The other strategy for engineering
the climate broadly speaking
is called Solar Radiation Management.
The idea is that aerosols scatter sunlight.
Aerosols released to the troposphere, like from power plants, don't last very long.
There's rain in the lower atmosphere, and the rain tends to scrub those
particles out on a time scale of a week, or two.
Whereas, if you can put the sulfate aerosols up
into the stratosphere, it never rains from up there.
You have to wait for those little droplets to slowly settle
through the air.
They last for several years.
It's been calculated that something like a tenth of
the amount of sulfate aerosols that we now produce as a byproduct of coal production,
if we just put those in the stratosphere, that would
be enough to make a big change in earth's climate.
And it's been tested, in fact, in a natural way by volcanic eruptions.
The last really large volcanic eruption was Mount Pinatubo,
in the Philippines in 1991. This was big enough that it
actually put aerosols into the stratosphere, and it lead to
a measurable cooling, for about a year and a half.
And so we know this would work.
The near term problem with this idea --
basically what we're doing, is we're substituting
stronger infrared energy coming down
from all the CO2 in the atmosphere,
and we're compensating for that, by making the
solar flux less intense, by putting these aerosols there.
The difference between sunlight coming down and infrared light coming
down, is that the sunlight makes it all the way to the ground,
and when it does that it provokes water
that's at the ground to evaporate,
to carry away that heat in the form of latent heat.
Infrared coming down, from greenhouse gases and clouds,
tends to get absorbed in the atmosphere,
in the boundary layer, just the bottom layers
of the atmosphere, by the water vapor in the atmosphere.
When the light is absorbed by water
vapor, it leads to heating, as opposed to evaporation.
In the case where we have
got more infrared from the greenhouse gas pollution,
and we're substituting less solar, by putting
the aerosols there, we're getting less evaporation overall.
This is expected.
It would be a dry, more arid planet.
And the Pinatubo years were in fact more arid than normal.
On a longer time scale the problem with solar radiation management, as a fix
for climate change, is the long life time of CO2 in the atmosphere.
We could use solar radiation management if the
climate got out of control, and we wanted to
cool it down for a few decades, put up
with the dryness, while we scrub CO2 from the atmosphere.
But it's wrong
to call the solar radiation management any kind of a fix for global warming.
Because we'd be leaving the planet to our descendants on life support.
If they failed to pay their climate bill, they would get clobbered with all
of the climate change that we've built in
to the atmosphere with a higher CO2 concentration.
At best, it's a band-aid,
It shouldn't be considered as a real fix.