[MUSIC] Another issue related to clouds is that of sulfate aerosols. So by aerosol we don't mean spray can. We mean little suspended droplets of things sort of hanging in the air, and we have lots of aerosols that are produced from the combustion of coal that has sulfur in it. So when you burn coal, it has sulfur. It releases SO2, which gets turned into sulfuric acid in a few weeks in the atmosphere. Sulfuric acid wants to be a liquid, and so it tends to form these little, tiny drops, about a micron in size. So these are much smaller than cloud droplets which are typically eight to ten microns in size. So these tiny ones, since they are comparable in size to the wave length of visable light, are really really great scatterers. They're very good at reflecting sunlight back out to space. So the presence of these aerosols as droplets of sulfuric acid of this size. This is called the direct effect of sulfate aerosols. As opposed to an indirect effect, which is where the aerosol forms, it 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. And this is called the indirect effect. So 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. So 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 sort of larger cloud droplets. Relative to the case where you have lots of these condensation nuclei, these little droplet seeds from these aerosols. So 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 these, 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 sulfate aerosols. So 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 that's caused by these various things. So here's carbon dioxide is about one and a half watts per square meter, and then other greenhouse gasses like methane and freons and things like that contribute some more warming. And there's some uncertainty in how much 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, and the albedo. And so, there's the direct effect here and then the indirect effect is, 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. So this will become important later when you start trying 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. Then 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 on the weak end of how important we think they are, that means that there's been a pretty relatively strong warming radiative forcing from just the greenhouse gases, which means that the earth is actually somewhat less sensitive to watts per square meter of forcing. So we know how much the temperature has changed, what we want to know is the watts per square meter of radiative forcing that led to that temperature change and then we can figure out how sensitive the Earth's climate is 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 radiative forcing is driving the planet temperature change that we observe. [MUSIC]
