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marți, 19 martie 2013

Geophysicist: A Katrina hurricane will strike every two years

Increases in Earth’s average temperature will result in far more hurricanes in the future, new study reveals.

Hurricane Katrina was one of the deadliest hurricanes in US history. The hurricane and the subsequent flooding killed 1,836 people.

The UN climate panel estimates that Earth’s average temperature may rise by 3.4 degrees Celsius over the next hundred years. This, scientists say, will increase the frequency of extreme weather events.

”If this trend continues, it is realistic to expect a ten-fold increase in hurricanes like Katrina. That amounts to once every two years,” says geophysicist Aslak Grinsted, of Copenhagen University’s Niels Bohr Institute.

Grinsted’s new study has just been published in the journal PNAS.

Using two data sets, the study confirms one of two conflicting hypotheses about our future climate.

Researchers discuss hurricane hypotheses

The two hypotheses seek to explain whether we can expect more hurricanes along the Gulf Coast and the US East Coast as Earth’s average temperatures are rising.

Hypotheses about the cause of hurricanes:

  •     The number of hurricanes varies with sea temperatures in the Atlantic. The warmer the sea, the greater the risk that great amounts of steam from the sea trigger an extreme weather event like a hurricane. In other words, this hypothesis states that rising temperatures will lead to more hurricanes.
  •     The other hypothesis states that it’s the difference in temperature between the Atlantic and the tropical oceans that determines whether or not a hurricane occurs. Since the temperature rises in the Atlantic and the tropical oceans are almost equal, this hypothesis says that there will not be an increase in hurricanes as a result of rising temperatures.

Atlantic Ocean temperature essential for hurricanes
Grinsted has examined data from hurricanes in US from 1923 up to the present day.

Unlike previous studies, he elected to use data from past water level observations rather than wind force measurements, as more solid data was available about the water levels.

The water level observations show the extent of flooding caused by past hurricanes, and also what impact these storms have had.

Grinsted compared these water level observations with past temperature measurements in the Atlantic and the tropical Pacific.

 Facts

With global temperatures being as they are today, we can expect to see a hurricane like Katrina every 20 years.

Aslak Grinsted’s new study shows that a one-degree (Celsius) rise in Earth’s average temperate could mean that the US East Coast will be hit by hurricanes three times as often as today.

His findings, adjusted for uncertainties, suggest that the number of hurricanes will increase by between two and seven for every one-degree rise in Earth’s average temperature.

On this basis, he believes it is realistic to say that the number of hurricanes will triple every time Earth’s average temperature rises by one degree Celsius.

Source: Aslak Grinsted


Combined model for predicting future hurricanes

He concludes that hurricanes of the past did not occur as a result of temperature differences between the two oceans.

His study supports the other hypothesis – that high sea temperatures in the Atlantic are sufficient to trigger more hurricanes.

“In addition, I have created a model in which I look at temperature measurements across the entire Earth so that I can circumvent both hypotheses and try to assemble a combined model. And it’s this model that I’m using to predict what happens as the temperatures are rising.”

Tiny changes can cause extreme weather
The researcher argues that small changes in what’s known as the background climate can bring about the extreme weather events.

The background climate is the climate that surrounds the local area – e.g. the Atlantic Ocean – where the hurricanes occur.

These changes in the background climate do not always result in a hurricane or flooding; however, Grinsted believes it is important to learn about the triggering potential of these factors, as this would help us make the right decisions in relation to infrastructure planning e.g. along the US East Coast.

Read the Danish version of this article at videnskab.dk

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