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Imagine a force so powerful that it
can change weather patterns around
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the world and even alter the
fate of ancient civilisations.
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Earth’s short term weather patterns and long term
climate are influenced by a complex collection of
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factors, from our place in the solar system and
the planet’s rotation, to atmospheric patterns and
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seasonal changes. To further complicate things,
every few years our planet experiences El Niño and
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La Niña events – two opposite ends of a cycle
that are part of the El Niño Southern Oscillation,
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or ENSO for short. Evidence of the ENSO
goes back tens of thousands of years,
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and may have even played a role in destabilising
some of the world’s great ancient civilisations.
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One of the five strongest El Niño
events ever recorded has finally
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come to an end as of June 2024, after months
of record-high ocean surface temperatures,
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unprecedented heat stress on coral
reefs, drought in the Amazon rainforest,
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and extreme rainfall with dangerous
consequences for North America.
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With La Niña predicted to begin in late 2024 or
early 2025, what changes can we expect globally,
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and locally? I’m Alex McColgan, and
you’re watching Astrum. Join me as we
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take a look at our changing planet, the
ENSO, and what La Niña will do to Earth.
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Our previous video about El Niño
explored one side of a global cycle
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that typically takes between 2 to 7 years to
swing from one extreme to the other. These
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El Niño and La Niña episodes usually last 9 to
12 months, but can last for several years. In
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this video we will take a closer look at how
this cycle works, what neutral periods are,
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and what might happen as we head into the
opposite extreme of La Niña in the coming months.
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When our Earth experiences average conditions,
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we call those periods “ENSO-Neutral.” But
every few years, fluctuations in wind and
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ocean surface temperatures can signal the
beginning of an El Niño or La Niña event,
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and a departure from Earth’s normal conditions.
These events alter worldwide atmospheric patterns
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and are known to wreak havoc by contributing
to extreme weather and environmental harm.
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Imagine we're on the International Space
Station orbiting Earth. From here we can see
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our planet’s spherical shape, and as you might
expect, sunlight affects the Earth’s surface
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unevenly. More light and heat reaches Earth at
the equator where sunlight strikes most directly,
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compared to the poles where sunlight
reaches our planet at a low angle.
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In the same way that a hot air balloon rises,
or hot steam rises over a pot of boiling water,
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the same thing happens along the equator..
Direct sunlight warms up the air, and that hot,
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moist, low pressure air rises up into the
atmosphere. As the warm air gets higher,
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it begins to cool off and condense
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into clouds – this is why we see an abundance
of tropical rainforests close to the equator.
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More warm air continues to rise, pushing the
cooler air away from the equator and out towards
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the north and south, where it will eventually sink
back down to the surface. Then, that cool air will
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move from higher pressure, along the surface
of the Earth, back to lower pressure near the
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equator to start the cycle all over again and
complete what we call Hadley Cell rotation. But
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how does this worldwide circulation of air, driven
by the Sun, relate to El Niño and La Niña events?
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The surface winds created by these Hadley
Cells are deflected towards the equator
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due to the Earth’s rotation, a phenomenon we
call the Coriolis Effect. It’s this effect
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that creates the trade winds on either side
of the equator, and it’s changes to these
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trade winds that indicate when we will
experience El Niño and La Niña events.
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Historically, the trade winds have been
so reliable that sailors have used them
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to navigate the globe for centuries, hence the
name, “trade winds.” Chemical signatures of the
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ENSO stretch back tens of thousands of years
in paleoclimate indicators like coral fossils,
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and we have written records of the ENSO as far
back as the 1500’s.. El Niño events may have
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aided Spain in their conquest of the Incan Empire
in the 1500’s, and in the late 1700’s, likely
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contributed to crop failures and unrest that
sparked the French Revolution. Despite this long
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record of ENSO activity and the massive impact
it has on worldwide weather and environments,
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it wasn’t until the 20th century that we finally
started to understand the mechanisms behind it.
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The first defining breakthrough came in the 1920s,
when a British scientist named Sir Gilbert Thomas
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Walker set out to better understand the strength
of monsoons in India. In his search for a way to
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predict monsoon strength, he ended up
documenting the Southern Oscillation,
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a repeating shift in air pressure that
happens across the equatorial Pacific
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Ocean. This oscillation was part of
another large-scale air circulation
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that had not been documented before, and
was later named the Walker Circulation.
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Remember how I said that Hadley Cells circulate
air north and south? The Walker Circulation is
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just like Hadley Cells, except instead of moving
air north and south, the Walker Circulation moves
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air to the east and west over the equatorial
Pacific. And instead of being driven by sunlight,
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the Walker Circulation is guided by the
easterly trade winds and ocean temperature.
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It would be 60 more years before scientists
were able to connect these changes in air
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pressure over the Pacific with the alternating
pattern of warm and cool surface water in the
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Pacific. Combined, these make up what
we now know as the El Niño Southern
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Oscillation – ENSO. El Niño refers to
the changes in sea surface temperature,
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and the Southern Oscillation refers to
the simultaneous changes in air pressure.
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Unlike Hadley Cells that reliably move air north
and south, the equatorial Walker Circulation is
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not consistent and can experience colossal
shifts as part of the Southern Oscillation.
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Every few years, the surface temperature and trade
winds over the Pacific experience fluctuations,
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signalling an oncoming shift in
the Walker Circulation. In turn,
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these shifts, which we refer to as El Niño
or La Niña events, can upset the balance of
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weather and ecosystems over the entire Earth. So
what happens to the Earth during each of these?
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During neutral ENSO periods, the sea surface
temperature and trade winds are near average
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conditions. Trade winds blow across the Pacific
Ocean, guiding warm surface waters to travel
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west from South America towards Australia and
Asia. As that warm surface water moves west,
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it makes way for deep, cooler waters
to rise up in its place. This ocean
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circulation brings nutrient-rich cool water
to the surface in a process called upwelling,
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where it feeds phytoplankton and in turn
supports other parts of the ecosystem like fish.
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In neutral periods, weather across
the world occurs, more or less,
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as expected. This can include normal
hurricane development in the Atlantic,
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and average monsoon rainfall across
southeast Asia. Walker Circulation
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drives columns of warm moist air to rise
above southern Asia,northern South America,
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and middle Africa, so it’s no coincidence
that these three regions are where we see
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a concentration of vast, lush rainforests. The
influence of this equatorial airflow is vast,
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so it’s easy to imagine how changes to this system
could cause a ripple effect around the world.
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The first signs of trouble are when
the trade winds begin to weaken and
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sea surface temperature rises in the Pacific,
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which can indicate an oncoming El Niño event
like the one we experienced in 2023 and 2024.
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During El Niño, the colossal columns of warm
air that rise above our world’s rainforests are
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shifted to the east or west.This change disrupts
Asia’s monsoon season with prolonged droughts and
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water scarcity, and affects the livelihoods
of billions of people in east Asia. The last
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El Niño also brought nine atmospheric rivers
to the western United States that led to major
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transportation issues, dangerous landslides, and
flooding. You can think of an atmospheric river
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like a river of moisture streaming through the
air. When these atmospheric rivers reach land,
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they release all of that moisture,
causing monumental precipitation.
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Everywhere on Earth, this shift in Walker
Circulation is felt during El Niño. However,
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the changes you experience in your local weather
conditions may be completely different from the
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changes another person sees in their local weather
elsewhere on our planet. El Niño typically brings
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a reversal of the normal conditions for a
given area. This is why places like east
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Asia or the Amazon rainforest, which typically
get plenty of rain, will experience drought
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during an El Niño event, or why usually
dry climates like western North America,
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will experience tremendous rainfall events.
The recent El Niño event was also responsible
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for worldwide shipping delays in 2023, as there
wasn’t enough water to feed the Panama Canal,
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which relies on consistent rainfall to accommodate
all of the cargo ships hoping to pass through.
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El Niño is described as the warm part of the ENSO
cycle because Pacific sea surface temperatures
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are higher than average during this time. In
addition to changing worldwide weather patterns,
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this also negatively affects
ecosystems.Take, for example,
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coral reefs. They rely on particular
sea surface temperatures to survive,
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and support some of the most important
and biologically diverse life on Earth.
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Corals have a symbiotic relationship with algae,
but an increase in water temperatures can cause
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the coral to expel this algae, leaving it drained
of colour and vulnerable. A reef can recover from
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this bleaching if conditions improve in
time, but their risk of dying is high,
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and the last El Niño event was no exception. An
unprecedented 99.7 percent of Atlantic tropical
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reefs were impacted by bleaching-level
heat stress during the 2023 to 2024 El
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Niño event, as part of the fourth worldwide
mass-bleaching event in recorded history.
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The warmer Pacific waters and weakened trade winds
from El Niño also cause the upwelling of cooler,
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nutrient-rich water to temporarily slow or
stop, leading to a dire situation where less
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phytoplankton means large numbers of fish
must migrate or perish. As you can imagine,
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this ripples across the food chain and can
impact other animals. For coastal families and
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communities who rely on those fish for nourishment
or income, this El Niño effect can be devastating.
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Now that we’ve discussed what it’s
like during a neutral ENSO period,
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and the destructive changes that can happen with
El Niño conditions like we saw in 2023 and 2024,
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what can we expect from this
upcoming La Niña phenomenon?
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La Niña is the other extreme. This period is
marked by stronger than usual trade winds,
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and cooler than average Pacific
sea surface temperatures.
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While El Niño usually causes the
reversal of neutral conditions,
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the best way to understand La Niña is to think
of it as a more intense version of neutral
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conditions for most parts of the world, with
a few exceptions. During La Niña, the neutral
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columns of rising warm air above south Asia and
eastern North America become more pronounced,
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while the typical column of warm air above
Africa reverses. Just as your experience of
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El Niño is highly dependent on where you are
located, the same is also true of La Niña.
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As of August 2024, the U.S. National
Oceanic and Atmospheric Administration
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predicts a 66 percent chance that La Niña will
develop between September to November of 2024,
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and a 74 percent chance it will last well into
the Northern Hemisphere’s winter of 2025 to 2026.
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And as of this video, models are predicting
a roughly 50 percent chance that this La Niña
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event will peak at a moderate strength. However,
while forecasts for a La Niña event happening are
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usually correct, the predicted strength for these
events will likely change from month to month.
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A strong El Niño ending in 2024 does not
necessarily mean the upcoming La Niña will
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be as extreme. Sometimes a strong El Niño leads
into a strong La Niña, but other times a strong
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El Niño is followed by a weak La Niña. With only
10 times in the historical record where the ENSO
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has changed between El Niño and La Niña within
a one year time period, as is expected with this
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year’s switch, there just isn’t enough historical
data to draw many conclusions. Besides, scientists
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warn that the strength of an ENSO event does not
always line up with the severity of its impacts.
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So what do we know about the upcoming La Niña?
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For the northern part of North America, La Niña
brings with it a colder, wetter winter, while the
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southern part of the continent might experience
a warmer and more dry winter. U.S. Winter Source
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And notably, La Niña will increase the likelihood
of a more active hurricane season in the Atlantic,
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with the potential for more,
and stronger, hurricanes.
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For east Asia and Australia, this typically
means a significant increase in rainfall.
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While in Africa, La Niña can mean
some areas to the west are more wet,
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while eastern Africa tends
to experience more drought.
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The connection between ENSO and
Europe isn’t quite as clear,
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since the continent is farther from
the source, but La Niña is expected
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to bring lower than average temperatures
to central and western Europe, with less
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precipitation across the mainland this winter,
and more precipitation to the north and south.
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There’s one final thing we need to talk
about when it comes to La Niña predictions:
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The El Niño and La Niña extremes of the
ENSO have been happening for millennia,
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but what’s less certain is how global warming
from climate change will impact this cycle.
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While we see short term, localised temperature
swings from ENSO, the all-over trend of global
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warming continues on an upward trajectory.
This means we are entering uncharted territory.
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There’s clear evidence that as our planet
continues to warm from climate change,
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the occurrence of severe weather will escalate.
But the ENSO is a complicated, worldwide,
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and in many ways, still an unpredictable
phenomenon. Just in recent history,
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El Niño and La Niña events have become stronger
and more frequent, leading to more droughts,
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floods, heat waves, wildfires, and severe
storms, like we saw during the last La Niña
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event that lasted for three years, from 2020
to 2023. Exactly how global warming may impact
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the ENSO cycle is unclear, but we do know that
climate change is likely to amplify that, too.
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Luckily, life on our planet is nothing
if not resilient and adaptable, and as
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our world continues to change and experience the
millennia-old ENSO swings, scientists will learn
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more each year and be able to make improved
predictions about the complex climate system.
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The approaching La Niña will undoubtedly teach us
more about our planet's climate. Let's hope we are
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paying attention and use these lessons to adapt
and prepare for our future in sustainable ways.
00:16:29
I’d love to hear in the comments what
question you have about our planet’s climate.
00:16:36
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