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The other strategy for engineering 

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the climate broadly speaking 

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is called Solar Radiation Management. 

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The idea is that 
aerosols scatter sunlight. 

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Aerosols released 
to the troposphere, like from power 
plants, don't last very long. 

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There's rain in the lower atmosphere, 
and the rain tends to scrub those 

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particles out on a time scale of a week, 
or two.

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Whereas, if you can put the sulfate 
aerosols up 

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into the stratosphere, it never rains from 
up there. 

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You have to wait for those little 
droplets to slowly settle 

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through the air. 

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They last for several years. 

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It's been calculated that something 
like a tenth of 

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the amount of sulfate aerosols that we now 
produce as a byproduct of coal production,

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if we just put those in the stratosphere, 
that would 

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be enough to make a big change in earth's 
climate. 

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And it's been tested, in fact, in a 
natural way by volcanic eruptions. 

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The last really large volcanic eruption 
was Mount Pinatubo, 

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in the Philippines in 1991. This was big 
enough that it 

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actually put aerosols into the 
stratosphere, and it lead to 

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a measurable cooling, for about a year and 
a half. 

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And so we know this would work. 

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The near term problem with this idea --

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basically what we're doing, is we're 
substituting

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stronger infrared energy 
coming down

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from all the CO2 in the atmosphere,

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and we're compensating for that, by making 
the 

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solar flux less intense, by putting these 
aerosols there. 

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The difference between sunlight 
coming down and infrared light coming 

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down, is that the sunlight makes it all 
the way to the ground, 

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and when it does that it provokes water 

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that's at the ground to evaporate, 


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to carry away that heat in the form of latent heat. 

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Infrared coming down, from greenhouse 
gases and clouds, 

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tends to get absorbed in the atmosphere,

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in the boundary layer, just the bottom layers 

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of the atmosphere, by the water vapor in 
the atmosphere. 

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When the light is absorbed by water 

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vapor, it leads to heating, as opposed to 
evaporation. 

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In the case where we have 

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got more infrared from the greenhouse gas 
pollution, 

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and we're substituting less solar, by 
putting 

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the aerosols there, we're getting less 
evaporation overall. 

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This is expected. 

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It would be a dry, more arid 
planet. 

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And the Pinatubo years were in fact more 
arid than normal. 

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On a longer time scale the problem with 
solar radiation management, as a fix 

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for climate change, is the long life time 
of CO2 in the atmosphere. 

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We could use solar radiation management 
if the 

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climate got out of control, and we wanted 
to 

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cool it down for a few decades, put up 

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with the dryness, while we scrub CO2 from 
the atmosphere. 

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But it's wrong 

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to call the solar radiation management
any kind of a fix for global warming. 

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Because we'd be leaving the planet to our 
descendants on life support. 

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If they failed to pay their climate 
bill, they would get clobbered with all 

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of the climate change that we've built in 

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to the atmosphere with a higher CO2 
concentration. 

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At best, it's a band-aid, 

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It shouldn't be considered as a real 
fix.