[Music]
sea-level might sound like a pretty
simple concept it's the height of the
sea but coastal water levels are always
shifting and even here in Missoula
Montana I can confidently say that I'm
nine hundred and seventy six meters
above sea level which what does that
even mean there's no ocean for hundreds
of kilometres defining and measuring sea
level is a surprisingly hard problem and
the reasons why are almost ridiculous
I'm talking problems with the moon the
lumpiness of the earth even basic laws
of gravitation get in the way but the
impressive thing is that in spite of all
that we can measure it at first glance
at measuring sea level doesn't seem like
too complicated a problem yeah there's
waves and stuff and weather and the
tides go up and down but you can average
all that stuff out right after all
storms pass and tides come in and go out
every few hours so you might think a few
weeks data should do the trick but the
cycles that pull and push the water
around the planet are way more
complicated than that for instance you
can watch tides change over a whole 29
day lunar orbit and factor out their
effect to get a number that's sort of
accurate that still doesn't account for
the Sun which creates tides about half
the size of the moon's to make matters
worse solar and lunar tides don't line
up
sometimes they amplify each other
sometimes they partly cancel each other
out in fact the Sun Moon and Earth only
line up in the same way about every 19
years so you have to wait at least that
long to get a reliable average and even
then one cycle isn't enough because of
the Sun Also drives
Earth's weather and weather alone can
change sea levels way more than some of
these tidal cycles so it can take
multiple decades to get a really solid
average and this is the simple stuff the
stuff that eventually averages out a lot
of things that affect sea level aren't
so neat and cyclical for instance the
shape and location of coastlines
constantly change making sea levels sort
of a moving target and in lots of places
land is subtly moving up and down yeah
sea level is hard to measure partly
because the land were standing on keeps
moving tens of thousands of years ago
parts of Europe and North America were
squashed under kilometers of ice and
even though that ice is long gone the
earth is still bouncing back land that
was crushed by glaciers is relaxing and
in places like Scandinavia and northern
Canada it's rising by about a centimeter
a year in some places like the
east coast of the United States land
buckled upward near all that ice so
today it's sinking down at a centimeter
or so a year
and that might not sound like much but
centimeters here and there add up and
each natural change makes it harder to
separate out the human driven changes in
sea level as the planet warms it also
means we can't just park instruments on
land because remember the land isn't
sitting still so to get around that
little detail scientists use satellites
with radar to measure sea level from
space radar works by timing how long
light takes to bounce off a target and
the radar on these satellites is so
sensitive that it could see a dime lying
flat on the ground from a dozen
kilometers in the air these satellites
give us maps of global sea levels every
couple weeks letting us watch how sea
levels change which sounds great throw a
satellite around the earth in a fixed
orbit and measure how long the light
takes to bounce back to the satellite
but there's another hitch Earth's
gravity isn't perfectly even all the way
around the globe different regions have
different rocks and different amounts of
water making some parts of the planet
denser than others these more or less
dense regions are known as gravitational
anomalies and they affect how water
spreads out across the surface areas
with stronger gravity pull more water
toward them so water bunches up in some
places meanwhile where there's less
gravity it fins out and I'm not just
nitpicking here some spots in the open
ocean have water that's almost a hundred
meters higher or lower than you'd expect
if gravity pulled evenly across the
globe so even if you could stick a
number on global sea level it wouldn't
actually describe the whole globe
satellites have revealed that there are
sea levels plural but that's still
useful after all one of the biggest
reasons we're looking at sea level
lately is to understand climate change
and that will affect sea level
differently in different places anyway
except the thing is as the climate warms
even that pattern of ups and downs is
changing in some surprising ways that's
because two things happen as ice melts
first as glaciers melt into the ocean
the water level immediately around them
drops the reason is glaciers are so
massive that they actually have a
significant gravitational pull it's like
having a bunch of mini moons on the
surface of the earth they pull water
toward them so when that mass melts away
the surrounding water recedes for
instance sea level is rising three
millimeters on average but if all of
Greenland's ice melted sea level would
actually drop by a couple
dozen meters in Iceland but on the flip
side it would rise more than the average
in the southern hemisphere second all
this melting redistributes mass on the
planet's surface and that changes how
the earth itself spins in space that's
right by heating our planet we are
melting glaciers which shifts the
Earth's mass which changes how our
planet moves in space and that's not the
end of the story Earth's rotation then
affects how water and air spread out
across its surface when its rotation
changes even a little it alters how all
that mass moves and as that mastery
distributes itself it changes the
Earth's rotation again this changing
spin can also lead to a drop in sea
level in some places even as the average
level rises so if you haven't noticed
yet this whole thing makes sea level a
really complicated thing to untangle or
even define for that matter sea level
means what we need it to mean sometimes
we need a global average that tells us
something about the overall impacts of
climate change and sometimes we need a
local estimate that tells us how ice
melts will affect a certain City as long
as we're consistent however we define
sea level can be meaningful and
instructive even if it isn't simple
thank you for watching this episode of
scishow and thank you to our patrons for
your support it takes a lot of people to
make a scishow video and your dollar
goes a long way if you like what we do
and are interested in joining our
community you can learn more at
patreon.com/scishow
[Music]