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Another issue related to clouds is that of
sulphate aerosols.

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By aerosol, we don't mean spray can,

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we mean little suspended droplets of
things,  hanging in the air.

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And we have lots of aerosols that are
produced

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from the combustion of coal that has
sulphur in it.

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When you burn coal, that has Sulphur in
it, it releases SO2

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which gets turned into sulphuric acid in a
few weeks in the atmosphere.

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Sulphuric acid wants to be a liquid, and
so it tends

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to form these little tiny drops about a
micron in size.

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These are much smaller than than cloud
droplets which

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are typically  8 to 10 microns in
size.

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These tiny ones,

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since they are comparable in size to the

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wavelength of visible light, are really,
really great scatterers.

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They're very good at reflecting sunlight
back out to space.

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The presence of these aerosols as
droplets of sulphuric acid of

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this size, this is called the direct
effect of sulphate aerosols.

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As opposed to an indirect effect which is
where the

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aerosol acts as a cloud
condensation nuclei.

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It's like a cloud seed.

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It enables water to accumulate on the
droplet.

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And it makes altered cloud droplets.
This is called the Indirect effect.

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If you have clean air that's rising up

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and so it's expanding and cooling down,
and so the

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water wants to condense, it's really hard
for water

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vapor to start a cloud droplet out of
clean air.

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Once one cloud droplet or a few cloud
droplets start to form, all the rest of

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the water goes to them, rather than
finding

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new cloud droplets to form because it's so
difficult.

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And so you tend to get not very many but
 larger cloud droplets.

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Relative to the case where you have lots
of these

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condensation nuclei, these little

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droplet seeds, from these
aerosols.

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In dirty air, if you have the same
trajectory, the same amount of water

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vapor, it tends to form a larger number of
smaller drops.

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And so, because these are smaller
than those, they are

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more effective at scattering, because the
smaller the better for cloud droplets.

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They're all too big to scatter as
efficiently as

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they could, but the smaller they
are, the better.

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00:02:57,460 --> 00:03:03,000
Also, if the drops are smaller, they will
tend to rain out

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more slowly, so the cloud may live longer.
This alteration of the cloud

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is called the indirect effect of the
sulphate aerosols.

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This is a diagram here of what's
called the radiative forcing.

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00:03:20,730 --> 00:03:23,390
This is the change in the energy

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balance of the earth in watts per square
meter.

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00:03:27,540 --> 00:03:30,360
It's caused by these aerial sinks.

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00:03:30,360 --> 00:03:35,890
Here, carbon dioxide is about one and
a half blocks per square meter.

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00:03:35,890 --> 00:03:40,370
And then other greenhouse gases, like
methane and freons,

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and things like that, contribute some more
warming.

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And there's some uncertainty in

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how much of radiative forcing that
produces.

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And then here are the aerosols.
Because what aerosols do is scatter light.

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They tend to cool the planet.

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00:03:57,100 --> 00:04:01,040
They're messing with the visible energy
budget in the Albedo.

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00:04:01,040 --> 00:04:05,412
There's the direct effect here and
the indirect

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effect is potentially even larger.

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00:04:08,630 --> 00:04:11,230
But what's important is that the

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uncertainties are huge for the aerosol
effects.

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Much larger for the cooling impacts of
aerosols than the

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uncertainties in the warming effects of
from the greenhouse gases.

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This will become important later when
we

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start try to figure out the climate
sensitivity of

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00:04:30,560 --> 00:04:35,532
the real earth based on the total amount
of radiative forcing.

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00:04:35,532 --> 00:04:40,700
We don't know exactly if the aerosols are
a strong cooling effect

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that counteracts most of the
warming effect from the greenhouse gases.

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00:04:46,320 --> 00:04:51,320
That means that the warming that we see is
due to a smaller total forcing.

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Whereas, if the aerosols are weaker,

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00:04:53,582 --> 00:04:55,844
on the weak end of how how important

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00:04:55,844 --> 00:04:57,110
we think they are,

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00:04:57,110 --> 00:04:59,430
that means that there's been a 
relatively

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00:04:59,430 --> 00:05:04,130
strong warming radiative forcing from just
the greenhouse gases.

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00:05:04,130 --> 00:05:09,370
Which means that the
the Earth

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00:05:09,370 --> 00:05:14,700
is somewhat less sensitive to
watts per square meter of forcing.

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00:05:14,700 --> 00:05:17,450
We know how much the temperature has
changed.

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00:05:17,450 --> 00:05:21,140
What we want to know is the watts per
square meter of radiative forcing

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00:05:21,140 --> 00:05:22,600
that lead to that temperature change.

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00:05:22,600 --> 00:05:24,790
And then we can figure out how sensitive
the

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00:05:24,790 --> 00:05:28,140
earth's climate is to to the watts per
square meter.

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00:05:28,140 --> 00:05:32,556
But the uncertainty in the aerosol effect
and especially

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00:05:32,556 --> 00:05:36,328
the indirect effect, is the thing that
limits us

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00:05:36,328 --> 00:05:40,008
the most in trying to understand how much

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00:05:40,008 --> 00:05:46,172
radiative forcing is driving the
planet's temperature change that

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00:05:46,172 --> 00:05:47,381
we observe.