater flooding - both of which, in Harvey’s case, have been wreaking major havoc in the greater Houston area.

‘‘There is a good chance it would have happened anyway,’’ Mann said of Harvey. However, he added: ’’The impacts were likely greatly amplified by climate change.’’

In this altered environment, with heavier rains, storms behave differently, according to Kevin Trenberth, a climate scientist with the National Centre for Atmospheric Research. ‘‘The storm itself grows a little bit more intense, it gets a little bit bigger and it helps it to last longer.’’

Hurricanes usually start to wane soon after they make landfall, because they are cut off from the supply of moisture from the ocean. Not so with Harvey, whose size allowed it to stay plugged into its power source in the Gulf of Mexico.

‘‘It still had spiral arm bands that were reaching out into the gulf and bringing a lot of moisture into the storm,’’ Trenberth said. This ’’enabled it to keep going where most storms would have petered out’’.

Harvey flooded Houston partly because it stalled over southeastern Texas for several days, rather than moving north.

Examining the dynamics shaping Hurricane Harvey could help researchers get a better handle on climate change, the scientists said.

Another idea that may require a revisit? Calculating the risk of such storms in the future.

Harvey already has been called a 500-year flood, and by the time the storm has dissipated it may well reach 1000-year status. But the last decade or two have brought a slew of these supposedly rare storms, including two 500-year floods in the Houston area in 2015 and 2016.

So what exactly does a 500-year storm mean? ‘‘This concept actually comes from hydrology, and it relates a lot more to precipitation than it does to hurricanes or to the storms themselves,’’ Trenberth said.

It is a concept typically used in assessing risk for flood insurance. It does not mean that a given storm can happen only once per century or twice per millennium. It means that for any given year, in stable climate conditions, a storm of this magnitude has a one-in-100 (or 500, or 1000) chance of taking place.

‘‘With climate change ... what used to be a 500-year event is becoming a 70-year event or a 50-year event,’’ Trenberth said. ‘‘They’re no longer anything like as rare.’’

As human populations continue to grow in coastal areas, some cities may not be built to handle such extreme ‘‘500-year’’ events, which may happen more often in the future, experts say.

- LA Times