How much reduction of insolation (solar energy) would be required to stop global warming?

Suppose a giant sheet of semi-reflective mylar (or some other kind of partial sun-screening material) was temporarily placed into space between the Earth and the Sun with the intent to lower the amount of solar energy reaching the surface of the planet.

How much solar radiation (and in what frequency ranges) would need to be filtered to cool the planet to the most-desirable global temperature? Also, would the entire planet need to be shaded or just a key area like the Equatorial region?

Obviously the plan would be to reduce the warming effect without dramatically darkening the day since so many plants require the visible spectrum of sunlight to live.

• @userzLTK Your point about the solar sail effect is well taken. In truth I said mylar or some other kind of material and I am actually rather fond of the idea of using a two-layer woven sheet of carbon nanotubes configured as a stenopeic lens. The physical mass of such material would be relatively trivial and the inherent strength of nanotubes would allow the sheet to be maintained with a fairly simple set of attitude jet "tugs", especially if the perimeter of the disk was made of a thick cable of nanotubes. Aug 2, 2015 at 2:46
• Maybe even a Texas-sized nanotube version of one of those collapsible popup reflector screens people put in their car windows, but with lots of holes in it and a few rockets on the perimeter to stabilize it.. Aug 2, 2015 at 2:54

It's not that hard to make an estimate.

CO2 traps about 2 watts per square meter. Direct sunlight at 1 Astronomical Unit is about 1,360 watts per square meter, but spread out over the earth, average night and day it's about 1/4th of that about 340 watts per square meter. (note, Casey's point is valid, it's probably better to use the number of watts that hits the Earth, so, my estimate is likely a bit low)

There may be more detailed and specific ways to look at it, but this is probably pretty close. If you shade enough sun to reduce 2 watts out of 340, or 1/170th, that should balance out climate change, at least at current levels.

From space, you can view the earth as a disk in terms of area needed for shade, so the 2D area of the earth is about 127 trillion square meters, and 1/170th of that, about 750 billion square meters, or 750,000 square KM, slightly larger than Texas and as you get closer to the sun, the area needed is reduced by the square of the relative distance.

Still, that's no easy task, to put up a shade the size of Texas into space and it's no easy task to keep it there as it would in effect by a huge solar sale and likely enormously heavy. It's far easier to simply build mirrors on earth than try to build something in space and I've seen that discussed as a possible solution to climate change. of-course, they'd need to be kept clean to be effective, it would require maintenance, but it's probably doable.

Source (1,360 watts): http://earthobservatory.nasa.gov/Features/EnergyBalance/page2.php

sources, 1.5-2 watts per square meter, CO2 (I went with the 2 watts estimate cause that includes CH4). http://www.easterbrook.ca/steve/2012/01/how-much-extra-energy-are-we-adding-to-the-earth-system/ and https://en.wikipedia.org/wiki/Radiative_forcing

• This is rough estimate but I think for a better estimate you'd need to consider how much solar energy makes it to the surface. Your 1360 W/m2 and 340 W/m2 numbers are at the top of atmosphere but only part of that is absorbed by either that atmosphere or the surface (the balance is reflected back into space). You'd also have to consider how that feeds back into terrestrial emission which is the energy the CO2 is absorbing. If I wasn't so busy I might consider running a simple 1D monte-carlo experiment to get at the estimated shortwave flux reduction necessary to equilibrate T. Aug 2, 2015 at 1:36
• @Casey, that occurred to me, and I think you're probably right. If 40% reflects directly off the upper atmosphere or earth in the form of visible light, 200 watts reach the earth and enter the "earth's thermal cycle" so to speak, which gets radiated off and about 2 watts of that are retained by CO2 - so, 1/100 not 1/170. That's probably better. I don't want to change it without specific estimates, but I'll look into that. As for effects with feedback, If you feel up for it, feel free, that's more complicated, but as I understand it, feedback mechanisms are largely temperature driven. Aug 2, 2015 at 2:55
• The Texas sized screen would have a diameter of 977 km. If it weighed 1 g/m2 it's mass would be 127.5 Mt. As userLTK mentions, big and heavy! Something like that would need to be made in outer space. The resources and effort required to make it would be phenomenal. Including the cost!
– Fred
Aug 2, 2015 at 4:30
• Obviously the cost would be damned if we felt we needed to do it to save the planet/species. And frankly I can't see us doing it for any other reason. Besides there is at least one strategy someone else has proposed that is well within our existing technology capabilities and is a lot less expensive. See this and this. Aug 2, 2015 at 6:21
• In the meantime its fun to try and imagine building something that big in outer space. Aug 2, 2015 at 6:23