You see my solution every summer day on the beaches.
Okay, so I'm not putting them on the beaches, I'm putting them up in space. The entire global warming problem, as of the worst estimates, is a few degrees Fahrenheit over a century. Let's say 4-5 so we get worst case scenario on the problem.
And yet, on any desert, the day-to-night temperature drop is huge - on the order of 40-50 degrees Fahrenheit. So that means that the sun is warming the earth by that much each day.
So, at worst, we need to cool the planet each century by 1/10 the amount of energy that the sun sends each day. What percentage is that?
EQ 1: 4/40 * 1/100 * 1/365 => 0.00027%
Okay, so that's the maximum percentage of incoming energy that we need to stop.
So how many square miles of energy are we getting, anyway? You remember pi times radius squared? Well, the radius of the earth is 4000 miles, so the area of sunlight that Earth intercepts is a circle with about 50 million square miles.
EQ 2: 3.15 * 4000 * 4000 = 50,265,482 square miles
Multiply that by our percent from Equation 1 and you get about 138 square miles of umbrella needed to solve the entire problem.
EQ 3: 50,265,482 * 0.00027% = 137.7136506
So, we can solve the whole alleged problem with current technology and a couple of hundred square miles of PVC and mylar. I make that out to be roughly three hundred shuttle loads, and even with NASA's current atrocious cost structure it's a tenth or less of the cost of the other proposed solutions.
The great thing about this particular solution is that, when the sun drops out of its current warming cycle and the coming ice age hits, we can reverse the mylar and reflect sunlight down to warm the Earth by a similar amount. Rough calculations tell me that we'll need about three times this many satellites at that time, and all the carbon dioxide we can put out.