When describing the Global Warming of Planet Earth, one should start by noting something called "heat balance". Most objects that are warmer than their surroundings will radiate heat energy, and thereby become cooler. Half the Earth does this every night, when it is exposed to a region of Outer Space
that doesn't have the Sun shining in it. And, of course, an object that is cooler than its surroundings tends to absorb heat. The average temperature of an object depends on the RATES of absorption and radiation, over the long run. Please note that one aspect of a hot object is that it also radiates faster than a merely warm object. Two objects that absorb heat at different rates will maintain (a long-run average of) different temperatures, simply because they will also radiate at different rates. That is the essence of heat balance.
Global warming is caused partly by humans adding extra energy to the surface of Planet Earth, over-and-above what the Sun supplies. Obviously the average temperature must go up a bit, so that faster radiation at that higher temperature will balance the extra supplied heat. Mostly, however, global warming is caused by interfering with the Natural heat-balance mechanisms, regarding Sun and Earth only. When humans dump lots of "greenhouse gases" into the atmosphere, they are causing a blockage of some of the normally-radiated heat from the planet's surface, so the natural result is that the surface warms up a bit, so it can radiate at a higher rate, and the amount that passes the blockage again becomes equal to the "constant" energy arriving from the Sun. It is that higher average temperature at the Earth's surface that will have worrisome consequences for many life-forms.
The key to this idea derives from the known fact that the Sun's energy is not actually truly constant. There was a period of time in the middle of the last millenium, known as the "Little Ice Age", in which sufficient Global Cooling occurred, for a long enough time, to put an end to the Vikings. It is known that the Sun exhibited almost no sunspots for the duration of that Little Ice Age. Nobody today knows why, but obviously what has happened before may happen again.
Anyway, since it should be equally obvious that if the amount of Solar Energy impinging upon the Earth directly affects the average temperature of the Earth, all we have to do is deliberately go about changing that quantity, to control the average temperature of the Earth.
Now consider a Solar Sail. This device is a vast (many kilometers wide) thin membrane of material, designed to intercept sunlight. Light (and Solar Wind) bounces off a Solar Sail somewhat like ordinary Earthly winds bounce off ordinary sailboat sails. Momentum is transferred, and the sail (with attached vessel) moves. Solar sails have to be extremely lightweight, because the "wind" it interacts with is feeble indeed. Too much mass, and the Sun's gravity will win! But that means that with just the right amount of mass, a Solar Sail can effectively "hover" at any position in the Sun's gravity gradient. (The inverse/square law affects both gravity and sunshine at exactly the same rate, with distance from the Sun.)
Okay, so let's place a Solar Sail in hover-position at a safe distance from the Earth, and let the shadow of that Sail fall upon the North Pole in the summertime (and move the Sail so its shadow falls upon the South Pole in its own summertime, six months later). Such a shadow means that the total amount of heat reaching the Earth from the Sun has been reduced a bit, right? More, we have reduced the incident Sunlight at the places most affected by current Global Warming!!! They will stay cold!
Additional Solar Shadows may be desired. How about one for the Sahara Desert (or the Gobi, the Austrailian Outback, the American West...). It would be both difficult and unnecessary to try to keep a Desert Shadow Sail positioned over those places all day long -- we DO want them to receive SOME sunlight, so that we can grow crops there (with water that won't immediately evaporate in excess heat).
The other side of the coin is to make Reflective Solar Sails, that can be used as mirrors to deliberatly send additional sunlight to places where people might want to be, but are just too cold to grow enough food. Alaska, Siberia and Nepal, for example. With enough Shades and Mirrors, we can have a Termperate Earth from the Equator almost to the Poles (which we keep frozen really solid!).
Yes, I know environmentalists will hate this idea. However, as long as humans continue to breed like mindless animals, they will continue to seek more ways to wrest more living space and food from the Earth. The real solution to Global Warming is population control, because the industries needed to support a world population of, say, 100 million people, is 1/60 that needed to support 6 billion. 1/60 the industry means 1/60 the pollution, and consequences thereof. Sure, it would be nice to have non-polluting technologies as another alternative, but one of the Laws of Thermodynamics essentially says that there will ALWAYS be pollution, even if it is nothing more than pure-heat-pollution. Which leads us right back to Global Warming, courtesy of the first couple of sentences in the second paragraph of this Idea. If there are enough people breeding like mindless animals (half a trillion, perhaps, as estimated by Larry Niven with respect to his fictional "puppeteers" in his novel "Ringworld"), then that alone will suffice, no sunlight required! Which in turn brings us back to population control as the only true long-term solution.