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Cyclones and climate change: connecting the dots
Topic Started: 28 Aug 2017, 12:11 AM (98 Views)
skibboy
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27 August 2017

Cyclones and climate change: connecting the dots

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© AFP / by Marlowe HOOD | Hurricane Harvey has left a trail of destruction in its wake and scientists say global warming means that the worst ones are becoming ever stronger

PARIS (AFP) - Scientists freely acknowledge they don't know everything about how global warming affects hurricanes like the one pummelling southeast Texas.

But what they do know is enough to keep them up at night.

The amplifying impact of sea level rise, warming oceans, and hotter air -- all incontrovertible consequences of climate change -- is basic physics, they say.

Likewise accelerated shifts in intensity, such as the sudden strengthening that turned Harvey from a Category 2 to a Category 4 hurricane -- on a scale of 5 -- just as it made landfall Friday.

What's missing is a detailed track record of hurricanes past, the kind of decades-long log of measurements that climate scientists need to discern the fingerprint of human influence.

Starting in the 1970s, satellite data allowed for a better tally, but even that wasn't enough.

"It is awfully difficult to see climate change in historical data so far because hurricanes are fairly rare," Kerry Emmanuel, a professor of atmospheric science at MIT in Boston, told AFP.

Experts, in other words, do not disagree on the potential of manmade global warming to magnify the destructive power of the tropical storms known variously around the world as cyclones, hurricanes and typhoons.

Rather, they are confounded -- for now -- by a lack of information.

"Just because the data don't allow for unambiguous detection yet, doesn't mean that the changes haven't been occurring," noted James Kossin, a scientist at the US National Oceanic and Atmospheric Administration's Center for Weather and Climate in Madison, Wisconsin.

Kossin figured out that cyclones have drifted poleward in their respective hemispheres over the last three decades, a finding hailed by other hurricane gurus as the most unambiguous evidence so far that global warming has already had a direct impact.

- Like a tsunami -

When it comes to cyclones and climate change, there are many points of near "universal agreement," said Emanuel.

One is the consequence of rising seas.

"The most lethal aspect of hurricanes -- wherever they occur in the world -- is storm surge," he said in an interview.

"It is physically the same phenomenon as a tsunami, except that it is excited by wind rather than a sea floor shaken by an earthquake."

If Hurricane Sandy -- which caused $50 billion in damage -- had happened a century earlier, it probably would not have flooded lower Manhattan because sea level was about 30 centimetres (a foot) lower, he pointed out.

Global warming is likely to add roughly a metre (three feet) to the global watermark by century's end, according to recently revised estimates.

"The surge from these storms will be more devastating -- higher and more penetrating," said James Elsner, an atmospheric scientists and hurricane expert at Florida State University.

A second point of consensus is that hurricanes will hold more water, raising the threat of lethal and destructive flooding.

"We calculate that one degree Celsius (1.8 degrees Fahrenheit) of warming translates into a seven percent increase in humidity in the atmosphere," said French scientist Valerie Masson-Delmotte, co-chair of the UN?s Intergovernmental Panel on Climate Change (IPCC).

The US National Hurricane Center predicts that Harvey could dump more than 40 inches (100 centimetres) by the time skies clear.

Hurricane Mitch -- the second deadliest Atlantic hurricane on record -- left some 19,000 dead in Central America, "all from fresh-water flooding," noted Emanuel.

"The irony is that hurricanes are known for wind, yet wind is third on the list of lethal aspects," after storm surges and flooding caused by rain.

- 'Fewer but stronger' -

Earlier this year, Emanuel published a study pointing to yet another worrying climate "signal" emerging from the noise of raw data.

Scientists have made great progress in anticipating the path a storm will follow, extending their predictive powers from a day or two to about a week.

At the same time they have made scant headway in forecasting hurricane strength.

"The thing that keeps forecasters up at night is the prospect that a storm will rapidly gain strength just before it hits land," Emanuel said, citing Harvey as an example.

In 2015, Hurricane Patricia in the Pacific Ocean intensified more rapidly -- "It just went 'Boom!'" -- than any storm on record.

"Global warming can accentuate that sudden acceleration in intensity," Emanuel said.

A finding oft cited as evidence that the jury is still out on whether climate change will boost cyclones is that scientists don't know if there will be more or fewer such storms in the future.

But even if there are fewer, which seems likely, that misses the point, the experts interviewed agreed.

Since 1971, tropical cyclones have claimed about 470,000 lives and caused some $700 billion in damages globally, according to the Centre for Research on the Epidemiology of Disasters.

But most of that death and destruction is attributable to a relative handful of storms.

Just three, for example, have caused well over half of all storm-related deaths in the US since 1900.

So even if the number of mostly smaller storms diminishes, that's not what counts.

"The idea of 'fewer but stronger' seems to be the fingerprint of climate change on tropical cyclones," Elsner concluded.

by Marlowe HOOD

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skibboy
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09 September 2017

Series of potent hurricanes stokes scientific debate

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© DUTCH DEFENSE MINISTRY/AFP / by Jean-Louis SANTINI | The devastation caused by Hurricane Irma on the Dutch Caribbean island of St Maarten

WASHINGTON (AFP) - The rapid-fire formation of four unusually potent Atlantic hurricanes, including Harvey and Irma, has stoked scientific debate over what role global warming is playing in this phenomenon.

First came Harvey, which unleashed massive floods in Texas, then three devastating hurricanes roared across the Atlantic simultaneously -- Irma, Katia and Jose.

"Currently we have three Atlantic hurricanes with 90-plus mile per hour winds -- only the fourth time on record in Atlantic this has occurred," Philip Klotzbach, a research scientist at Colorado State University, said on Twitter.

The last time three hurricanes were active at once was 2010, when hurricanes Igor, Julia and Karl were classified as hurricanes, according to the National Oceanic and Atmospheric Administration (NOAA).

Katia weakend to a tropical depression on Saturday.

Hurricane Irma, now taking aim at Florida, has stunned experts with its sheer size and strength, churning across the ocean with sustained Category 5 winds of 183 miles per hour (295 kilometers per hour) for more than 33 hours, making it the longest-lasting, top-intensity cyclone ever recorded.

Meanwhile Jose, a Category 4 on the Saffir Simpson scale of 1 to 5, is fast on the heels of Irma, pummeling the Caribbean for the second time in the span of a few days.

Many have wondered what is contributing to the power and frequency of these extreme storms.

"Atlantic hurricane seasons over the years have been shaped by many complex factors," said Jim Kossin, a NOAA hurricane scientist at the University of Wisconsin.

"Those include large scale ocean currents, air pollution -- which tends to cool the ocean down -- and climate change."

- Active cycle since 1995 -

For Gabriel Vecchi, professor of geosciences at Princeton University's Environmental Institute, the surge in cyclones is evidence of an "active era" for storms in the Atlantic since the mid 1990s, even if not every year saw strong storms.

A period of relative calm for hurricanes, stretching from 2013 to 2016, can be explained by the presence of the equatorial Pacific warming trend, El Nino, which produces wind shear that tends to discourage the formation of hurricanes.

There was also little hurricane activity in the 1960s, '70s and '80s.

"There is still a lot of debate in the scientific community," over what causes this shift between calm and tumultuous times for storms, Vecchi said.

Some think a surge in industrial pollution after World War II may have produced more pollutant particles that blocked the Sun's energy and exerted a cooling effect on the oceans.

"The pollution reduced a lot of hurricane activity," he told AFP.

Pollution began to wane in the 1980s due to regulations such as the Clean Air Act, allowing more of the Sun's rays to penetrate the ocean and provide warming fuel for storms.

Vecchi said the "big debate" among scientists is over which plays a larger role -- variations in ocean currents or pollution cuts.

There is evidence for both, but there isn't enough data to answer a key question.

"We don't know how long the cycle may last," Vecchi said.

"We have a lack of historical perspective."

- Warming's role -

The burning of fossil fuels, which spew greenhouse gases into the atmosphere and warm the Earth, can also be linked to a rise in extreme storms in recent years.

Warmer ocean temperatures yield more moisture, more rainfall, and greater intensity storms.

"It is not a coincidence that we're seeing more devastating hurricanes," climatologist Michael Mann of Penn State University told AFP in an email.

"Over the past few years, as global sea surface temperatures have been the warmest on record, we've seen the strongest hurricanes -- as measured by peak sustained winds -- globally, in both Southern and Northern Hemisphere, in both Pacific and now, with Irma, the open Atlantic," he added.

"The impacts of climate change are no longer subtle. We're seeing them play out in real time, and the past two weeks have been a sadly vivid example."

by Jean-Louis SANTINI

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