The top map shows annual average lightning
density at a resolution of about 10 kilometers (6 miles), as recorded by
the WWLLN, from 2005 to 2016. The bottom map shows aerosol emissions
from ships crossing routes in the Indian Ocean and South China sea from
2010.
Credit: Thornton et al/Geophysical Research Letters/AGU
Date:
September 7, 2017
Source:
American Geophysical Union
Summary:
Thunderstorms directly above
two of the world's busiest shipping lanes are significantly more powerful than
storms in areas of the ocean where ships don't travel, according to new
research.
[IMAGE]
Thunderstorms directly above
two of the world's busiest shipping lanes are significantly more powerful than
storms in areas of the ocean where ships don't travel, according to new
research.
A new study mapping lightning around
the globe finds lightning strokes occur nearly twice as often directly above
heavily-trafficked shipping lanes in the Indian Ocean and the South China Sea
than they do in areas of the ocean adjacent to shipping lanes that have similar
climates.
The difference in lightning
activity can't be explained by changes in the weather, according to the study's
authors, who conclude that aerosol particles emitted in ship exhaust are
changing how storm clouds form over the ocean.
The new study is the first to
show ship exhaust can alter thunderstorm intensity. The researchers conclude
that particles from ship exhaust make cloud droplets smaller, lifting them
higher in the atmosphere. This creates more ice particles and leads to more
lightning.
The results provide some of
the first evidence that humans are changing cloud formation on a nearly
continual basis, rather than after a specific incident like a wildfire,
according to the authors. Cloud formation can affect rainfall patterns and
alter climate by changing how much sunlight clouds reflect to space.
"It's one of the clearest
examples of how humans are actually changing the intensity of storm processes
on Earth through the emission of particulates from combustion," said Joel
Thornton, an atmospheric scientist at the University of Washington in Seattle
and lead author of the new study in Geophysical Research Letters, a journal of
the American Geophysical Union.
"It is the first time we
have, literally, a smoking gun, showing over pristine ocean areas that the lightning
amount is more than doubling," said Daniel Rosenfeld, an atmospheric
scientist at the Hebrew University of Jerusalem who was not connected to the
study. "The study shows, highly unambiguously, the relationship between
anthropogenic emissions -- in this case, from diesel engines -- on deep
convective clouds."
Mapping lightning and exhaust
All combustion engines emit
exhaust, which contains microscopic particles of soot and compounds of nitrogen
and sulfur. These particles, known as aerosols, form the smog and haze typical
of large cities. They also act as cloud condensation nuclei -- the seeds on
which clouds form. Water vapor condenses around aerosols in the atmosphere,
creating droplets that make up clouds.
Cargo ships crossing oceans
emit exhaust continuously and scientists can use ship exhaust to better
understand how aerosols affect cloud formation.
In the new study, co-author
Katrina Virts, an atmospheric scientist at NASA Marshall Space Flight Center in
Huntsville, Alabama, was analyzing data from the World Wide Lightning Location
Network, a network of sensors that locates lightning strokes all over the
globe, when she noticed a nearly straight line of lightning strokes across the
Indian Ocean.
Virts and her colleagues
compared the lightning location data to maps of ships' exhaust plumes from a
global database of ship emissions. Looking at the locations of 1.5 billion
lightning strokes from 2005 to 2016, the team found nearly twice as many
lightning strokes on average over major routes ships take across the northern
Indian Ocean, through the Strait of Malacca and into the South China Sea,
compared to adjacent areas of the ocean that have similar climates.
More than $5 trillion of world
trade passes through the South China Sea every year and nearly 100,000 ships
pass through the Strait of Malacca alone. Lightning is a measure of storm
intensity, and the researchers detected the uptick in lightning at least as far
back as 2005.
"All we had to do was
make a map of where the lightning was enhanced and a map of where the ships are
travelling and it was pretty obvious just from the co-location of both of those
that the ships were somehow involved in enhancing lightning," Thornton
said.
Forming cloud seeds
Water molecules need aerosols
to condense into clouds. Where the atmosphere has few aerosol particles -- over
the ocean, for instance -- water molecules have fewer particles to condense
around, so cloud droplets are large.
When more aerosols are added
to the air, like from ship exhaust, water molecules have more particles to
collect around. More cloud droplets form, but they are smaller. Being lighter,
these smaller droplets travel higher into the atmosphere and more of them reach
the freezing line, creating more ice, which creates more lightning. Storm clouds
become electrified when ice particles collide with each other and with unfrozen
droplets in the cloud. Lightning is the atmosphere's way of neutralizing that
built-up electric charge.
Ships burn dirtier fuels in
the open ocean away from port, spewing more aerosols and creating even more
lightning, Thornton said.
"I think it's a really
exciting study because it's the most solid evidence I've seen that aerosol
emissions can affect deep convective clouds and intensify them and increase
their electrification," said Steven Sherwood, an atmospheric scientist at
the University of New South Wales in Sydney who was not connected to the study.
"We're emitting a lot of
stuff into the atmosphere, including a lot of air pollution, particulate
matter, and we don't know what it's doing to clouds," Sherwood said.
"That's been a huge uncertainty for a long time. This study doesn't
resolve that, but it gives us a foot in the door to be able to test our
understanding in a way that will move us a step closer to resolving some of
those bigger questions about what some of the general impacts are of our
emissions on clouds."
Story Source:
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