Narrator: Water here at the largest reservoir in the US
has dropped more than 140 feet in the last two decades.
That's about the height of the Statue of Liberty
without its pedestal.
Now, water levels at Nevada's Lake Mead
are at their lowest ever.
But what's causing this drought?
Where else is it happening?
And what's the long-term damage?
We decoded the climate emergency
that's devastating parts of the country.
Lake Mead was formed in the 1930s by the Hoover Dam.
Now in 2021,
scientists say the reservoir may never fill up again.
Whole sections of the lake,
like here, north of the Overton Arm, are completely dry.
And for the first time ever,
the federal government has declared
an official water shortage here.
Arizona's Lake Powell is also at record lows.
Water levels have fallen almost 152 feet
below full capacity.
Both reservoirs are on the Colorado River
and feed water to large parts of the West.
More than 40 million people depend on them.
And starting next year,
they're going to experience water cuts.
Nevada will receive about 7% less water,
and Arizona's supply will be cut by about 18%,
mostly affecting farmers.
Other reservoirs in the region are also drying up.
The two largest in California are at 24% and 22% capacity.
And the cracked earth around them can be seen from space.
This is Lake Shasta, the state's largest reservoir, in 2019.
And here it is in 2021.
This brown, barren area is expanding,
partly because water is evaporating.
And the same is happening 100 miles away
in Lake Oroville.
Water levels are so low
that more than 130 houseboats,
seen here at the Bidwell Canyon Marina,
had to be pulled out of the water.
And the reservoir's hydropower plant shut down this summer
for the first time in its history.
More than half the US is experiencing some level
of abnormally dry conditions or drought,
but it's not just here.
All around the world,
severe droughts are happening 1.7 times more often
than in 1850 to 1900.
So why is that happening?
It has to do more with temperature than rain.
Precipitation was actually above average
for many parts of the country this summer,
with places like Tucson, Arizona,
seeing their wettest month on record in July.
But even record rain can't stop rising temperatures
from evaporating more water.
The UN says this extreme heat
is mostly because of human-caused climate change.
Its latest report states that 2.4 trillion tons of CO2
have been added to the atmosphere since the mid-1800s.
And because of that,
the average global temperature
has gone up by almost 2 degrees Fahrenheit,
or 1.1 degrees Celsius.
And if it goes up by 1.5 degrees Celsius,
almost 1 billion people could endure severe heat waves
every five years.
As temperatures rise,
it creates something called a heat dome.
That's when sinking air from the Earth's atmosphere
traps warm air that's rising from the ground,
making hot air even hotter.
This June was the hottest the country has ever seen,
with temperatures reaching 115 degrees or higher in Phoenix
for six straight days,
116 degrees in Portland, breaking a record,
and 130 degrees in California's Death Valley.
This severe heat combined with the ongoing drought
is perfect fuel for wildfires.
As vegetation dries up, one spark is all it takes.
This is a map of all the wildfires burning in the US
as of late September.
They span 12 states.
Wildfire seasons are stretching
as intense summer heat comes sooner and stays later.
In the 1970s,
the fire season was typically under five months,
or around 150 days.
Now it can be more than eight months long.
Fires are also getting both bigger
and more destructive over time.
Of California's 10 largest wildfires,
nine have happened in the last decade.
Satellite images show this year's Bootleg Fire
burying Oregon in smoke and polluting the air.
That smoke traveled to 2,500 miles
and even reached New York city.
As winds carry smoke across states,
it can turn bodies of water more acidic,
destroy nutrients and soil, and damage ecosystems.
The US Air Quality Index measures air pollution
on a scale from zero to 500.
Generally, a value of 100 or below is fine,
but anything above that is unhealthy.
This year, Reno's AQI hit a high of 291
because of smoke from California's Caldor fire.
And the San Martin Airport in the Bay Area reached 215.
And even some of the most polluted cities in the world,
like Delhi and Beijing,
have been measuring lower AQIs than that.
This year's extreme weather
has already cost hundreds of lives in the US.
One study predicts that if nothing is done
to address the warming climate,
around 100,000 Americans could die each year
by the end of the century.
Cities on the west coast
are going to have to learn how to adapt in the long term.
But so far, nature's changes have outpaced society's.