1
99:59:59,999 --> 99:59:59,999
In 1992,

2
99:59:59,999 --> 99:59:59,999
a cargo ship carrying bath toys 
got caught in a storm.

3
99:59:59,999 --> 99:59:59,999
Shipping containers washed overboard,

4
99:59:59,999 --> 99:59:59,999
and the waves swept 28,000 rubber ducks 
and other toys into the North Pacific.

5
99:59:59,999 --> 99:59:59,999
But they didn’t stick together.

6
99:59:59,999 --> 99:59:59,999
Quite the opposite–

7
99:59:59,999 --> 99:59:59,999
the ducks have since washed up 
all over the world,

8
99:59:59,999 --> 99:59:59,999
and researchers have used their paths

9
99:59:59,999 --> 99:59:59,999
to chart a better understanding
of ocean currents.

10
99:59:59,999 --> 99:59:59,999
Ocean currents are driven 
by a range of sources:

11
99:59:59,999 --> 99:59:59,999
the wind, tides, changes in water density,

12
99:59:59,999 --> 99:59:59,999
and the rotation of the Earth.

13
99:59:59,999 --> 99:59:59,999
The topography of the ocean floor 
and the shoreline modifies those motions,

14
99:59:59,999 --> 99:59:59,999
causing currents to speed up,

15
99:59:59,999 --> 99:59:59,999
slow down, or change direction.

16
99:59:59,999 --> 99:59:59,999
Ocean currents fall into 
two main categories:

17
99:59:59,999 --> 99:59:59,999
surface currents and deep ocean currents.

18
99:59:59,999 --> 99:59:59,999
Surface currents control the motion

19
99:59:59,999 --> 99:59:59,999
of the top 10 percent 
of the ocean’s water,

20
99:59:59,999 --> 99:59:59,999
while deep-ocean currents mobilize 
the other 90 percent.

21
99:59:59,999 --> 99:59:59,999
Though they have different causes,

22
99:59:59,999 --> 99:59:59,999
surface and deep ocean currents 
influence each other

23
99:59:59,999 --> 99:59:59,999
in an intricate dance that keeps 
the entire ocean moving.

24
99:59:59,999 --> 99:59:59,999
Near the shore,

25
99:59:59,999 --> 99:59:59,999
surface currents are driven 
by both the wind and tides,

26
99:59:59,999 --> 99:59:59,999
which draw water back and forth 
as the water level falls and rises.

27
99:59:59,999 --> 99:59:59,999
Meanwhile, in the open ocean, wind is the 
major force behind surface currents.

28
99:59:59,999 --> 99:59:59,999
As wind blows over the ocean,

29
99:59:59,999 --> 99:59:59,999
it drags the top layers 
of water along with it.

30
99:59:59,999 --> 99:59:59,999
That moving water pulls on 
the layers underneath,

31
99:59:59,999 --> 99:59:59,999
and those pull on the ones beneath them.

32
99:59:59,999 --> 99:59:59,999
In fact, water as deep as 400 meters

33
99:59:59,999 --> 99:59:59,999
is still affected by the wind 
at the ocean’s surface.

34
99:59:59,999 --> 99:59:59,999
If you zoom out to look at the patterns 
of surface currents all over the earth,

35
99:59:59,999 --> 99:59:59,999
you’ll see that they form 
big loops called gyres,

36
99:59:59,999 --> 99:59:59,999
which travel clockwise 
in the northern hemisphere

37
99:59:59,999 --> 99:59:59,999
and counter-clockwise 
in the southern hemisphere.

38
99:59:59,999 --> 99:59:59,999
That’s because of the way 
the Earth’s rotation

39
99:59:59,999 --> 99:59:59,999
affects the wind patterns that
give rise to these currents.

40
99:59:59,999 --> 99:59:59,999
If the earth didn’t rotate,

41
99:59:59,999 --> 99:59:59,999
air and water would simply 
move back and forth

42
99:59:59,999 --> 99:59:59,999
between low pressure at the equator

43
99:59:59,999 --> 99:59:59,999
and high pressure at the poles.

44
99:59:59,999 --> 99:59:59,999
But as the earth spins,

45
99:59:59,999 --> 99:59:59,999
air moving from the equator to the 
North Pole is deflected eastward,

46
99:59:59,999 --> 99:59:59,999
and air moving back down 
is deflected westward.

47
99:59:59,999 --> 99:59:59,999
The mirror image happens 
in the southern hemisphere,

48
99:59:59,999 --> 99:59:59,999
so that the major streams of wind

49
99:59:59,999 --> 99:59:59,999
form loop-like patterns 
around the ocean basins.

50
99:59:59,999 --> 99:59:59,999
This is called the Coriolis Effect.

51
99:59:59,999 --> 99:59:59,999
The winds push the ocean beneath 
them into the same rotating gyres.

52
99:59:59,999 --> 99:59:59,999
And because water holds onto heat 
more effectively than air,

53
99:59:59,999 --> 99:59:59,999
these currents help redistribute 
warmth around the globe.

54
99:59:59,999 --> 99:59:59,999
Unlike surface currents,

55
99:59:59,999 --> 99:59:59,999
deep ocean currents are driven primarily 
by changes in the density of seawater.

56
99:59:59,999 --> 99:59:59,999
As water moves towards the North Pole,

57
99:59:59,999 --> 99:59:59,999
it gets colder.

58
99:59:59,999 --> 99:59:59,999
It also has a higher 
concentration of salt,

59
99:59:59,999 --> 99:59:59,999
because the ice crystals that form 
trap water while leaving salt behind.

60
99:59:59,999 --> 99:59:59,999
This cold, salty water is more dense,

61
99:59:59,999 --> 99:59:59,999
so it sinks,

62
99:59:59,999 --> 99:59:59,999
and warmer surface water takes its place,

63
99:59:59,999 --> 99:59:59,999
setting up a vertical current called 
thermohaline circulation.

64
99:59:59,999 --> 99:59:59,999
Thermohaline circulation of deep water 
and wind-driven surface currents

65
99:59:59,999 --> 99:59:59,999
combine to form a winding loop 
called the Global Conveyor Belt.

66
99:59:59,999 --> 99:59:59,999
As water moves from the depths of 
the ocean to the surface,

67
99:59:59,999 --> 99:59:59,999
it carries nutrients that nourish the 
microorganisms

68
99:59:59,999 --> 99:59:59,999
which form the base of many 
ocean food chains.

69
99:59:59,999 --> 99:59:59,999
The global conveyor belt is the
longest current in the world,

70
99:59:59,999 --> 99:59:59,999
snaking all around the globe.

71
99:59:59,999 --> 99:59:59,999
But it only moves a few 
centimeters per second.

72
99:59:59,999 --> 99:59:59,999
It could take a drop of water 
a thousand years to make the full trip.

73
99:59:59,999 --> 99:59:59,999
However, rising sea temperatures are 
causing the conveyor belt

74
99:59:59,999 --> 99:59:59,999
to seemingly slow down.

75
99:59:59,999 --> 99:59:59,999
Models show this causing havoc with 
weather systems

76
99:59:59,999 --> 99:59:59,999
on both sides of the Atlantic,

77
99:59:59,999 --> 99:59:59,999
and no one knows what would happen if it 
continues to slow

78
99:59:59,999 --> 99:59:59,999
or if it stopped altogether.

79
99:59:59,999 --> 99:59:59,999
The only way we’ll be able to forecast
correctly and prepare accordingly

80
99:59:59,999 --> 99:59:59,999
will be to continue to study currents 
and the powerful forces that shape them.