>The wavelength they're proposing usage of is significantly large enough to make the amount of energy negligible to any life in the path of the beam. The receiver dish is between 3-10km in diameter, and if you were to walk out on the dish during the night you would be exposed to about half the amount of radiation as you would walking around in the midday sun. So no, not very dangerous. You do realize that the Sun bombards us with massive amounts of microwave radiation already? This is simply enhancing and focusing that energy to a point where we can more efficiently collect it 24/7 instead of simply during the daylight. And the loss to the atmosphere is estimated to about about 1.5-2%. We're talking miniscule amounts compared to the energy the sun pours into our atmosphere. Not to mention, you would also be reducing consumption of fossil fuels by a significant amount, thus actually overall reducing the amount of energy added to the atmosphere. Assuming 2% of a 30GW space solar energy plant, you're looking at 600MW of energy being lost into the atmostphere. To produce 30GW of energy on fossil fuels, you're dumping about 70,000 metric tonnes of carbon dioxide into the atmosphere each year. CO2 is dangerous in the atmosphere because it happens to vibrate and absorb energy in the Infared spectrum, not Microwave spectrum. Microwaves at the energy level proposed here would not excite water molecules or CO2 molecules in the atmosphere, thus it would not increase the thermal energy in the atmosphere by any significant amount.

Copying my response to someone else above.