1
00:00:06,687 --> 00:00:07,668
You've probably heard that

2
00:00:07,668 --> 00:00:10,452
carbon dioxide is warming the Earth,

3
00:00:10,452 --> 00:00:11,823
but how does it work?

4
00:00:11,823 --> 00:00:13,832
Is it like the glass of a greenhouse

5
00:00:13,832 --> 00:00:15,733
or like an insulating blanket?

6
00:00:15,733 --> 00:00:17,852
Well, not entirely.

7
00:00:17,852 --> 00:00:19,272
The answer involves a bit

8
00:00:19,272 --> 00:00:22,010
of quantum mechanics, but don't worry,

9
00:00:22,010 --> 00:00:23,959
we'll start with a rainbow.

10
00:00:23,959 --> 00:00:25,818
If you look closely at sunlight separated

11
00:00:25,818 --> 00:00:27,134
through a prism,

12
00:00:27,134 --> 00:00:30,365
you'll see dark gaps where bands of color went missing.

13
00:00:30,365 --> 00:00:31,753
Where did they go?

14
00:00:31,753 --> 00:00:33,168
Before reaching our eyes,

15
00:00:33,168 --> 00:00:35,080
different gases absorbed those

16
00:00:35,080 --> 00:00:37,688
specific parts of the spectrum.

17
00:00:37,688 --> 00:00:39,847
For example, oxygen gas snatched up

18
00:00:39,847 --> 00:00:41,695
some of the dark red light,

19
00:00:41,695 --> 00:00:44,914
and sodium grabbed two bands of yellow.

20
00:00:44,914 --> 00:00:46,308
But why do these gases absorb

21
00:00:46,308 --> 00:00:48,362
specific colors of light?

22
00:00:48,362 --> 00:00:51,279
This is where we enter the quantum realm.

23
00:00:51,279 --> 00:00:53,637
Every atom and molecule has a set number

24
00:00:53,637 --> 00:00:56,968
of possible energy levels for its electrons.

25
00:00:56,968 --> 00:00:58,979
To shift its electrons from the ground state

26
00:00:58,979 --> 00:01:00,297
to a higher level,

27
00:01:00,297 --> 00:01:03,701
a molecule needs to gain a certain amount of energy.

28
00:01:03,701 --> 00:01:06,119
No more, no less.

29
00:01:06,119 --> 00:01:08,123
It gets that energy from light,

30
00:01:08,123 --> 00:01:11,292
which comes in more energy levels than you could count.

31
00:01:11,292 --> 00:01:14,657
Light consists of tiny particles called photons

32
00:01:14,657 --> 00:01:16,761
and the amount of energy in each photon

33
00:01:16,761 --> 00:01:19,126
corresponds to its color.

34
00:01:19,126 --> 00:01:22,149
Red light has lower energy and longer wavelengths.

35
00:01:22,149 --> 00:01:25,823
Purple light has higher energy and shorter wavelengths.

36
00:01:25,823 --> 00:01:28,642
Sunlight offers all the photons of the rainbow,

37
00:01:28,642 --> 00:01:30,663
so a gas molecule can choose

38
00:01:30,663 --> 00:01:33,270
the photons that carry the exact amount of energy

39
00:01:33,270 --> 00:01:35,024
needed to shift the molecule to

40
00:01:35,024 --> 00:01:37,381
its next energy level.

41
00:01:37,381 --> 00:01:38,715
When this match is made,

42
00:01:38,715 --> 00:01:40,644
the photon disappers as the molecule

43
00:01:40,644 --> 00:01:41,965
gains its energy,

44
00:01:41,965 --> 00:01:44,765
and we get a small gap in our rainbow.

45
00:01:44,765 --> 00:01:47,887
If a photon carries too much or too little energy,

46
00:01:47,887 --> 00:01:49,366
the molecule has no choice but

47
00:01:49,366 --> 00:01:51,219
to let it fly past.

48
00:01:51,219 --> 00:01:53,639
This is why glass is transparent.

49
00:01:53,639 --> 00:01:55,711
The atoms in glass do not pair well

50
00:01:55,711 --> 00:01:58,384
with any of the energy levels in visible light,

51
00:01:58,384 --> 00:02:00,647
so the photons pass through.

52
00:02:00,647 --> 00:02:03,760
So, which photons does carbon dioxide prefer?

53
00:02:03,760 --> 00:02:05,585
Where is the black line in our rainbow

54
00:02:05,585 --> 00:02:07,960
that explains global warming?

55
00:02:07,960 --> 00:02:09,789
Well, it's not there.

56
00:02:09,789 --> 00:02:11,952
Carbon dioxide doesn't absorb light directly

57
00:02:11,952 --> 00:02:13,286
from the Sun.

58
00:02:13,286 --> 00:02:14,507
It absorbs light from a totally

59
00:02:14,507 --> 00:02:16,232
different celestial body.

60
00:02:16,232 --> 00:02:19,016
One that doesn't appear to be emitting light at all:

61
00:02:19,016 --> 00:02:20,739
Earth.

62
00:02:20,739 --> 00:02:22,175
If you're wondering why our planet

63
00:02:22,175 --> 00:02:23,876
doesn't seem to be glowing,

64
00:02:23,876 --> 00:02:27,138
it's because the Earth doesn't emit visible light.

65
00:02:27,138 --> 00:02:29,306
It emits infared light.

66
00:02:29,306 --> 00:02:30,920
The light that our eyes can see,

67
00:02:30,920 --> 00:02:32,992
including all of the colors of the rainbow,

68
00:02:32,992 --> 00:02:35,422
is just a small part of the larger spectrum

69
00:02:35,422 --> 00:02:37,825
of electromagnetic radiation,

70
00:02:37,825 --> 00:02:40,198
which includes radio waves, microwaves,

71
00:02:40,198 --> 00:02:43,286
infrared, ultraviolet, x-rays,

72
00:02:43,286 --> 00:02:45,103
and gamma rays.

73
00:02:45,103 --> 00:02:47,744
It may seem strange to think of these things as light,

74
00:02:47,744 --> 00:02:49,291
but there is no fundamental difference

75
00:02:49,291 --> 00:02:53,027
between visible light and other electromagnetic radiation.

76
00:02:53,027 --> 00:02:54,127
It's the same energy,

77
00:02:54,127 --> 00:02:56,394
but at a higher or lower level.

78
00:02:56,394 --> 00:02:58,374
In fact, it's a bit presumptuous to define

79
00:02:58,374 --> 00:03:01,969
the term visible light by our own limitations.

80
00:03:01,969 --> 00:03:04,865
After all, infrared light is visible to snakes,

81
00:03:04,865 --> 00:03:07,827
and ultraviolet light is visible to birds.

82
00:03:07,827 --> 00:03:09,907
If our eyes were adapted to see light of

83
00:03:09,907 --> 00:03:12,283
1900 megahertz, then a mobile phone

84
00:03:12,283 --> 00:03:13,423
would be a flashlight,

85
00:03:13,423 --> 00:03:14,425
and a cell phone tower

86
00:03:14,425 --> 00:03:17,157
would look like a huge lantern.

87
00:03:17,157 --> 00:03:19,315
Earth emits infrared radiation

88
00:03:19,315 --> 00:03:21,240
because every object with a temperature

89
00:03:21,240 --> 00:03:24,468
above absolute zero will emit light.

90
00:03:24,468 --> 00:03:26,726
This is called thermal radiation.

91
00:03:26,726 --> 00:03:28,158
The hotter an object gets,

92
00:03:28,158 --> 00:03:30,857
the higher frequency the light it emits.

93
00:03:30,857 --> 00:03:32,561
When you heat a piece of iron,

94
00:03:32,561 --> 00:03:35,956
it will emit more and more frequencies of infrared light,

95
00:03:35,956 --> 00:03:40,322
and then, at a temperature of around 450 degrees Celsius,

96
00:03:40,322 --> 00:03:43,082
its light will reach the visible spectrum.

97
00:03:43,082 --> 00:03:45,310
At first, it will look red hot.

98
00:03:45,310 --> 00:03:46,606
And with even more heat,

99
00:03:46,606 --> 00:03:48,112
it will glow white

100
00:03:48,112 --> 00:03:51,166
with all of the frequencies of visible light.

101
00:03:51,166 --> 00:03:52,857
This is how traditional light bulbs

102
00:03:52,857 --> 00:03:54,309
were designed to work

103
00:03:54,309 --> 00:03:56,187
and why they're so wasteful.

104
00:03:56,187 --> 00:04:00,265
95% of the light they emit is invisible to our eyes.

105
00:04:00,265 --> 00:04:02,174
It's wasted as heat.

106
00:04:02,174 --> 00:04:04,827
Earth's infrared radiation would escape to space

107
00:04:04,827 --> 00:04:07,358
if there weren't greenhouse gas molecules

108
00:04:07,358 --> 00:04:09,080
in our atmophere.

109
00:04:09,080 --> 00:04:11,834
Just as oxygen gas prefers the dark red photons,

110
00:04:11,834 --> 00:04:14,512
carbon dioxide and other greenhouse gases

111
00:04:14,512 --> 00:04:17,153
match with infrared photons.

112
00:04:17,153 --> 00:04:18,914
They provide the right amount of energy

113
00:04:18,914 --> 00:04:22,085
to shift the gas molecules into their higher energy level.

114
00:04:22,085 --> 00:04:24,410
Shortly after a carbon dioxide molecule

115
00:04:24,410 --> 00:04:26,727
absorbs an infrared photon,

116
00:04:26,727 --> 00:04:29,182
it will fall back to its previous energy level,

117
00:04:29,182 --> 00:04:32,714
and spit a photon back out in a random direction.

118
00:04:32,714 --> 00:04:34,619
Some of that energy then returns

119
00:04:34,619 --> 00:04:36,098
to Earth's surface,

120
00:04:36,098 --> 00:04:37,639
causing warming.

121
00:04:37,639 --> 00:04:39,447
The more carbon dioxide in the atmosphere,

122
00:04:39,447 --> 00:04:41,380
the more likely that infrared photons

123
00:04:41,380 --> 00:04:43,558
will land back on Earth

124
00:04:43,558 --> 00:04:45,124
and change our climate.