Another issue related to clouds is that of sulphate aerosols.
By aerosol, we don't mean spray can,
we mean little suspended droplets of things, hanging in the air.
And we have lots of aerosols that are produced
from the combustion of coal that has sulphur in it.
When you burn coal, that has Sulphur in it, it releases SO2
which gets turned into sulphuric acid in a few weeks in the atmosphere.
Sulphuric acid wants to be a liquid, and so it tends
to form these little tiny drops about a micron in size.
These are much smaller than than cloud droplets which
are typically 8 to 10 microns in size.
These tiny ones,
since they are comparable in size to the
wavelength of visible light, are really, really great scatterers.
They're very good at reflecting sunlight back out to space.
The presence of these aerosols as droplets of sulphuric acid of
this size, this is called the direct effect of sulphate aerosols.
As opposed to an indirect effect which is where the
aerosol acts as a cloud condensation nuclei.
It's like a cloud seed.
It enables water to accumulate on the droplet.
And it makes altered cloud droplets. This is called the Indirect effect.
If you have clean air that's rising up
and so it's expanding and cooling down, and so the
water wants to condense, it's really hard for water
vapor to start a cloud droplet out of clean air.
Once one cloud droplet or a few cloud droplets start to form, all the rest of
the water goes to them, rather than finding
new cloud droplets to form because it's so difficult.
And so you tend to get not very many but larger cloud droplets.
Relative to the case where you have lots of these
condensation nuclei, these little
droplet seeds, from these aerosols.
In dirty air, if you have the same trajectory, the same amount of water
vapor, it tends to form a larger number of smaller drops.
And so, because these are smaller than those, they are
more effective at scattering, because the smaller the better for cloud droplets.
They're all too big to scatter as efficiently as
they could, but the smaller they are, the better.
Also, if the drops are smaller, they will tend to rain out
more slowly, so the cloud may live longer. This alteration of the cloud
is called the indirect effect of the sulphate aerosols.
This is a diagram here of what's called the radiative forcing.
This is the change in the energy
balance of the earth in watts per square meter.
It's caused by these aerial sinks.
Here, carbon dioxide is about one and a half blocks per square meter.
And then other greenhouse gases, like methane and freons,
and things like that, contribute some more warming.
And there's some uncertainty in
how much of radiative forcing that produces.
And then here are the aerosols. Because what aerosols do is scatter light.
They tend to cool the planet.
They're messing with the visible energy budget in the Albedo.
There's the direct effect here and the indirect
effect is potentially even larger.
But what's important is that the
uncertainties are huge for the aerosol effects.
Much larger for the cooling impacts of aerosols than the
uncertainties in the warming effects of from the greenhouse gases.
This will become important later when we
start try to figure out the climate sensitivity of
the real earth based on the total amount of radiative forcing.
We don't know exactly if the aerosols are a strong cooling effect
that counteracts most of the warming effect from the greenhouse gases.
That means that the warming that we see is due to a smaller total forcing.
Whereas, if the aerosols are weaker,
on the weak end of how how important
we think they are,
that means that there's been a relatively
strong warming radiative forcing from just the greenhouse gases.
Which means that the the Earth
is somewhat less sensitive to watts per square meter of forcing.
We know how much the temperature has changed.
What we want to know is the watts per square meter of radiative forcing
that lead to that temperature change.
And then we can figure out how sensitive the
earth's climate is to to the watts per square meter.
But the uncertainty in the aerosol effect and especially
the indirect effect, is the thing that limits us
the most in trying to understand how much
radiative forcing is driving the planet's temperature change that
we observe.